Investigator disturbance, chick movement, and aggressive behavior in ring-billed gulls

Infanticidal attacks on conspecific young can be a major cause of chick mortality in ground nesting colonial seabirds, and past studies have shown that investigator disturbance scatters chicks, subjecting them to attacks by neighbors. In 1991, we studied the effects of investigator disturbance on aggressive behavior and chick movement at a Ring-billed Gull (Larus delawarensis) colony near Port Colborne, Ontario. The frequency of charges, jabs, and wingpulls increased during the period of investigator activity in the colony, while the duration of wingpulls and beakpulls decreased. Few chicks ( 85%) remained crouched on the natal territory. However, chick mobility increased with age, and older (>7days) chicks strayed off their natal territories and were attacked more frequently during disturbances than before disturbance. In contrast to Fetterolf (1983; Wilson Bull. 95:23-41) who found that investigator disturbance increased chick mortality and de- creased fledging success, in our study, no chicks were fatally attacked during investigator disturbance and stray chicks returned to their natal territories quickly after disturbance. Furthermore, the mean (+ 1 SD) fledging success of pairs disturbed during this study (1.5 + 0.8 chicks per pair) was not different from that recorded for undisturbed pairs (1.6 + 0.8). As our results vary somewhat from Fetterolf's, we contrast the two sets of data in the context of experimental procedures that yielded them. In this study, suitable hiding places and termination of investigator visits into the colony after the hatching period were attributed to preventing human-induced chick mortality. We show that awareness of and attention to the potential impacts of investigator activity can reduce and even eliminate the negative consequences associated with such activity. Received 14 March 1994, accepted 22 Aug. 1994.

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