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Artificial bird nests are often used in studying birds whose nests are difficult to find, such as those of many tropical species. Yet the underlying assumption that predation on artificial nests accurately estimates predation on real nests may be invalid. We compared rates at which contents of real and artificial nests were lost to predators in a Panamanian rainforest. We attempted to make artificial nests as realistic as possible by moving real, undamaged nests to species-typical nest sites in a study area where the same species were actively breeding. Characteristics of new sites for the moved nests were statistically similar to nest sites chosen by birds. We baited nests with two quail eggs and monitored them for species-specific incubation periods. Predation on real and artificial nests was dissimilar in three of four species, revealing that predation on artificial nests correlated poorly with predation on real nests. In a fourth species, artificial and real nests were lost at similar rates. The latter result may have occurred by chance, because depredated real nests rarely showed any sign of damage; whereas depredated artificial nests were torn, which suggests that real and artificial nests attracted different predators. Our results indicate that artificial nests, even when built by the species themselves and placed in realistic situations, are poor predictors of real nest success and we caution against their use in the tropics. Key words: artificial bird nests, daily predation rate, nest predation, Panama, tropical forest.

Latitudinal sexual segregation during the nonbreeding season, in which males tend to winter farther north than females, is known to occur in many temperate-zone bird species. This pattern, however, has rarely been reported in species wintering in tropical regions. We investigated latitudinal sexual segregation in 45 Nearctic-Neotropical migratory bird species that winter in Mexico, using specimen records from 35 natural-history museums. We found significant latitudinal segregation in nine species: Yellow-bellied Sapsucker (Sphyrapicus varius), Swainson's Thrush (Catharus ustulatus), Orange-crowned Warbler (Vermivora celata), Yellow Warbler (Dendroica petechia; aestiva group), Yellow-rumped Warbler (Dendroica coronata; coronata group), Black-and-white Warbler (Mniotilta varia), Ovenbird (Seiurus aurocapilla), Wilson's Warbler (Wilsonia pusilla), and Indigo Bunting (Passerina cyanea). In most cases, males predominated in the northern part of species' winter ranges and females in the south, but the trend was reversed in Indigo Buntings. Discovering the frequency of latitudinal sexual segregation in Neotropical migratory birds will influence understanding of avian ecology and migration systems. Key words: differential migration, Nearctic-Neotropical migration, Neotropics, sexual segregation.

Recent field experiments tested the hypothesis that variations in the aggressiveness of territorial cocks drive Red Grouse (Lagopus lagopus scoticus) population cycles. The implications of these results were previously explored with parametrically flexible models that made specific assumptions about the functional form of direct density dependence and the form and timing of delayed density dependence. Although these models were characterized by apparently different stability conditions, they pointed at the same conclusion: that the occurrence of population cycles under this hypothesis relies on the strength of the interaction between density and aggressiveness around the system's equilibrium. To investigate if this important result is valid more generally, we develop a minimally specified model by lifting most of the assumptions on direct and delayed density dependence. Stability analysis of this functionally flexible model confirms that unstable dynamics are indeed more likely if small perturbations from equilibrium density have a strong impact on aggressiveness, and it unifies the stability conditions previously derived for the more specific models under a single, general condition. Further, we derive global, necessary, and sufficient conditions for instability and express them in terms of proportional changes in aggressiveness and density. For the first time since the inception of the hypothesis, the necessary condition quantifies the minimum strength of the intrinsic mechanism that would be required to cause unstable dynamics. We predict that unstable population dynamics are possible if proportional perturbations from equilibrium density are at least matched by proportional changes in aggressiveness. Existing field data indicate that the necessary condition for intrinsic cycles is satisfied in Red Grouse populations. In contrast, the sufficient condition is considerably more strict, implying that intrinsic instability is not an inevitable feature of the system. We conclude that the model is consistent with the demographic patterns of cyclic population fluctuations in Red Grouse and other birds of the grouse family. Key words: aggressive behavior; density dependence; intrinsic hypotheses; kinship hypothesis; Lagopus lagopus scoticus; mathematical model; necessary and sufficient conditions for instability; population cycles; Red Grouse; territoriality; unstable population dynamics.

Behavior and electrophysiological studies have demonstrated a sensitivity to characteristics of the Geomagnetic field that can be used for navigation, both for direction finding (compass) and position finding (map). The avian magnetic compass receptor appears to be a light-dependent, wavelength-sensitive system that functions as a polarity compass (i.e., it distinguishes poleward from equatorward rather than north from south) and is relatively insensitive to changes in magnetic field intensity. The receptor is within the retina and is based on one or more photopigments, perhaps cryptochromes. A second receptor system appears to be based on magnetite and might serve to transduce location information independent of the compass system. This receptor is associated with the ophthalmic branch of the trigeminal nerve and is sensitive to very small (

We studied fat stores in passerine migrants at a high-latitude site in Fairbanks, Alaska (64[degrees]50'N, 147[degrees]50'W). We examined fat-deposition strategies during the final (spring) and initial (autumn) stages of long-distance migration, 1992-1998, to (1) improve understanding of geographic fat-deposition patterns by adding a high-latitude perspective; (2) determine whether there are age-related differences in fat-deposition strategies in autumn; and (3) test the 'spring fatter' hypothesis of seasonal fat-deposition, which suggests that migrants should carry more fat in spring when they near their breeding areas than in autumn when they depart. Our analyses examined factors affecting daily fat scores during migration and compared between-season differences in fat stores among a total of 18,685 individuals of 16 migrant species. In autumn, adults had higher visible subcutaneous fat scores than immatures in 11 of 16 species. However, in all but two species, those differences were attributable to the effects of overnight low temperature, day length, and time of day, rather than age, probably because of later departures by adults. Fat scores were higher in autumn than in spring in 6 of 16 species, and body-condition indices were higher in autumn in 5 of 16 species. Only one species showed higher fat scores in spring, but that difference was not reflected in a seasonal comparison of body-condition indices. No species arrived with high fat loads in spring, and generally low fat levels in autumn suggest that high-latitude passerine migrants in North America are paying most of the energetic costs of long-distance migration with resources obtained en route to their wintering grounds. Among passerine migrants near these high-latitude breeding grounds, seasonal fat-deposition strategies appear to be responding to energetic needs at the level of daily maintenance, rather than to hypothesized insurance needs in spring or to the forthcoming needs of a long-distance migration in autumn. Received 16 October 2003, accepted 10 November 2004. Key words: birds, energetics, migration, Nearctic-Neotropic migrants, stopover ecology, subarctic stopover sites.

