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Confirming previous observations in the gull-colony the parental attraction-call 'crooning' selectively enhanced activity while the alarm-calls 'kow' and 'uk-uk' suppressed activity and vocalizations, even in the absence of additional visual clues. These calls may have had the observed effects in part because of certain acoustic characteristics to which chicks preferentially respond and in part because of specific experiences that chicks may have had with these calls prior to testing. Some of the difference between experiment (a) and (b) in the performance towards 'uk-uk' calls may have been due to the fact that the chicks in (b) were tested later in the season than the chicks in (a). Later hatching chicks may have been physically less well developed and thus less responsive than earlier hatched chicks, or they may have had differential kinds or amounts of auditory experience before they were tested. The present results differ from findings of an earlier pilot-investigation (BEER, 1973) in which recordings of single adult birds evoked no clearcut responses in chicks. However most of those chicks had been several days old and in other experiments it was shown that responsiveness to calls of adults changes with age (BEER, 1970b).Observations. Adult Laughing Gulls utter several distinct calls during the incubation of their eggs and the raising of their chicks. One call referred to as 'uhr' call is frequently heard during incubation in conjunction with rising from the eggs or resettling, and in response to the mate's activities near the nest. 'Crooning' is heard during mate-reliefs in incubation. After hatching this call functions to attract the young to the parent. `Uk-uk' and 'kow' alarm-calls are both uttered when the colony is disturbed by a potential predator, but 'kow' calls can also be heard in purely conspecific disturbances. Experiment I . The responses of day-old chicks, reared by their parents, were investigated towards recordings of some of these calls. Confirming observations in the wild, `crooning' selectively enhanced activity and elicited approach; 'uk-uk' suppressed activity and vocalization and elicited crouching; 'kow' calls had similar effects but to a lesser extent. Experiment 2. Chicks collected at hatching from nests in the gull-colony were compared with chicks hatched in an incubator in order to discover whether the prenatal conditions affected early postnatal responsiveness to 'crooning'. It was found that parent-hatched chicks showed increased activity in the presence of 'crooning' and some of them approached the speaker, whilst incubator-hatched chicks were not activated by these calls. Experiment 3. Younger chicks were compared with older chicks (all hatched in the incubator) for the purpose of finding out to what extent their responsiveness to 'crooning' would change with postnatal age. The results showed that to parentally inexperienced chicks this call increasingly acquires the effects of alarm-calls in that it suppresses vocalization and activity and elicits crouching. Experiment 4. The role of parental calls as experienced in the wild during incubation was examined experimentally. During the last 2 1/2 to 3 days of incubation eggs were repeatedly exposed to different types of calls (or no calls at all), in order to see to what extent and with what degree of selectivity such exposures would affect responsiveness to 'crooning' and 'kow' calls in newly hatched chicks. It was found that prenatal exposure to 'crooning' leads to enhancement of activity and vocalization in the presence of such calls postnatally. Effects of prenatal experience with disyllabic 'uhr' call point into the same direction. Prenatal experience with 'kow' calls does not lead to early postnatal activation in response to 'crooning'. Responsiveness to 'kow' calls was only affected by prenatal exposure to these calls. Discussion. Some previously published studies in this field of research are briefly reviewed and compared with the present findings. The relative contribution of prior experience of parental and filial vocalizations to later responsiveness to parental calls is discussed. Earlier research in the Laughing Gull had shown that parentally inexperienced embryos are selectively activated by 'crooning' till close to hatching. In the present study it was shown that repeated prenatal experience with 'crooning' leads to its attractiveness after hatching. Therefore repeated prenatal (and maybe early postnatal) exposure to this call and related calls seems to function to maintain and. consolidate responsiveness to this call in the neonate. In contrast to 'crooning', 'kow' alarm-calls heard prenatally do not enhance nor suppress motility. Prior exposure to this call does not maintain this apparent indifference, but merely reduces the extent to which it acquires activiy suppressing effects.

i. To playback of calls of one of their chicks and calls of a foreign chick, parent Laughing Gulls responded by orientation towards the sound, approach, and calling in reply. They showed no discrimination between calls of their own chick and calls of the foreign chick. Response to the playback waned during testing when all the parent's chicks were absent, but not when one was present. 2. When gulls were tested by having one of their own chicks and a foreign chick placed in covered boxes near the nest, they responded more to their own chick than to the foreigner. This appeared to be because their own chick called more than did the foreigner. The results were consistent with the possibility that individual recognition of the parent's voice by a chick causes the chick to behave in ways that tell the parent whether the chick belongs to it or not. 3. The parents' behaviour in response to chick calls in these two sets of tests differed between families of different ages in ways that exemplified the natural progression in patterns of parent-chick interactions, and suggested some speculation about the course of social development in this species.

