Acts of aggression by Roseate Tern (Sterna dougallii) adults on conspecific chicks were examined on Aride Island, Seychelles, from 13 June-10 July 1998. Data were used to examine two hypotheses to explain adult aggression towards chicks: (1) the adoption avoidance hypothesis, and (2) the feeding unfamiliar chicks hypothesis. Throughout the entire nestling development, adults pecked mostly unattended chicks (96%). The intensity of pecking was higher on b-chicks than on a-chicks and 72% of the pecking acts were on chicks that were on their own nests. During each pecking act, young chicks (0-6 days) were pecked by a greater number of adults than were old chicks (>6 days). When pecked, young chicks moved significantly greater distances away from nests than did old chicks (1.41 ± 1.92 m vs. 0.18 ± 0.42 m). Of the 23 chicks killed by adults, 11 were a-chicks, representing 8.5% of the a–chicks in the study area. Parents sometimes rescued a-chicks, but did not attempt to rescue b-chicks from adult attacks. Chicks that died had moved significantly greater distances away from nests than chicks that survived (2.8 ± 2.5 m vs. 0.3 ± 0.67 m). This study presents evidence that both hypotheses may be important in explaining intraspecific adult aggression by Roseate Terns on chicks. In particular, the feeding unfamiliar chicks hypothesis may be invoked to explain the significant negative correlation between daily intensity of adult aggression and the amount of food delivered to chicks. The adoption avoidance hypothesis may explain why b-chicks were disproportionately involved in pecking in relation to a-chicks. Adult aggression is an additive factor of chick mortality, operating mostly on days of food shortage.
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