Various studies have shown that experiments on nest defense and enemy recognition (e.g. recognition of adult brood parasites) can be confounded by many factors. However, no study has described a confounding effect of control dummy type. Here, I show experimentally that the choice of control dummy may influence the results of an experiment and lead to erroneous conclusions. I tested recognition abilities of the Blackcap (Sylvia atricapilla), currently a host rarely used by the Common Cuckoo (Cuculus canorus). Blackcaps responded very differently to two kinds of control dummies: they ignored the Eurasian Blackbird (Turdus merula) dummy, but attacked the Rock Pigeon (Columba livia) dummy as frequently as they attacked the Common Cuckoo. The differing results may be explained by the fact that the Rock Pigeon is more similar to the Common Cuckoo than the Eurasian Blackbird is, and consequently elicited more aggressive behavior than the latter. Thus, absence of discrimination in enemy-recognition studies may reflect a methodological artifact resulting from varying abilities of particular hosts to discriminate along a continuum of recognition cues. This result has serious methodological implications for further research on enemy recognition and aggression in general: a control dummy should not be too similar to the dummy brood parasite; otherwise, the chance of detecting existing recognition abilities is low. Further, I argue that coevolution only increases pre-existing aggression in the particular host species. Therefore, increment analysis (assessing changes in host antiparasitic responses during the nesting cycle while controlling for background aggression to control dummies) provides a more accurate picture of hosts' recognition abilities than the traditional approach (when the total level of antiparasitic response is analyzed). Received 30 January 2004, accepted 3 November 2004. Key words: brood parasitism, coevolution, discrimination, methodology, nest defense, recognition.

Aspen stands and riparian areas are important to breeding birds in the southwestern U.S. because they provide resources such as food and shelter. We investigated how this importance varies throughout the year for both resident and migratory birds. We sampled birds in 96 sites, half in small isolated aspen stands and half in the ponderosa pine forest in northern Arizona during the summer of 1996, and a subset of those plots during fall of 1996 and the spring of 1997. Bird species richness and abundance vailed seasonally. During the summer there were more birds and more bird species in aspen stands. This relationship appears to be driven by an affiliation between Neotropical migrants and aspen trees. During fall, residents were associated with riparian areas. We demonstrate the importance of small inclusions of aspen to Neotropical migrants in the Southwest during the breeding season and we show that preference for habitat types among migratory groups can vary seasonally.

We hypothesized that nest attendance characteristics in Common Goldeneyes (Bucephala clangula) at the northern limit of their range differ from those of more southern populations. In 2002 and 2003, we used artificial eggs containing temperature-sensing data loggers to obtain nest attendance data from 20 incubating females over 515 days. On average ([+ or -] SE), each female spent 79.8 [+ or -] 0.3% of the day on the nest, and took 2.9 [+ or -] 0.1 recesses per day, each averaging 100.7 [+ or -] 1.5 minutes. These recess characteristics were comparable to those reported for other Common Goldeneye populations. Most recesses (88%) occurred between 09:00 and 22:00 Alaskan Daylight Time although recesses were initiated at all times of day. Female incubation behavior does not appear to be strongly influenced by coarse-level environmental variables or the female-specific variables that we measured, but could be related to a complex assortment of line-scale environmental or endogenous factors. Key words: Alaska, Bucephala clangula, Common Goldeneye, incubation, nest attendance, nesting behavior.

Migration is an important part of many temperate bird species' annual life history. We used a time-point census method to describe the changes in bird communities during the autumn migration in central Iowa. Site selection within a second-growth forest varied significantly based on diet and migration habits of birds. Temporal changes in individual activity and species presence show preferences that may be related to availability of fruit. We discuss the effect of the invasive shrub Rosa multiflora on bird abundances on both temporal and spatial scales.

Understanding movements of individual birds between breeding sites (breeding dispersal) or between natal sites and the site of first breeding (natal dispersal) is crucial to the modeling of population dynamics. Unfortunately, these aspects of demography are poorly understood for avian species in general, and for migratory songbirds in particular. This is because it is often impossible to sample broadly enough to relocate marked birds that have moved. We used stable-hydrogen ([delta]D) and carbon ([delta][sup.13]C) isotope analyses of the feathers of 139 American Redstarts (Setophaga ruticilla) and 193 Ovenbirds (Seiurus aurocapillus) to evaluate evidence for individuals molting feathers at locations other than their breeding sites from the previous year. We sampled outer rectrices from breeding populations at three extensive boreal forest sites (Prince Albert National Park and Duck Mountain, Saskatchewan. and Lac La Biche, Alberta) and at three isolated forest tracts (Cypress Hills, and Moose Mountain, Saskatchewan, and Turtle Mountain, Manitoba) in western Canada. Based on outlier analysis of [delta]D measurements, we found evidence for long-distance dispersal ranging from 0-29% of individuals. For both species, second-year birds had higher variance in [delta]D values suggesting they had a higher probability of originating from elsewhere compared to after-second-year birds. Key words: carbon-13, deuterium, dispersal, feathers, philopatry, source-sink populations, stable isotopes.

In many bird species, Arctic-breeding and temperate-zone-breeding populations may have different behavioral and physiological adaptations that reflect trade-offs between immediate survival and reproductive success. Compared with their temperate-zone-breeding counterparts, Arctic breeders face shorter breeding seasons and less predictable environments. As a result, it may be necessary for Arctic breeders to employ immediate life-saving responses (facilitated through an acute adrenocortical response) that might come at the expense of immediate reproductive success. Regardless of latitude, the sexes may also face differences in the trade-off between immediate survivorship and reproductive effort. To investigate those hypotheses, we (1) measured the adrenocortical response of male and female Yellow Warblers (Dendroica petechia) at both ends of a north-south breeding distribution and (2) examined within- and between-sex variation in corticosterone secretion during distinct stages of the breeding season that represent significant differences in parental investment (e.g. incubation and nestling provisioning). As predicted, Arctic-breeding males showed a significantly reduced adrenocortical response during the nestling period (when their parental contribution was highest; mean reduction = 39%), whereas temperate-zone-breeding males showed no significant reduction (mean reduction = 12%). Arctic-breeding and temperate-zone-breeding females did not differ in their adrenocortical responses during incubation and provisioning stages, showed reduced responses when compared with temperate-zone-breeding males, and showed responses similar to those of Arctic-breeding males during the provisioning stage. These data support the hypothesis that, during the breeding season, a reduced adrenocortical response (1) is more likely to occur when the potential for brood loss is greatest and (2) is associated with the degree of parental investment by the sexes. The balance between cost(s) and benefit(s) of the adrenocortical response may vary with the relative risks to survivorship and breeding success, and may vary between populations facing different time and energy constraints. Received 25 April 2003, accepted 12 May 2004.

