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Newly hatched laughing gulls, Larus atricilla , pecked at herring gull models more than at models of their own species, and preferred an unusual laughing gull model to a more conventional one. Chicks learned to peck preferentially at laughing gull models that fed them and to reduce their pecking at models that did not; they learned best when the model was unusual. Chicks were also able to learn to prefer a particular laughing gull mew call over a mew call recorded from a different laughing gull. When auditory cues were present during conditioning, the degree and importance of visual learning sharply declined.

After the eggs hatch, about 63% of the large eggshell fragments can be found near laughing gulls' nests. For the shells that were found, the mean distance from the rim of the nest was less than 80 cm at three study sites. The removal distance was positively correlated with the size of the nest platform, which suggests that birds often walk to the edge of the platform and simply drop the shell. Within the colony, eggshells have a natural variation in darkness that is related to the range of darkness of the nest material. In preference tests we have matched shell models to the darkness of the eggs in the nest. When both natural shells and orange shells pegged to the ground were presented simultaneously, gulls preferentially attempted to remove the orange shell. The strength of the preference for the pegged orange shell varied from site to site within the colony, so the results were validated in a separate experiment by placing shells in the nest bowl one at a time to be removed in as natural a manner as possible. Orange was stiff preferred; fear of the orange model seemed to promote the removal response. Contrary to the expectations from prior studies, the natural model was also no stronger stimulus than a solid white model in pegged shell tests. Control tests with white, grey, and black models indicate that brightness plays a very weak role, so the removal preference must be based on colour. When pegged models of any colour were on the nest and the bird engaged in nest building, the nest building was usually biased toward the models, but not precisely directed at them. Our results suggest that removal behaviour can be elicited via different perceptual mechanisms. One mechanism specifically recognizes the egg-shell, while the other mechanism is stimulated by a broad range of foreign objects that contrast with the nest background. In the laughing gull, the contrast mechanism appears to be at least as strong as the recognition mechanism. This may be related to the low level of predation experienced by these gulls.

Hailman reported that wild-reared laughing gull, Larus atricilla, chicks peck preferentially at models of laughing gulls over models of herring gulls, L. argentatus, but newly hatched chicks have no species preference, and respond equally well to an isolated red bill. The experiments reported here confirm these observations. They also suggest that the pecking of newly hatched chicks is guided by heterogeneous summation. A new working definition of heterogeneous summation is presented. Furthermore, the results suggest that species preference does not develop spontaneously with maturation or on the basis of imprinting, but rather from reinforcement and conditioned inhibition or habituation. It appears that learning can take place only to a model with high releasing value for pecking, and that the crooning call of adult laughing gulls has no major effect on pecking in newly hatched chicks.

Predation by laughing gulls, Larus atricilla , on black skimmer, Rynchops niger , chicks was observed in a Texas nesting colony. The patterns of predation differed from random in two respects: (1) significantly more attacks (both attempts and successes) were directed toward nests with two parents present than toward nests with only one parent and (2) significantly more attempts occurred when female parents were brooding than males. Differences in gull attack rates according to sex of the parent skimmer may have been due to the considerable sexual size dimorphism in skimmers. Male skimmers are larger than laughing gulls, while female skimmers are smaller. Predation on skimmer eggs was uncommon. Solo incubating by females was significantly less frequent after the first egg's hatching than before. The decreased frequency of female solo attendance after the first hatching may represent an adaptive response to predator pressure at a time when chicks are most vulnerable to predation by gape-limited predators such as laughing gulls and would be better protected by males or both parents. The higher observed frequencies of both predation attempts and successes directed toward nests with bi-parental attendance was not predicted and may be attributable to certain unavoidable high risk activities, such as brooding exchanges and chick feeding when broods are young, which typically occur only when both parents are present.