The authors studied the piracy behavior of four species of Larus gulls in New Jersey and Mexico to learn more about the effects of age on piracy behavior. Piracy refers to the stealing of food from another individual and usually involves aerial chasing. For the four species observed feeding at a garbage dump: adults engaged in piracy more often than young gulls, there were no differences in piracy success rates, adult herring and ring-billed gulls were chased less than young of these species but there were no age-related differences for the other two species, and only herring gulls showed an age difference in victim rates with adults being victimized less often than young. Herring gulls were the most frequent pirates although all species were both pirates and victims. There were few piracy attempts on great black-backed gulls. Several measures of piracy behavior improved from September to February in herring gulls: the percentage of food dropped decreased, the victim rate decreased, and victim losses decreased. Ring-billed gulls pirating from starlings foraging on the ground near a dump showed age differences in piracy success for different methods. When provisioned with pre-cut pieces of liver in Mexico, laughing gull adults were more successful than subadults and young. Young were unable to obtain and keep larger pieces. Similarly, young gulls pirated from young rather than from adults. Taken altogether, this study indicates differences in piracy rates, piracy success rates, victim rates and victim losses as a function of age and methods used by some species. Improvement occurred with age and experience.

A series of four field experiments which tested the camouflage properties of different eggshell pigmentation and marking patterns was conducted. Neither natural (Laughing Gull, Larus atricilla, eggs) nor artificial (khaki, khaki-randomly spotted, and black painted eggs) camouflage patterns conferred any selective advantage, when crows (Corvus brachyrhynchos and C. ossifragus) were allowed extended periods (over 2 hours) to hunt for the eggs. However, a natural eggshell pattern (Japanese Quail, Coturnix c. japonica, eggs) conferred a very strong survival advantage during field tests which were terminated either after half the eggs were discovered or after I hour, whichever came first. The selection index (SI) obtained for the quail eggs was higher than those yielded from other experimental investigations of the survival value of camouflage. The results of these experiments were discussed in view of the different test procedures employed, and the extent to which such procedures are reflective of naturally occuring patterns of crow predation, such as in a gull or a tern colony. A fifth experiment which tested the capacity of pigmented and unpigmented eggshells to shield egg contents from solar radiation was also conducted. Pigmented eggshells provided far less effective solar shielding than did unpigmented ones, and this effect appears to hold over a wide range of eggshell thicknesses. In view of these findings, the dark eggshell pigmentation patterns of open ground nesting birds are seen as an "adaptive compromise." That is, under different circumstances dark eggshell pigmentation may be either advantageous or disadvantageous to species and individuals which possess them. Based upon functional considerations an hypothesis was generated which predicts that natural selection should favor darker eggshell pigmentation patterns among certain avian species with particular types of reproductive strategies nesting in particular types of breeding habitat. Some general evidence in support of this hypothesis was offered, and some suggestions for further comparative tests of this hypothesis are suggested.

Laughing Gulls (Larus atricilla) were studied in a tidal salt marsh located in Brigantine, New Jersey and on an island in the Gulf of Mexico to determine how nesting gulls respond to conspecific intruders. Gulls were observed in Texas and at Brigantine during the pre-egg stage. In Texas, Laughing Gulls set up pairing territories in the sand beach area next to the colony, and defended these areas against conspecifics. In New Jersey, pairing territories were often located on Spartina mats within the colony area. Twelve pairs of gulls were marked, sexed, and observed for eight to fourteen hours per day during the incubation period. An intruder was defined as any gull other than the nesting pair which landed within three meters of the nest. The interactions between the nesting pair and intruders were recorded for 1145 sequences. Eighty-five percent of all intruders were responded to with the following patterns: Long Calls (18%), Intruder Displays (45%), and Chases (22%). Long Calls and Intruder Displays were given by an incubating bird, while Chasing was performed by the mate standing nearby. Gulls flying over the nest were usually reacted to with a Long Call although Intruder Displays were sometimes given. Intruders landing within one meter of the nest were always given an Intruder Display by the incubating bird and were frequently chased by its mate. Intruders landing from one to two meters from the nest were usually given the Intruder Display: those landing two to three meters from the nest were usually chased. Gulls landing over three meters from the nest were usually ignored. The probability that an intruder was responded to decreased as the distance from the nest increased. The behavior of intruders was as follows. Fifty-three percent of the intruders landing within a meter of the nests were rapists or potential rapists. These intruders remained on the nest longer than did other intruder, and sometimes evoked pecking and attack from the incubating bird. Nest-material stealers (18%) remained on the nest for less time. Neighbors (7%) infrequently landed on the wrong nest but quickly flew to their own nest. In 22 percent of the cases the activities of the intruders could not be determined, since they were quickly repelled.