We studied the applicability of Lanchester's laws of combat to explain interspecific dominance in birds. We focused on 10 species of Australian birds in the arid zone of New South Wales that foraged at an established locust trap. Consistent with the 'linear law,' larger species usually dominated smaller species in one-on-one encounters. We found no support for the 'N-square law,' which predicted that large numbers of smaller species could dominate larger species when more abundant. Further analysis of the most abundant species revealed that it was less likely to visit the locust trap when larger, more dominant heterospecifics were present. Body size, and not numerical superiority, seems to be an important determinant in interspecific foraging decisions in birds. Key works: body size, foraging behavior, group size, interspecific competition, Lanchester's laws.

Action imitation, once thought to be a behavior almost exclusively limited to humans and the great apes, surprisingly also has been found in a number of bird species. Because imitation has been viewed by some psychologists as a form of intelligent behavior, there has been interest in how it is distributed among animal species. Although the mechanisms responsible for action imitation are not clear, we are now at least beginning to understand the conditions under which it occurs. In this article, I try to identify and differentiate the various forms of socially influenced behavior (species-typical social reactions, social effects on motivation, social effects on perception, socially influenced learning, and action imitation) and explain why it is important to differentiate imitation from other forms of social influence. I also examine some of the variables that appear to be involved in the occurrence of imitation. Finally, I speculate about why a number of bird species, but few mammal species, appear to imitate.

I analyzed the species composition, structure, and trophic organization of bird assemblages in the forest canopy around Manaus, Brazil. Using three canopy towers located in primary terra firme forests, I conducted 117 censuses over a 13 month period, recording 160 bird species. The three study sites had similar species richness (between 126 and 128 species), sharing 95 species in common, which accounted for >90% of all individual records. Most species recorded in the canopy (110) belonged to the core canopy avifauna. The other 50 species included migrants (13), species that feed overhead (8), species from lower strata (15), night birds (3), and vagrants or visitors from open areas and second-growth forests (11). Most of those species, however, appeared in very low numbers in the forest canopy, except understory frugivores, which regularly fed on canopy fruits. Among the migratory species were some Nearctic (6), Austral (3), and species probably holding both migratory and resident populations in the area (4). Additionally, I found evidence that two species of parrots perform seasonal movements, leaving the forest canopy around Manaus during the dry season. Frugivores dominated the canopy avifauna in abundance, but contradicting results reported from other Neotropical forest canopies, insectivores and frugivores had equal species richness. The canopy avifauna was highly homogeneous among sites in terms of species richness, distribution among abundance classes, and number of species in each feeding guild, habitat, and microhabitat. As in Costa Rica, the canopy avifauna of Manaus was mostly composed by typical canopy bird species, and not by edge-living species as reported from Panama. I suggest that habitat fragmentation may affect the bird community composition in the forest canopy, allowing a higher influx of opportunistic species. Received 3 March 2003, accepted 16 September 2003.

Edge effects on grassland-nesting birds should be less pronounced or absent near cropland edges of grasslands that lack wooded-edge habitat often used by nest predators and brood parasites. We compared nest predation, brood parasitism and densities of dickcissel (Spiza americana) nests in relation to distance from woodland and cropland edges of Kansas tallgrass prairie. Daily nest predation rates did not differ (P > 0.25) among distance intervals ([less than or equal to] 50 m, 51-100 m, [less than or equal to] 100 m and >100 m) from either edge type or among 50-m intervals adjacent to each edge type. Brood parasitism rates by the brown-headed cowbird (Molothrus ater) were higher [less than or equal to] 100 m vs. >100 m from woodland edges (P = 0.04), being highest [less than or equal to] 50 m from woodland edges (P = 0.09). Parasitism rates were not related to distance from cropland edges, although parasitism rates [less than or equal to] 50 m from woodland and cropland edges were statistically similar (P = 0.16). Dickcissel nest densities were lower [less than or equal to] 50 m from woodland edges relative to farther distance intervals (P = 0.004), indicating dickcissel avoidance of this edge type. There was no similar pattern of nest density on cropland-edged sites, but nest densities [less than or equal to] 50 m from woodland and cropland edges were statistically similar (P = 0.17). Thus, some woodland edge effects on this grassland bird species were apparent but might vary geographically.

Nest predation has been hypothesized as a major factor influencing clutch size in birds. Indirect and correlative evidence for clutch size reduction under high nest predation risk is accumulating, but such a reduction has never been shown experimentally. We experimentally tested this 'nest predation hypothesis' by manipulating nest predation rate in the Collared Flycatcher, Ficedula albicollis. As predicted, clutch size decreased in the year following the experiment in 'depredated' areas (in which 20-35% of the nests were totally depredated) compared to control ones (no predation). This could result from (1) phenotypic plasticity in clutch size, or (2) differential breeding habitat selection, with dominant individuals or individuals laying large clutches preferentially leaving depredated areas and settling in control ones. There was no evidence for differential dispersal from areas of the two treatments, nor differential competitive abilities of breeders in areas of the two treatments. Conversely, all individuals had a smaller clutch size in depredated areas, supporting the hypothesis of clutch size plasticity. These results provide experimental support for the nest predation hypothesis of clutch size variation in birds and suggest the importance of information available to individual birds on local nest predation risk and previous experience for determining clutch size. Key words: breeding habitat selection; Collared Flycatcher; clutch size reduction; Ficedula albicollis; individual strategy; intraspecific competition; life-history traits; nest predation; phenotypic plasticity; reproductive cost; reproductive success; trade-offs.

