Assessing the Effects of Introduced Norway Rats (rattus Norvegicus) on Survival and Productivity of Least Auklets (aethia Pusilla)

A .—We assessed potential eff ects of introduced Norway rats (Ra us norvegicus) on Least Auklets (Aethia pusilla) breeding at Sirius Point on Kiska Island, the largest auklet colony in the Aleutian Islands, Alaska. We compared productivity, chick growth, and adult survival of Least Auklets during 2001–2003 at Kiska and two nearby, rat-free Least Auklet colonies on Buldir and Kasatochi islands. During 2001 and 2002 (when rats were abundant), productivity at Kiska was the lowest ever recorded for this species (0.09–0.16 chicks fl edged per eggs laid), primarily because of high mortality of newly hatched chicks. Growth rates and mean fl edging mass were both lower on Kiska than on rat-free islands, though there were some interannual diff erences in these pa erns. Adult survival rates were highly variable among years but strongly concordant among colonies, and survival from 2001 to 2002 on Kiska (0.881 ± 0.033) did not diff er signifi cantly from long-term averages on either Buldir (0.853 ± 0.014, 1990–2003) or Kasatochi (0.893 ± 0.027, 1996–2003) islands. Although we found li le evidence at nesting crevices of predation on adults, eggs, or chicks, low productivity and slow chick growth were both consistent with disturbance caused by rats, particularly through disruption of adults a empting to brood or provision young chicks. Breeding failure may have been exacerbated by low prey availability for chick provisioning, but the lack of concordance in either productivity or chick growth rates between Kiska Island and nearby rat-free Buldir Island cast doubt on this possibility. Received 16 March 2004, accepted 14 September 2005.