Abstract: Carry-over effects, i.e. when processes in one season influence processes in the next, are believed to have important effects on behavior and fitness in animals. Despite an increasing interest in the identification of carry-over effects, there are few experimental studies of the underlying mechanisms. We investigated how a short-term experimental elevation of the stress hormone corticosterone (CORT) during breeding influences the migratory behavior of black-legged kittiwakes Rissa tridactyla. We exposed chick-rearing kittiwakes to a 3 d elevation of CORT, which is usually secreted in response to food shortages. The migratory behavior of CORT-treated kittiwakes and a control group was then tracked using geolocators. We found that CORT treatment affected subsequent autumn migration in a sex-specific manner. CORT-treated females left the breeding grounds earlier and spent a longer period at the wintering grounds than control birds and CORT-treated males. The CORT treatment did not affect the timing of spring arrival or total length of migration. Our findings indicated that physiological stress incurred during breeding can carry over to affect key parameters of migratory behavior in autumn. Identifying carry-over mechanisms, such as those described here, is important to understand how performance and fitness in animals are determined by interactions between different parts of their life cycle.

Abstract: Late breeding usually occurs during years of poor environmental conditions, but the proximate mechanisms underlying this phenological pattern have been poorly documented. Here, we combined the deployment of GPS devices (from 2008 to 2010) and the monitoring of breeding parameters and baseline corticosterone levels (from 2007 to 2011) during the pre-laying period to investigate the proximate regulation of breeding date in an Arctic population of black-legged kittiwake Rissa tridactyla. The timing of breeding varied considerably during the course of the study and late breeding was associated with reduced clutch size and low breeding success at the individual level. Foraging strategies differed considerably between males and females and among years. All but one of the females tracked using GPS during the pre-laying period foraged inside the fjord, whereas tracked males foraged both inside and outside the fjord, using the deep waters of the Greenland-Svalbard ridge. Trips lasted longer and were to greater distances in 2009, the year of late breeding, compared to 2008 and 2010, highlighting a food scarcity in 2009. Corticosterone levels differed among years, and were the lowest in 2010, the year of earliest breeding. Moreover, kittiwakes exhibiting higher corticosterone levels tended to undertake longer trips when foraging outside the fjord. Breeding decision and laying date were not related to corticosterone levels at the individual level, but were positively influenced by body condition, suggesting that complex proximate mechanisms may affect timing of breeding in kittiwakes.