Most forest birds include arthropods in their diet, sometimes specializing on arthropods that consume plant foliage. Experimental tests of whether bird predation on arthropods can reduce plant damage, however, are few and restricted to relatively low-diversity systems. Here, we describe an experimental test in a diverse tropical forest of whether birds indirectly defend foliage from arthropod herbivores. We also compare how the indirect effects of bird predation vary with different levels of foliage productivity in the canopy vs. the understory. For three Neotropical tree species, we observed that birds decreased local arthropod densities on canopy branches and reduced consequent damage to leaves. In contrast, we observed no evidence of bird--arthropod limitation on conspecific saplings in the less productive understory of the same forest. Our results support theory that predicts trophic cascades where productivity is high and suggest that birds play an important role in Neotropical communities by means of their indirect defense of some canopy tree species.

Egg-capping is the slipping of a hatched eggshell fragment over an intact shell. It is a rare phenomenon among avian species, probably because most eggs in the same clutch hatch synchronously, are similar in size, and parents typically remove debris--including shell fragments--from nests. Hatching asynchrony and egg-size differences are typically more pronounced in the context of interspecific brood parasitism, making conditions for egg-capping more likely. Indeed, egg-capping of host eggs occurred in 33% of clutches of Eastern Phoebes (Sayornis phoebe) parasitized by Brown-headed Cowbirds (Molothrus ater), always by the earlier hatched cowbird's eggshell, whereas it was hot detected in nonparasitized clutches near Ithaca, New York. To determine experimentally if host parents rejected their own eggs capped by the parasite's shell, cowbird shell fragments were introduced into nonparasitized phoebe clutches either as slipped over intact phoebe eggs (caps) or simply placed into the nests (noncaps). The latency for cowbird egg-fragments to disappear was greater for caps (1-13 days) because all noncaps were removed in 1 day had a greater probability of hatching failure (0.6) than noncapped eggs (0). Those data suggest that eggcapping by shell-fragments of earlier hatched cowbird eggs reduces phoebes' reproductive success and may represent a fitness cost to hosts in other parasitized species as well.

Effects of bacteria on avian hosts in the wild have received little attention until recently. Whereas the pathogenic effects of bacteria are well known, positive effects of symbiotic bacteria are more rarely considered. Nestling growth has important repercussions for offspring fitness in avian populations and may be affected by microbial colonization of the gut. Enterococcus faecalis is a common opportunistic pathogen, whereas E. faecium has been used as a growth promoter because it interacts competitively with pathogenic bacteria, E. faecalis included. We followed the growth in tarsus length and mass of 18 Pied Flycatcher (Ficedula hypoleuca) broods. Chicks were weighed and measured on days 4, 8, 10, and 13 after hatching. On day 13, wing length was also measured and cloacal swabs were taken of two chicks in each brood for detection of enterococci. In all, the methods used allowed us to detect six species of bacteria among a possibly much richer community. Most chicks had E. faecalis, whereas E. faecium was less prevalent. There was a negative association between scores for E. faecalis and for the rest of the species pooled. The presence of E. faecalis showed no detectable association with nestling mass or size at any age, whereas the presence of the other species showed significantly positive associations with mass and size on day 13, but not before. Presence of E. faecium on its own was positively associated with nestling mass and size shortly before fledging. E. faecium may act as a growth promoter in the wild through its competitive interactions with facultative pathogenic bacteria. The presence of some microbes are critically important in avian growth and development.

Cooperative breeding, in which more than two individuals live in a group and raise offspring, usually in a single nest, is found in only 3% of avian species. On the basis of a review of the literature, we found reports of groups (usually trios) at nest sites in 42 species of diurnal raptors. At least one example of cooperative breeding was found in 29% of genera and 14% of species, distributed in both Accipitridae and Falconidae. Given the difficulty of obtaining behavioral observations necessary to detect cooperative breeding in most raptor species, combined with the large number of species that have been poorly studied, cooperative breeding in diurnal raptors may be more common than our data indicate. However, when data on the sex of the extra bird(s) or relationships among group members were available, patterns were quite varied. For 7 of 13 species, groups primarily contained multiple adult males, though three of those species also had groups formed from offspring that had delayed dispersal; three species had a low, but regular, occurrence of multiple females (females of groups laid eggs in the same nest); and the remaining three species were characterized by having extra birds that were yearlings or subadults. In over half of species, groups did not appear to be composed of related individuals, contrary to many cooperatively breeding passerine species where groups are primarily composed of offspring that have delayed dispersal. Our review suggests that the evolution of group living in many raptors may be independent of delayed dispersal, and that the factors important in explaining the evolution of that behavior depend upon the benefits of group living.

There is growing awareness that costs associated with egg production play a significant role in shaping avian life histories. The life-history strategy of the Thick-billed Murre (Uria lomvia), a colonial, cliff-breeding seabird of Arctic waters, is characterized by a high annual adult survival rate, deferred breeding, and laying of a single-egg clutch. The single-egg clutch is a widespread phenomenon among seabirds and is generally thought to reflect demands of chick provisioning, rather than egg production. We compared composition of eggs laid by Thick-billed Murres most likely to be physiologically constrained in their capacity to produce eggs (young females with no prior experience, and females forming replacement eggs) to that of first eggs laid by early laying females (typically older, more experienced members of the population). Young, inexperienced females laid 4-18 days past the populationwide median laying date, and their eggs averaged 13% lighter in mass than those laid by early layers. Compared to early laid eggs, shell mass on young females' eggs was similar to that predicted from egg mass, but their eggs had a lower yolk-to-albumen ratio. There was little difference between the two groups in relative protein content of albumen, relative protein or lipid content of yolk, or amino acid makeup of protein in yolk or albumen. Replacement eggs averaged 6% lighter in mass than first eggs laid by the same females earlier that season. As with young females' eggs, replacement eggs had shells similar in mass to that predicted from egg mass, but lower yolk-to-albumen ratios, when compared to early laid eggs. Both protein and lipid concentrations in yolk were similar in first and replacement eggs, but replacements were deficient in albumen protein. Amino acid makeup of protein in yolk and albumen was similar in the two groups. Those results suggest that any limitations on egg production acting on young, inexperienced females are manifested in delayed laying and reductions in overall egg mass and proportional yolk content, but not in variation in biochemical composition of their eggs. Limitations on females forming replacement eggs are manifested not only in reduced egg mass and yolk content (as with young females), but also in changes in the biochemical composition of eggs; in particular, evidence suggests that relaying females may face a deficit of endogenous protein. The existence of such limitations suggests that demands of egg production can be significant even in a species that lays a single-egg clutch.

