Responses of Laughing Gull Chicks (Larus Atricilla) To Parental Attraction- and Alarm-Calls, and Effects of Prenatal Auditory Experience On the Responsiveness To Such Calls
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. 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...