Are corticosterone levels a good indicator of FOOD STRESS AND reproductive success in a seabird colony?

Richard B. Lanctot*1, Scott A. Hatch1, Verena A. Gill1, and Marcel Eens2  1USGS-BRD, Alaska Biological Science Center, 1011 E. Tudor Road, MS 701, Anchorage, Alaska 99503 USA, Richard_lanctot@usgs.gov ; 2 Department of Biology, University of Antwerp, Universiteitsplein 1, 2610 Wilrijk, Belgium.

To determine if stress hormone levels in adult seabirds are related to local food conditions and indicate the reproductive health of a colony, we contrasted base-line levels of corticosterone in supplementally fed and unfed adult Black-legged Kittiwakes (Rissa tridactyla) breeding on a radar tower on Middleton Island, AK in 1999 and 2000. Blood was obtained from adults shortly after returning from the wintering grounds (~1 ½ months prior to breeding), and during pre-egg laying, incubation and chick-rearing stages of breeding. A variety of reproductive parameters were also measured at fed and unfed sites throughout the colony. Base-line levels of corticosterone were 2-3 times higher in birds sampled shortly after arriving on the island compared to those sampled during the remainder of the breeding season (levels decreased from an average of 8-16 ng/ml to 2-6 ng/ml). Differences were most pronounced in 1999 – a season when pre-laying conditions were some of the worst for many years – and were greater for individuals that subsequently failed to breed. During pre-egg laying, incubation, and chick-rearing stages, corticosterone levels were consistently low and not significantly different between sexes, or between fed and unfed kittiwakes (except chick-rearing in 1999). Further, corticosterone levels during these three stages were a poor predictor of reproductive success in the colony. Taken together, these results suggest that base line-levels of corticosterone have a limited ability to predict local food conditions and the reproductive health of a colony.  Useful sampling appears to be limited to when birds first arrive from their wintering grounds.

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