Several European studies have indicated advances in breeding dates of birds in the last 30 years, which authors attributed to climate change. In North America, Dunn and Winkler (1999) reported a continent-wide advance of rive to nine days in breeding dates of Tree Swallows (Tachycineta bicolor) between 1959 and 1991. I present results of an intensive study of Tree Swallows breeding on and near Long Point, Ontario, at four sites monitored 1969-2001, 1977-1986, 1978-2001, and 1987-2001. Local air temperatures varied among sites, according fo distance from Lake Erie, but there was no significant regional trend in April and May temperatures between 1969 and 2000. Median and 10th percentile dates of clutch initiation by females that were at least two years old varied among years by up to 20 days overall and 18 days within sites, and differed by 0-14 days between sites in the same year. There were no significant differences in average timing of laying among the 1970-1980, 1980-1990, and 1990-2000 periods. Nevertheless, there was a significant trend towards earlier laying in 1990-2000, which was attributable to exceptionally early laying and warm May weather in 1998-2001. Across all sites, median dates of clutch initiation were strongly correlated with average daily maximum temperatures in the first four, five-day periods in May, but clutches atone site were initiated three days earlier than predicted by temperatures alone. I estimate that climate warming of 5[degrees]C in May could result in an average advance of about seven days in the median date of laying of Tree Swallows at those sites. My results indicate that spring temperatures are strongly correlated with timing of laying, but show that spring temperatures have not increased in the Long Point region and timing of the breeding season has not advanced throughout North America.

House Wrens (Troglodytes aedon) remove eggs from the nests of other birds, including conspecifics and heterospecifics and both cup and cavity nests. Egg removal by males before females arrive increases the number of empty nests in and around a male House Wren's territory, and females might use this trait in mate choice. We manipulated the presence of empty nests in House Wren territories prior to female settlement by adding artificial nests with or without plastic eggs. We used the timing of female settlement as an index of mate preference. Our manipulation had no effect on the timing of female settlement or on variables related to maternal investment such as clutch size, egg volume, or provisioning effort. Differential investment in offspring was based on the timing of a reproductive attempt, which was unrelated to the experimental manipulation. Key words: egg removal, empty nests, House Wren, mate choice, nest destruction, parental investment, Troglodytes aedon.

Across their range, Horned Larks (Eremophila alpestris) consistently construct their nests adjacent to and north of a conspicuous object such as a tuft of grass, shrub, or rock. We studied the relative importance of a northern nest orientation to nest microclimate in Horned Larks breeding in northeastern California. Nests showed a significant northern bias in orientation angle and were 49% shaded in the early afternoon, the hottest part of the day. Artificial nests of eastern, western, and southern orientations exhibited little to no shade during this time. A northern nest orientation also allowed nests to face prevailing winds during the day and avoid them in the evening. The Horned Larks' preference for a northern nest orientation offers multiple advantages for regulation of nest microclimate. Key words: Eremophila alpestris, Horned Lark, microclimate, nest entrance, nest orientation, shade, solar radiation.

In a field experiment, we measured the giving-up densities and activity of co-occurring American Robins (Turdus migratorius) and European Starlings (Sturnus vulgaris). Across six distances from cover, both species had lower giving-up densities near cover (safe) than away (risky). In terms of activity, both species were more active near than away from cover. Preference for cover was more pronounced for robins. Both species foraged more in the morning than in the afternoon. This pattern was more pronounced for starlings. Relative to the background environment (mowed grass), robins favored the food patches more than starlings. For starlings and robins, respectively, it took 72.6 [m.sup.2] and 382.3 [m.sup.2] of background environment to generate the same feeding activity in a 1-[m.sup.2] food patch filled with 210 mealworms (Tenebrio molitor). The greater degree of herbivory by starlings may explain the robins' greater affinity for food patches (invertebrates only) relative to the background environment (both invertebrates and plants). Key words: American Robin, European Starling, foraging theory, giving-up density, habitat preference, patch use, predation risk.

Storks were previously known in Cuba only from the living Wood Stork (Mycteria americana) and two bones of the extinct species Ciconia maltha from Cienfuegos Province. Newly explored Quaternary tar seep deposits in Matanzas Province have yielded fossils of M. americana, the extinct wood stork M. wetmorei, and an unidentified species of Ciconia smaller than C. maltha. These specimens provide the first verifiable fossil record of M. americana anywhere, the first of M. wetmorei outside of Florida and California, and the first instance of these two species occurring sympatrically. Key words: Ciconia, Cuba, fossil birds, Mycteria, Quaternary, storks.

We examined the value of paternal care in the socially monogamous Chestnut-collared Longspur (Calcarius ornatus). We removed parental males from their territories after eggs hatched, and compared fledging success of these nests with that of nests at which both parents attended young. We demonstrate that males were essential for nestling survival: no young fledged from female-only nests. Thus, constraints of the breeding environment apparently impose a need for biparental care in this species. We relate this study to previous research, and suggest that ecological variables constraining males to care for young may also dictate patterns of male behavior in response to the steroid hormone testosterone. Key words: Calcarius ornatus, male removal, monogamy, paternal care, testosterone.

We monitored 18 nests of Cooper's Hawks (Accipiter cooperii) in Tucson, Arizona, and 18 nests in rural areas of southeastern Arizona from 1999-2000 to compare feeding behavior of urban- and rural-nesting hawks. We recorded the frequency of prey deliveries, the approximate size and type of prey items, and the behavior of hawks during each delivery. Differences between rates of prey delivery at urban and rural nests decreased as nestlings grew. Rate of prey delivery at urban nests exceeded that at rural nests most during the morning and least at midday. Urban hawks delivered 2.0 [+ or -] 1.2 times more prey biomass [nestling.sup.-1] [hr.sup.-1] to nests than rural hawks. The odds of males delivering prey directly to nests, and of prey items being refused, were 13.6 [+ or -] 2.3 and 2.5 [+ or -] 1.6 times greater, respectively, at urban nests than at rural nests. Male and female hawks also vocalized more at rural nests than at urban nests. Our data suggest that prey is more abundant and available to hawks in Tucson than in surrounding rural areas. Diet composition of urban- and rural-nesting hawks also differed. Doves comprised 57% of urban prey deliveries, but only 4% of rural prey deliveries, and may explain the high rate of nestling mortality from trichomoniasis, an avian disease caused by a parasitic protozoan, in Tucson. Key words: Accipiter cooperii, behavior, diet, prey delivery, trichomoniasis, urban wildlife, vocalizations.

