Components of productivity in black-legged kittiwakes Rissa tridactyla:: response to supplemental feeding

In contrast to the high productivity of black-legged kittiwakes in Britain, kittiwakes at many colonies in Alaska have failed chronically to reproduce since the mid 1970s. To determine if food is limiting productivity and, if so, at what stages of nesting food shortages are most severe, in 1996 and 1997 we supplementally fed kittiwakes nesting on an abandoned building. The effects of feeding were stronger in 1997 than in 1996, possibly because naturally occurring prey were of poorer quality in 1997. Consumption of supplemental herring declined as egg laying approached then increased slowly during incubation and more rapidly after hatching. All of the six components of productivity we studied were improved by supplemental feeding to some degree. Supplemental food did not significantly alter laying success in either year, although fed pairs bred at slightly higher rates than unfed pairs in 1997, the poorer food year. In 1996 and 1997, extra food noticeably increased clutch size and hatching success, but significantly so only in 1997. Fledging success and productivity were substantially augmented by feeding in both years. Fed pairs fledged twice as many chicks per nest as did unfed pairs in 1996 and three times as many in 1997. Fed and unfed pairs lost most of their potential productivity through the inability to hatch eggs, and secondarily because of their poor success at raising chicks. The benefits of supplemental feeding did not carry over from one stage of breeding to another. Pairs cut off from supplemental food after laying or hatching performed similarly to pairs that had not been previously fed. This study provides benchmark values of breeding performance attainable by kittiwakes in Alaska under optimal conditions. These values are comparable to highly productive colonies in Britain and suggest that differences in life-history characteristics between Pacific and Atlantic breeding populations are primarily controlled by food supply.