Laughing gulls Larus atricilla , nesting in saltmarshes subject to tidal inundations, build substantial nests and continue adding material throughout the reproductive cycle. Under normal conditions, nest size increased until gulls had 5-day-old chicks. I manipulated nests at various stages in the reproductive cycle. Until their chicks reached 14 days of age, gulls added material to artificially wetted nests, making them significantly deeper and wider than before treatment. During all reproductive stages examined, gulls repaired nests having half of the material removed. The adaptive significances of nest building and relative nest size are discussed in terms of washouts and the requirements of the eggs and chicks at different stages in the reproductive cycle.

We recorded the choice of victims by a population of adult and first-winter laughing gulls ( Larus atricilla ) making attempts to steal food from adult and juvenile brown pelicans ( Pelecanus occidentalis ). Adult gulls made food theft attempts on juvenile pelicans with a disproportionately great frequency; first-winter gulls appeared to select pelican victims at random. Adult pelicans were more frequently successful in their foraging attempts than juvenile pelicans, suggesting that adults may be more reliable as potential food sources for gulls making theft attempts. However, juvenile pelicans attempted to evade kleptoparasitizing gulls less frequently than did adult pelicans, suggesting that the fish prey of juveniles may be more easily stolen. These patterns are discussed in relation to optimal victim choice by gulls, and to deferred maturity in both species.

laughing gulls emit a call referred to as ‘crooning’ with increasing frequency shortly before the hatching of their young. Uttered to newly hatched chicks this call appears to enhance pecking for food. The adult birds ‘kow’ call in response to disturbances near the nest. Incubator raised chicks respond differentially to these two calls, both pre- and post-natally. An experiment was designed to discover whether the hearing of such vocalizations prenatally would have any bearing on their postnatal effectiveness in eliciting pecking. With respect to ‘crooning’ an effect was found but it would not be clearly demonstrated that ‘crooning’ heard prenatally enhances pecking. Chicks pecked less often when hearing ‘kow’ calls regardless of prenatal experience with this call.

Three age-groups of laughing gull chicks were tested with playback of recordings of calls of their parents and calls of neighbouring adults. The chicks in each group showed response to the playback calls, and recognition of the voices of their parents. The chicks in the early group (1 to 3 days post-hatching) showed the least evidence of parent recognition: the chicks of the late group (12 to 28 days post-hatching) showed the lowest levels of response in general. Evidence of relationships between variation in the behaviour in the tests and variation in the test recordings was found from correlation analysis, the type of call initiating positive response, and analysis of temporal association. This evidence, and the results of tests which allowed responses to the parents' crooning and responses to the parents' ke-hah calls to be compared for chicks in the late group, indicate that whereas the filial approach behaviour of very young chicks is stimulated by crooning but suppressed by ke-hah and long-calls, this behaviour in older chicks requires the occurrence of ke-hah calls or long-calls for its elicitation. However the ke-hah calls and long-calls of adults other than the parents continue to have negative effects on the behaviour of older chicks. It is argued that this progression in discrimination of the voices of the parents and the changes in responsiveness to the different types of adult calls are consistent with functional requirements, both from the point of view of changes in a growing chick's social situation and from the point of view of the characteristics of the calls. The changes are also discussed from a developmental point of view.

Twelve matched pairs of parent-raised and hand-raised laughing gull chicks from the same clutches were tested, at 6 to 8 days post-hatching, with successive presentations of playback of a recording of calls of their parents and recording of calls of parents from a neighbouring nest, in a situation in which they could approach or withdraw from the sound. The parent-raised chicks oriented towards the sound, moved towards the sound and called when the calls of their own parents were played; they tended to withdraw from the sound and sit silently when the calls of the neighbouring parents were played. The hand-raised chicks fled from the sound and crouched in silence in response to both calls of their parents and the calls of neighbours. They were more vocal and active in periods without sound than were the parent-raised chicks. It is concluded that laughing gull chicks in nature learn to recognize the individual characteristics of the calls of their parents, and to react positively to the calls, through interaction with their parents prior to the age of 6 days post-hatching.