I quantified the foraging ecology of omnivorous Tangara tanagers with three methods commonly used in the study of foraging behavior. The relative importance of two food types, arthropods and fruits, varied largely depending on which method was used for data analyses. Arthropod foraging was more important than fruit foraging when calculated using the duration of foraging. In contrast, fruit foraging was more important when characterized by the food taken at initial observation and the total number of food items taken. This bias was probably caused by the difference in distribution and abundance of these two food types. Although numerous studies have used the frequency of initial observations to quantify bird for aging behavior, this method tends to underestimate the importance of highly rewarding but scarce food types in time budgets and tends to overestimate the same food type in the number of food items in birds' diets. Key words: duration, Ecuador, foraging behavior, frequency, omnivorous birds, Tangara.

We assessed the effect of body size and bill length on the acoustic properties of the rattle calls of 15 corvid species. First, we correlated body traits and the acoustic properties of rattles, regardless of the evolutionary history of the study species. Then we repeated the analysis, taking into account phylogeny and thus excluding its effect on morphological and acoustic characters. When the potential effects of phylogeny were ruled out, the second frequency peak was negatively correlated to body size. The carrying frequency also decreased with decreasing body size, but the relationship was weaker than that resulting from a phylogeny-free analysis. This pattern is consistent with the demonstrated body-size-frequency allometry in the vocalizations of many birds. Bill length also influenced rattle spectral properties. We emphasize that patterns and processes elucidated with comparative studies can be biased if species relatedness is not considered. Key words: bill size, body size, corvids, phylogeny, short-range communication, vocalizations.

During 1995-1998 we studied the dynamics of nesting dispersion in a population of 160-200 Red-necked Grebes (Podiceps grisegena) on Lake Osakis, a large, wind-blown lake in central Minnesota. Breeding grebe pairs arriving on the lake were highly attracted to several large floating cattail (Typha angustifolia) mats that had broken off and drifted away from the mainland to become 'islands' snagged within rooted emergent beds. These mats provided prime nesting habitat protected from waves and predators that was chosen by nearly 40% of the breeding population. Most pairs establishing nests on the islands averaged 5-10 simultaneously active neighbors within 50 m, compared to pairs that established territories along the shoreline, most of which had no neighbors within 50 m. The overall success of the two strategies varied widely from year to year, depending on the water levels, nesting habitat availability, timing of windstorms, and predator activity. During most years, however, pairs that established nests on the mats hatched earlier in the season and had larger clutches and broods. The dense mats appeared to provide protection from wind and wave action, the primary cause of early nest loss. Our blood analysis work showed no genetic structure of the population and no tendency for neighbors to be closely related. However, pairs often returned to the same area and type of nesting situation in subsequent years. The behavioral flexibility of Red-necked Grebe pairs to nest in groups within prime habitat areas may enable them to breed successfully within unpredictable emergent wetland habitats. Key words: coloniality, intraspecific variability, nesting dispersion, Podiceps grisegena, Red-necked Grebe, spacing, territoriality.

We examined winter (November-January) shorebird use at 19 sites around Humboldt Bay, California, an important site for nonbreeding shorebirds. We analyzed species richness (number of species), species densities, and incidences (presence/absence) in relation to habitat characteristics (tidal flat width, channelization, standing water, timing of tidal ebb, and sediment particle size). We included site area in analyses of incidence, and site area and substrate heterogeneity in the species richness analysis. We observed a total of 19 species, 8-16 at individual sites, and this variation correlated with substrate heterogeneity. Substrate particle size correlated positively with Sanderling (Calidris alba) incidence and negatively with American Avocet (Recurvirostra americana) incidence. Amount of standing water correlated positively with Whimbrel (Numenius phaeopus) and negatively with dowitcher (Limnodromus griseus and L. scolopaceus) incidence. Width of tidal flat correlated negatively with Whimbrel incidence. Sites at which tides ebbed earliest had higher incidences of Whimbrel and Sanderling and higher densities of Long-billed Curlew (Numenius americanus), but lower yellowlegs (Tringa melanoleuca and T. flavipes) densities. The amount of channelization correlated positively with curlew densities. These habitat relationships suggest that alteration of tidal fiats at Humboldt Bay and elsewhere in coastal habitats has the potential to adversely affect patterns of shorebird distribution. Key words: bird-habitat relationships, Charadrii, habitat characteristics, Humboldt Bay, intertidal habitats, nonbreeding distribution, shorebirds.

I examined the hypothesis that male Ovenbirds (Seiurus aurocapillus) make breeding dispersal decisions based on prior breeding experience at a site. I determined the reproductive success of color-banded male Ovenbirds at sites in fragmented and unfragmented landscapes in Missouri from 1992-1995. I documented which individuals returned and I recorded their reproductive success upon return as well as the success of birds new to each plot. I obtained similar results in both landscapes. Males with different histories of reproductive success returned at different rates. Only 2 of 22 males that were paired but failed to raise young returned in the following year, whereas the return rates of unpaired males (41%, n = 37) and males that successfully raised young (54%, n = 57) did not differ significantly (P = 0.19). The patterns were more consistent with the hypothesis that return rates were due to decisions about dispersal rather than difference in survival of individuals in these groups. Key words: breeding dispersal, forest fragmentation, Ovenbirds, reproductive success, return rate, Seiurus aurocapillus.

