Aim  An understanding of the non‐breeding distribution and ecology of migratory species is necessary for successful conservation. Many seabirds spend the non‐breeding season far from land, and information on their distribution during this time is very limited. The black‐legged kittiwake, Rissa tridactyla, is a widespread and numerous seabird in the North Atlantic and Pacific, but breeding populations throughout the Atlantic range have declined recently. To help understand the reasons for the declines, we tracked adults from colonies throughout the Atlantic range over the non‐breeding season using light‐based geolocation. Location  North Atlantic. Methods  Geolocation data loggers were deployed on breeding kittiwakes from 19 colonies in 2008 and 2009 and retrieved in 2009 and 2010. Data from 236 loggers were processed and plotted using GIS. Size and composition of wintering populations were estimated using information on breeding population size. Results  Most tracked birds spent the winter in the West Atlantic, between Newfoundland and the Mid‐Atlantic Ridge, including in offshore, deep‐water areas. Some birds (mainly local breeders) wintered in the North Sea and west of the British Isles. There was a large overlap in winter distributions of birds from different colonies, and colonies closer to each other showed larger overlap. We estimated that 80% of the 4.5 million adult kittiwakes in the Atlantic wintered west of the Mid‐Atlantic Ridge, with only birds from Ireland and western Britain staying mainly on the European side. Main conclusions  The high degree of mixing in winter of kittiwakes breeding in various parts of the Atlantic range implies that the overall population could be sensitive to potentially deteriorating environmental conditions in the West Atlantic, e.g. owing to lack of food or pollution. Our approach to estimating the size and composition of wintering populations should contribute to improved management of birds faced with such challenges.

Mercury, a ubiquitous toxic element, is known to alter expression of sex steroids and to impair reproduction across vertebrates but the mechanisms underlying these effects are not clearly identified. We examined whether contamination by mercury predicts the probability to skip reproduction in black-legged kittiwakes (Rissa tridactyla) from Svalbard. We also manipulated the endocrine system to investigate the mechanism underlying this relationship. During the pre-laying period, we injected exogenous GnRH (gonadotropin-releasing hormone) to test the ability of the pituitary to release luteinizing hormone (LH, a key hormone for the release of sex steroids and hence breeding) in relation to mercury burden. Birds that skipped reproduction had significantly higher mercury concentration in blood than breeders. Endocrine profiles of these birds also varied based on breeding status (breeders versus non-breeders), mercury contamination and sex. Specifically, in skippers (birds that did not breed), baseline LH decreased with increasing mercury concentration in males, whereas it increased in females. GnRH-induced LH levels increased with increasing mercury concentration in both sexes. These results suggest that mercury contamination may disrupt GnRH input to the pituitary. Thus, high mercury concentration could affect the ability of long-lived birds to modulate their reproductive effort (skipping or breeding) according to ongoing environmental changes in the Arctic, thereby impacting population dynamics.

Summary1. An emergency life-history stage is expressed in breeding vertebrates when the immediate survival is threatened by poor energetic conditions (i.e. allostatic overload). This emergency life-history stage shifts energy investment away from reproduction and redirects it toward immediate survival. 2. In birds, this emergency life-history stage is promoted by a release of the stress hormone corticosterone. However, how corticosterone reduces the expression of parental cares remains to be clarified. One hypothesis is that the release of corticosterone may also affect prolactin levels, a pituitary hormone widely involved in regulating parental behaviours. 3. We tested this hypothesis by experimentally increasing corticosterone levels of chick-rearing black-legged kittiwakes ( Rissa tridactyla ) over a 2-day period and by monitoring prolactin levels over an 8-day period. We also investigated whether this hormonal manipulation affected nest attendance, the motivation to come back to the nest after a short-term stress and breeding success. 4. Corticosterone treatment resulted in a significant increase in plasma corticosterone levels during the first 2 days, which returned to pre-treatment values at day 3. This short-term corticosterone increase was accompanied by a 30% decrease in prolactin levels. Prolactin levels were reduced in a progressive and persistent manner and did not return to their initial levels when corticosterone levels returned to pre-treatment levels. Moreover, although corticosterone levels had returned to pre-treatment values, low prolactin levels were associated with a reduced nest attendance and a greater latency to come back to the nest after a short-term stress. This hormonal treatment also significantly reduced breeding success. 5. This experimental treatment strongly supports the idea that the secretion of these two hormones might be mechanistically linked. Thus, we showed that even a relatively short-term increase in corticosterone levels can durably affect plasma prolactin levels. Therefore, the well-established suppressive action of corticosterone on parental behaviour is probably mediated and reinforced through an effect on prolactin levels. This study highlights the need to consider the potential synergistic effects of these two hormones when studying on the hormonal basis of parental decisions.