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During the course of a plague epizootic, decimation of rodent host populations may result in the transfer of fleas to alternate or phoretic hosts, including to sympatric raptors that prey on rodents. We studied flea abundance and flea species assemblages on burrowing owls (Athene cunicularia hypugaea) in southwestern Idaho before (2012 – 2014), during (2015 – 2016), and after (2017) an epizootic of plague in Piute ground squirrels (Urocitellis mollis). We examined (1) if a larger proportion of burrowing owl nests contained fleas, (2) the likelihood that owls within a high flea abundance class increased, and (3) if owls harbored ground squirrel fleas during the epizootic. Using a flea abundance index assigned to 1,184 owls from 236 nests, the proportion of nests and the likelihood that owls had high flea abundance decreased rather than increased during epizootic years. Moreover, of 3,538 collected fleas from owls at 143 nests, no fleas were species that Piute ground squirrels typically harbor. Instead, Pulex irritans was the predominant flea collected in all study years (> 99%). Thus, although raptors may play a role in plague, there was no evidence that the die-off of ground squirrels resulted in higher flea intensity in burrowing owls or that they served as frequent accidental or phoretic hosts for ground squirrel fleas that could potentially be infectious with Yersinia pestis.

Western Burrowing Owls (Athene cunicularia hypugaea) frequently nest near agricultural lands. In southwestern Idaho, greater population density in agricultural landscapes appears to be driven in part by reliable and abundant prey populations. However, these potential benefits may be offset if agricultural land use increases pesticide exposure, especially during the sensitive reproduction period. Thus, we investigated the extent to which Burrowing Owls nesting near croplands within the Morley Nelson Snake River Birds of Prey National Conservation Area (NCA), Idaho, were exposed to organophosphate (OP), carbamate (CB), and/or organochlorine (OC) pesticides. We examined plasma cholinesterase activity, OP and CB residues in foot-wash samples, and OCs in soils and whole egg contents collected from owls and their nests along a distance gradient from agricultural fields. We further measured eggshell thickness to assess potential for thinning from OC exposure. There was no inhibition of cholinesterase activities in adult or nestling owls near agriculture, and foot-wash samples from adults tested negative for OP and CB pesticides. The OC p,p′-DDE, a metabolite of DDT, occurred in eggs at 27 of 58 nests, but there was some evidence that concentrations increased with increasing distance from agriculture. Concentrations of p,p′-DDE in eggs were relatively low compared to harmful levels in other avian species, were not correlated with eggshell thickness, and did not appear to reduce nesting success. Neither DDT, its metabolites, nor any other OCs were detected in soil samples from local owl breeding areas. These results indicate that p,p′-DDE, organophosphate, and carbamate exposure were not causing toxicity or reproductive impairment of Burrowing Owls in the NCA during our study years.

Bird species, including raptors, can often be categorized into two groups depending upon their response to alterations in clutch size while laying. For some, clutch size is predetermined prior to the start of laying (i.e., determinate species). In contrast, the clutch size of indeterminate layers can be influenced by external factors present at the time of laying. Using field experiments, our objective was to examine the egg-laying responses of female Western Burrowing Owls (Athene cunicularia hypugaea) to manipulations of clutch size. To assess whether Burrowing Owls are determinate or indeterminate egg-layers, we altered female clutch size by removing or adding one egg during the laying period. At the time of egg removal or addition, the resident female had 1–5 of her own eggs present in her clutch. We compared the size of completed clutches of both removal and addition nests to the size of completed clutches at control nests. Mean clutch size at removal nests (x = 8.3 eggs, SE = 0.44, n = 9) ...

Increases in yolk androgens within and among avian clutches have been correlated with decreased incubation time, increased aggression within a nest, increased begging behaviour, decreased immune response, and decreased life span. Although the mechanisms that lead to variability in yolk androgens within and between clutches are not completely known, yolk androgens can be a function of both social and environmental conditions. We were interested in if and how nesting density, laying date, and laying order influenced yolk androgens in Western Burrowing Owls ( Athene cunicularia hypugaea (Bonaparte, 1825)) in which nest density varies considerably. In 2006 and 2007, we used radioimmunoassay to quantify the concentrations of testosterone, 5α-dihydrotestosterone, and androstenedione in the egg yolks from one early and one late-laid egg in 47 nests of Burrowing Owls located in the Morley Nelson Snake River Birds of Prey National Conservation Area in southern Idaho. Nesting density had no detectable effect on yolk androgens. Yolk androgens varied temporally and peaked in the middle of the laying season while being low before and after this time period. Within nests, late-laid eggs had higher testosterone and dihydrotestosterone than early-laid eggs; adrostendione exhibited a similar pattern in one but not both years of our study. It is possible that the seasonal pattern in yolk androgens that we observed is related to aspects of mate quality for females or declining chances of fledging success for later nesting females, whereas rises in egg androgens between early and late eggs within clutches could reflect a mechanism to assist nestlings from late-laid eggs that hatch one to several days after their siblings to better compete for resources within the nest or promote survival in the presence of larger siblings.