We investigated temporal and spatial variability in the diet of chick-provisioning Adelie Penguins (Pygoscelis adeliae) breeding at all colonies within one isolated cluster in the southwestern Ross Sea, Antarctica, 1994-2000. We wished to determine if prey quality explained different population growth and emigration rates among colonies. Diet composition was described both by conventional means (stomach samples) and by analysis of stable isotopes in chick tissues (toenails of individuals killed by skuas [Stercorarius maccormicki]). Diets were similar among the four study colonies compared to the disparity apparent among 14 widely spaced sites around the continent. Calorimetry indicated that fish are more energetically valuable than krill, implying that if diet varied by colony, diet quality could attract recruits and help to explain differential rates of colony growth. However, a multiple-regression analysis indicated that diet varied as a function of year, time within the year, and percent of foraging area covered by sea ice, but not by colony location. Stable isotopes revealed similarity of diet at one colony where conventional sampling was not possible. We confirmed that sea ice importantly affects diet composition of this species in neritic waters, and found that (1) quality of summer diet cannot explain different population growth rates among colonies, and (2) stable isotope analysis of chick tissues (toenails) is a useful tool to synoptically describe diet in this species over a large area. Key words: Adelie Penguin, Antarctic, colony choice, diet variation, diet quality, Pygoscelis adeliae, stable isotope.

We estimated annual apparent survival, recruitment, and rate of population growth of breeding Ruby-throated Hummingbirds (Archilochus colubris), while controlling for transients, by using 18 years of capture-mark-recapture data collected during 1984-2001 at Hilton Pond Center for Piedmont Natural History near York, South Carolina. Resident males had lower apparent survival (0.30 [+ or -] 0.05 SE) than females (0.43 30 [+ or -] 0.04). Estimates of apparent survival did not differ by age. Point estimates suggested that newly banded males were less likely than females to be residents, but standard errors of these estimates overlapped (males: 0.60 30 [+ or -] 0.14 SE; females: 0.67 30 [+ or -] 0.09). Estimated female recruitment was 0.60 30 [+ or -] 0.06 SE, meaning that 60% of adult females present in any given year had entered the population during the previous year. Our estimate for rate of change indicated the population of female hummingbirds was stable during the study period (1.04 30 [+ or -] 0.04 SE). We suggest an annual goal of [greater than or equal to] 64 adult females and [greater than or equal to] 64 immature females released per banding area to enable rigorous future tests for effects of covariates on population dynamics. Development of a broader cooperating network of hummingbird banders in eastern North America could allow tests for regional or meta-population dynamics in this species. Key words: Archilochus colubris, mark-recapture, population growth, recruitment, Ruby-throated Hummingbird, survival, transients.

Barred Owls (Strix varia) have expanded their range into the Pacific Northwest, and anecdotal evidence suggests that they may be displacing the federally threatened Northern Spotted Owl (Strix occidentalis caurina). Our objectives were to describe the current status of Barred Owls in Oregon and compare occupancy of Spotted Owls in historic Spotted Owl territories before and after Barred Owls were first detected in those territories. Between 1974 and 1998, we estimated that 706 different Barred Owl territories were located in Oregon. From 1989-1998 an average of 60 new Barred Owl territories were located in Oregon each year. In Spotted Owl demographic study areas in Oregon and Washington, Barred Owl detections increased at Spotted Owl territories from 1987-1999. Occupancy of Spotted Owl territories declined after Barred Owls were detected within 0.80 km of the territory center. When Barred Owls were detected 0.81-2.40 km from Spotted Owl territory centers, occupancy of Spotted Owls was only marginally less than at territories without Barred Owls. This suggests that the frequency and intensity of interactions between the two species is negatively associated with distance between them. Our results suggest that land managers and regulatory agencies should regard Barred Owls as a threat to Spotted Owls, particularly if Barred Owls continue to increase in number as they have during the past 25 years. Key words: Barred Owl, competition, distribution, Northern Spotted Owl, occupancy, Strix occidentalis caurina, Strix varia.

The Screaming Cowbird (Molothrus rufoaxillaris) is a specialized brood parasite that primarily parasitizes the Bay-winged Cowbird, (Agelaioides badius; Screaming Cowbirds parasitize 80-100% of this species' nests). In contrast, the Shiny Cowbird (M. bonariensis) parasitizes more than 200 hosts. According to the differential reproductive success hypothesis, we expect that Screaming Cowbirds would have a lower reproductive success than Shiny Cowbirds when parasitizing other hosts. We assessed the breeding success of the Screaming Cowbird using an alternative host, the Brown-and-yellow Marshbird (Pseudoleistes virescens). This species is a common host of the Shiny Cowbird with 60-70% of nests parasitized, and is also regularly parasitized by the Screaming Cowbird but with lower frequency (6-20% of the nests). We compared the breeding success of Screaming and Shiny Cowbirds parasitizing this host species. No differences were found in the number of fledglings produced per egg laid between cowbird species. About 8-10% of cowbird eggs produced fledglings. The daily survival rate of Screaming Cowbird eggs was higher than daily survival rates for Shiny Cowbird eggs, but no differences were detected in the nestling daily survival rates. Moreover, we could not detect any difference in the hatching success (nestlings per egg), fledging success (fledglings per nestling), or growth rates of the two parasitic cowbird chicks. Furthermore, breeding success and growth rates of Screaming Cowbird chicks were similar to those previously reported while parasitizing the Bay-winged Cowbird. Our results are not consistent with the differential reproductive success hypothesis proposed as an explanation for the specialized parasitism of Screaming Cowbirds. Key words: breeding success, brood parasitism, Brown-and-yellow Marshbird, host specialization, Screaming Cowbird, Shiny Cowbird.

Virtually all female Common Murres (Uria aalge) continued to visit the colony after their mate had taken the chick to sea. There were significant differences among years, but the average time between a chick fledging find a female last being seen at the colony was 13 days (range 0-36). In over 99% of instances, the female was at her breeding site. On N ~5% of days she was joined by another male, and in a few cases (8% of those days) copulation was observed. None of those transient matings persisted into the next season, even when the original male did not return; thus, we found no support for the hypothesis that females might be looking for replacement mates in case they were widowed. The most successful females (in terms of breeding output over several years) tended to have the longest periods of postfledging visiting, apparently because such birds fledged their chicks early in the season, but there was no difference in daily frequency of attendance. We conclude that successful males and females were maximizing time spent occupying the best breeding sites, even to the extent that only one adult took the chick to sea to complete its development. Received 2 February 2002, accepted 14 August 2002.