Most of this paper is a description and analysis of the hostile behavior patterns of adult Ring-billed Gulls and Franklin's Gulls on the ground or water. Both species have elaborate repertories of these patterns, including a variety of unritualized reactions and an even larger number of ritualized displays. The causation of these patterns is analyzed in terms of their motivation. All of them appear to be produced by simultaneous activation of the attack and escape drives; each pattern being produced by its own particular combination, i.e. relative and actual strength, of these drives, (with the addition of other drives, such as sex, in some cases). An attempt is also made to trace the derivation and social signal function(s) of the display patterns. A few of the corresponding patterns of the California Gull and Bonaparte's Gull are described very briefly. The comparative, evolutionary, implications of some of the displays are discussed at greater length, as they seem to be particularly suggestive, (more so than most of the aerial hostile patterns). The non-aerial hostile repertory of adult Ring-bills is very reminiscent of the other "typical Larus" gulls; but it does include a few peculiar features which have not been found in either the Herring Gull or the Common Gull. The California Gull is also very similar; and some of its displays are almost exactly intermediate in form between the homologous displays of the Herring Gull and the Ring-bill. The adult non-aerial hostile displays of Franklin's Gull are more distinctive. They are most reminiscent of the Laughing Gull; but the repertories of the two species are strikingly different in several important features. The repertories of the Ring-bill and Franklin's Gull are rather more similar than would be expected in view of the great morphological and ecological differences between the two species. This might suggest that the displays they hold in common are primitive in the family Laridae, (and the arrangement of these displays may also be primitive). It is possible, in fact, to make a tentative and hypothetical reconstruction of the probable complete hostile repertory, both aerial and non-aerial, of the ancestral gull, based upon comparison of the adults of all species whose behavior has been described in this and other publications.

The Ring-billed Gull, the Herring Gull, Franklin's Gull, and Bonaparte's Gull have many hostile aerial behavior patterns, produced by simultaneous activation of the attack and escape drives. Among these patterns are superficially, and probably deceptively, "pure" attack and escape reactions, and a whole host of more obviously ambivalent reactions, including ritualized patterns or displays. The four species seem to have similar repertories of aerial hostility; i.e. most of their aerial hostile patterns appear to be homologous. Many (at least) of the same patterns are found in the european Black-headed Gull and the Little Gull. It is probable, therefore, that the range of aerial hostile behavior is essentially similar in all gulls. This does not mean, however, that there are no minor differences between the repertories of different species. Some species have peculiar patterns all their own; and even the strictly homologous patterns of different species may differ in details of physical form, orientation, frequency, strength of motivation, degree of ritualization, etc. The Ring-billed Gull, for instance, can be distinguished by the relative rarity and simplicity of its aerial reactions. Its only aerial hostile displays are a few calls. The aerial hostility of the Herring Gull is particularly reminiscent of the Ring-bill; and most of its aerial patterns are almost equally rare. The Herring Gull does, however, have several distinctive. display postures. The hooded gulls, as a group, can be distinguished from both the Ring-bill and the Herring Gull by the fact that their aerial reactions are performed much more frequently. They all have numerous aerial hostile displays (including many ritualized movements and postures as well as calls), most of which are apparently threat. Some of these displays are of considerable comparative interest. Some of the aerial hostile displays of Franklin's Gull are peculiar in physical form, and many of them are frequently re-orientated or redirected in a most peculiar fashion; but several of them also show particular, and probably significant, resemblances to certain patterns of such diverse species as the Ring-bill, the Laughing Gull, the Black-headed Gull, and the Little Gull. Some of the aerial hostile displays of Bonaparte's Gull appear to be equally distinctive in physical form, if more conventional in orientation ; but the whole aerial hostility of this species is more like that of the Black-headed Gull and less like that of the other gulls.

This paper includes descriptions of the pair-formation and later pairing behavior of Franklin's Gull and the Ring-billed Gull ; and some notes on the pair-formation of the Laughing Gull, Bonaparte's Gull, and the american Herring Gull. The pairing of all these species is similar in many fundamental characters. Pair-formation begins with a selective response by unmated females to a series of hostile displays by unmated males. It is continued by a series of repeated mutual hostile performances between the sexes, gradually diminishing in strength. There are considerable differences in the form, frequency, and orientation of some of the homologous hostile displays used by different species during this phase. Sexual behavior, with its associated patterns, (many of which are still partly or completely hostile, but relatively weak), appears after the initial strong hostile reaction between the sexes has begun to decline. These sexual patterns and associated activities are almost identical in all gulls. The observed forms of pairing in the Laughing Gull and the Herring Gull were rather different from previously published descriptions of the process in these species; which would seem to indicate that some aspects of pairing (the social situation in which it occurs) may vary considerably within the same species.