Western burrowing owls (Athene cunicularia hypugaea) are ground-dwelling owls distributed through- out western North America. Because of population declines, this species is considered endangered in Canada, and burrowing owls are listed as a species of conservation concern in states of the western USA. Korfanta et al. (2002) previously presented primers for seven microsatel- lite loci in burrowing owls. Parentage and relatedness studies require a larger number of markers for accuracy and precision. Here, we developed and characterized 18 addi- tional microsatellite DNA loci, and we tested these loci in 23 individuals. The number of alleles per locus ranged from 2 to 11; two loci deviated from Hardy-Weinberg equilibrium following Bonferroni correction; we did not detect linkage disequilibrium following Bonferroni cor- rection; and the probability of exclusion for parent pairs using all loci was (0.9999. We envision these loci will facilitate detailed analyses of the genetic mating system of burrowing owls, which is poorly understood.

Broadcasts of conspecific vocalizations are used to increase detection rates on surveys of secretive bird species, yet the assessment of detectability necessary to fully interpret such survey data is frequently lacking. We used radiotelemetry to evaluate the probability of detection of 17 radio-tagged male Flammulated Owls (Otus flammeolus) using conspecific broadcast surveys in Idaho during 2005 and 2006. Probability of detection among the 11 paired, five unpaired, and one unknown pairing status owls was 100% during the pair-bonding and incubation periods of the breeding season, after which it declined to less than 15% during the postfledging period. Paired males showed a different pattern than unpaired males. Following hatching of eggs, detectability of paired males declined gradually over a 6-week period, whereas detectability of unpaired males dropped sharply for a 4-week period before increasing during the postfledging period. We suggest that surveys for Flammulated Owls be conducted during the pair-bonding and incubation periods when high detectability permits stronger inference concerning the presence or absence of owls based on broadcast survey detections.

Three hypotheses that attempt to explain latitudinal sex ratio variation on wintering grounds are: 1) the behavioral dominance hypothesis, 2) the arrival time/sexual selection hypothesis, and 3) the body size/physiological hypothesis. Testing these hypotheses is difficult because many predictions are not mutually exclusive. By examining migratory behavior and winter distributions of House Finches (Carpodacus mexicanus), a species in which females dominate males despite their smaller size, we controlled for confounding effects of body size on behavioral dominance. We examined U.S. Fish and Wildlife Service records of House Finches banded throughout the eastern United States following their introduction in 1940. These data suggest the evolution of partial migration among the introduced population of finches, with a trend in recent years (1975-1989) for females to migrate farther than males. The proportion of males on wintering grounds varied significantly and consistently with latitude in the East. Sex ratios north of 380N latitude were malebiased, while those south of this were female-biased. Similar patterns were not apparent among western House Finches during winter or in the eastern population during the breeding season. Of the three hypotheses, only the body size hypothesis correctly predicts the pattern observed among wintering finches. Therefore, different energetic constraints facing male and female House Finches, perhaps related to body size, may be influencing migratory behavior and winter distributions.

A model that explains natal dispersal in resident screech-owls is presented and examined. The model is based on interactions among hormonal changes, body condition and social stimuli. It predicts that corticosterone, an adrenal glucocorticoid known to stimulate locomotor and foraging activity, increases in blood plasma prior to dispersal through a combination of endogenous and exogenous events. This mediates the locomotor activity that underlies dispersal behaviour. Juveniles in good body condition (i.e. those with sufficient fat reserves) will disperse when corticosterone increases. Birds in poor body condition will not, but they will increase their foraging activity under the influence of corticosterone. Dispersal of siblings will reduce aggression and/or competition for food, enabling the remaining juveniles to improve their body condition and disperse. Initial studies on screech-owls, Otus asio and O. kennicottii, have produced results that are generally consistent with the model. For example, captive juvenile screech-owls showed increased locomotor activity in the weeks leading up to the time when free-living juveniles are dispersing, and activity levels declined thereafter. Peaks in corticosterone corresponded with periods of high locomotor activity (i.e. at the time of dispersal) in captive owls. Finally, field studies indicate that dominant juveniles, which are presumably in better physical condition, initiated dispersal before their more subordinate siblings. Copyright 1998 The Association for the Study of Animal Behaviour.

Eastern Screech-Owls (Otus asio) possess a repertoire of two song types, the bounce and the whinny. We examined (1) seasonal variation in the use of these songs in central Kentucky and (2) the responses of male and female screech-owls to the playback of bounce songs. The spontaneous use of bounce songs increased during February and March, declined in April and May, increased again in June and continued through November. The spontaneous use of whinny songs increased dramatically during August and September and continued through November. Playback experiments revealed that (1) males uttered significantly more bounce songs than did females, and (2) owls responding with bounce songs were located significantly closer to the speaker than were those responding with whinny songs. Our results suggest, therefore, that bounce songs are directed to nearby conspecifics while whinny songs are directed to more distant individuals. Based on patterns of seasonal change in spontaneous use, we tentatively conclude that bounce songs are used in both aggressive and nonaggressive contexts while whinny songs are used in aggressive contexts.