Previous studies suggest that Ovenbirds (Seiurus aurocapillus) are area sensitive and apparently avoid forest edges. In 1999 and 2000, we used radiotelemetry to investigate how breeding male Ovenbirds respond to forest edges. Twenty-one males with home ranges abutting edges of seven forest fragments surrounded by agriculture were tracked for an average of two weeks. We found that sightings of males were situated 8 [+ or -] 10 m closer to edges than random locations within each home range. However, the mean time of day for edge sightings (1139 hours, 95% CI = 1052-1227 hours) occurred significantly later than the mean for sightings in the interior of forest fragments (0936 hours, 95% CI = 0856-1016 hours). That indicates that previous studies focusing on morning singing locations to delineate home-range use have likely underestimated use of edges by birds. Habitat characteristics also varied in relation to edges. Forest canopy was lower, shrubs were denser, leaf-litter thicker, and soils dryer near edges than in the portion of home ranges facing the interior of forest fragments. Arthropod biomass varied little in relation to edges, except biomass of larvae, which was greatest at edges. Boreal forest edges abutting agricultural fields do not appear to reduce habitat use or quality for breeding male Ovenbirds, and so we suggest that the generalized association between area sensitivity and edge avoidance for Ovenbirds in forest fragments be reassessed. Received 4 February 2002, accepted 1 October 2002.

Lark Sparrows (Chondestes grammacus) are declining throughout most of their range. Effective management for this species is hampered because relatively little is known about nesting ecology. We studied habitat characteristics affecting Lark Sparrow nest-site selection and nest success at nine study pastures in a southern mixed-grass prairie in Oklahoma. We used a neural-network technique to discriminate between nest and random locations, and bootstrapping with 95% confidence intervals to compare habitat features of successful and unsuccessful nests. We quantified habitat features at the nest and random points during the breeding seasons of 1999 and 2000 among three grazing treatments (control, moderate, and heavy). We located 40 nests during two years of the study, for which crude nest-success was 26.3%. Most nests were located in either moderately grazed pasture (55%) or heavily grazed pasture (40%). The neural model correctly identified nest and random points 91% of the time. Percentage of structural cover, distance to nearest structural element, bare-ground exposure, and percentage of litter cover were the most important nest-site selection criteria according to the model. Simulation analysis indicated points were classified as nest sites if they were 9% structural cover. Successful nests had less bare-ground exposure ([bar] x = 6.2 [+ or -] 1.9% [SE]) and more litter cover ([bar] x = 18.0 [+ or -] 4.6%) compared to unsuccessful nests ([bar] x = 17.5 [+ or -] 3.8% and 10.1 [+ or -] 1.6%, respectively). These results suggest that habitat management for Lark Sparrows in mixed-grass prairie should focus on creating abundant structural cover with moderate levels of litter accumulation and bare ground. Received 4 February 2002, accepted 12 September 2002.

Over a period of 19 years, we studied 237 breeding attempts of Spanish Imperial Eagles (Aquila adalberti) in Donana National Park (southwestern Spain), including 29 pairs with at least one immature member, to investigate age-related effects on population fecundity. Without considering effect of territory quality, adult pairs were significantly more productive than immature pairs. Highly significant differences in breeding performance among territories were independent of age-classes of birds occupying them. Low-quality territories were more frequently occupied by immature pairs, whereas high-quality territories were occupied mostly or exclusively by adult birds. Therefore, age and territory quality appear to be interrelated. We found no effect on average population fecundity due to breeding by immature pairs. Received 13 November 2001, accepted 26 October 2002.

If Northern Flickers (Colaptes auratus) defend feeding territories was investigated by examining home-range overlap, social interactions at foraging sites, and the predictability and subsequent defendability of their main food source, ants. Flickers did not defend feeding territories, foraging with conspecifics in 29% of observations without visible aggressive behavior. Mean ([+ or -] SD) percent cumulative home-range overlap between radiotagged individuals was 50 [+ or -] 32%; competitor density was positively related to overlap. Rain and temperature extremes reduced surface activity of potential ant prey. Simulated foraging over time decreased the number of active anthills in plots when compared to controls. Food predictability and competitive pressure may act together to make territories uneconomical for flickers to defend. Received 15 February 2002, accepted 1 October 2002.

Over a 14 year period, we determined offspring sex ratios in a population of Savannah Sparrows (Passerculus sandwichensis) breeding on Kent Island, an isolated 80 ha island in the Bay of Fundy, New Brunswick, Canada, based on morphological measurements of 318 independent juveniles and 361 returning adults of known parentage. The mean annual offspring sex ratio was exactly 1.00 ([+ or -] 0.23, range= 0.61-1.41, n = 14). In no year did offspring sex ratio deviate significantly from 50:50, regardless of the sex ratio of breeding adults. Offspring sex ratio did not vary as a function of the timing of breeding, maternal or paternal age, or parental mating status (monogamy vs. polygamy). Overall, our data provide strong support for 50:50 offspring sex ratios at the population level (Fisher 1930), but no support for the notion of adaptive modification of offspring sex ratios by individuals (Trivers and Willard 1973). Received 25 July 2001, accepted 12 September 2002.

The recent decline of Golden-winged Warblers (Vermivora chrysoptera) correlates with the loss of suitable nesting habitat, range expansion by Blue-winged Warblers (V. pinus), and eastward expansion of Brown-headed Cowbirds (Molothrus ater). Multivariate statistics were used to examine effects of those factors on Golden-winged Warbler reproduction in north central New York. Herb and shrub cover were positively correlated with clutch size. Blue-winged Warbler proximity was negatively correlated with Golden-winged Warbler clutch size. Tree cover and perhaps herb cover, after adjusting for brood size, correlated with a reduced number of Golden-winged Warbler fledglings. Herbaceous cover correlated with a greater number of cowbird eggs in Golden-winged Warbler nests. Cowbird parasitism correlated with a reduction in the number of Golden-winged Warbler eggs incubated and proportion of incubated eggs that hatched. However, cowbird parasitism, after adjusting for brood size, did not significantly affect nestling success rate. Cowbirds parasitized 30% of Golden-winged Warbler nests, which reduced the number of Golden-winged Warblers fledged by ~17%. Average herb and tree cover values were 69 and 22 in Golden-winged Warbler territories and 60 and 23 in Blue-winged Warbler territories, respectively, with herb cover significantly greater for Golden-winged Warblers. Territories in the earliest stages of succession used by Golden-winged Warblers supported larger clutches and a reduction in the strong, negative effect of Blue-winged Warbler proximity and an increase in the negative effect correlated with cowbirds, if cowbirds were locally abundant. Received 10 January 2002, accepted 23 October 2002.