A bimodal foraging strategy has previously been described for procellariiform seabird species and is thought to have evolved in response to local resource availability being too low for adult birds to meet chick requirements and simultaneously maintain their own body condition. Here, we examine the dual foraging trip pattern of an alcid, the little auk Alle alle, at five colonies with contrasting oceanographic conditions. In spite of large variation in local conditions, little auks at all colonies showed the same general pattern of alternating a single long‐trip with several consecutive short‐trips. However, we found that the foraging pattern was flexible and could be adjusted at three levels: (1) the length of long‐trips, (2) the frequency of short‐trips, and (3) the total time spent foraging. Birds facing unfavorable conditions increased the duration of long‐trips and reduced the number of short‐trips. These adjustments resulted in reduced provisioning rates of chicks despite the fact that birds also increased the time allocated to foraging. Travel times during foraging trips were positively correlated to the total duration of the trip suggesting that differences in trip length among colonies were partly driven by variation in the distance to foraging areas. Most birds spent substantially more time traveling during long compared to short‐trips, indicating that they accessed distant foraging areas during long‐trips but remained close to the colony during short‐trips. However, the difference in travel times was small at the site with the most favorable conditions suggesting that bimodal foraging in the little auk may be independent of the existence of high‐quality areas at distance from the breeding ground.

The timing of breeding is a life-history trait that can greatly affect fitness, because successful reproduction depends on the match between the food requirements for raising young and the seasonal peak in food availability. We analysed phenology (hatch dates) in relation to climate change for 2 seabird species breeding in the high-Arctic, little auks Alle alle and black-legged kittiwakes Rissa tridactyla, for the periods 1963-2008 and 1970-2008, respectively. We show that spring climate has changed during the study period, with a strong increase in both air temperature (TEMP) and sea surface temperature (SST) and a decrease in sea ice concentration. Little auks showed a trend for earlier breeding over the study period, while kittiwakes showed a non-significant trend for later breeding, demonstrating different phenological responses in these 2 species. Little auks and kittiwakes adjusted their timing of breeding to different environmental signals. Spring TEMP was the best predictor of little auk phenology, with a significant negative effect. Spring SST was the strongest predictor of kittiwake phenology, with a non-significant negative effect. Spring sea ice concentration and the North Atlantic Oscillation (NAO) winter index had a low relative variable importance. Furthermore, in kittiwakes, years with late breeding were associated with low clutch size and mean annual breeding success, indicating poor investment and food availability. This study identifies some spring environmental factors important for regulating the timing of breeding in the high-Arctic, most likely through effects on snow cover limiting access to nest sites and the development of the polar marine food web. It remains to be investigated whether environmental factors are reliable predictors of marine prey phenology, and whether the decision to start breeding is constrained by food availability.

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.

Climate warming is rapidly altering marine ecosystems towards a more temperate state on the European side of the Arctic. However, this “Atlantification” has rarely been confirmed, as long-term datasets on Arctic marine organisms are scarce. We present a 19-year time series (1982–2016) of diet samples from black-legged kittiwakes as an indicator of the changes in a high Arctic marine ecosystem (Kongsfjorden, Svalbard). Our results highlight a shift from Arctic prey dominance until 2006 to a more mixed diet with high contribution of Atlantic fishes. Capelin, an Atlantic species, dominated the diet composition in 2007, marking a shift in the food web. The occurrence of polar cod, a key Arctic fish species, positively correlated with sea ice index, whereas Atlantic species demonstrated the opposite correlation indicating that the diet shift was likely connected with recent climate warming. Kittiwakes, which gather available fish and zooplankton near the sea surface to feed their chicks, can act as messengers of ecosystem change. Changes in their diet reveal that the Kongsfjord system has drifted in an Atlantic direction over the last decade.

Thyroid hormones (TH) are known to stimulate in vitro oxygen consumption of tissues in mammals and birds. Hence, in many laboratory studies a positive relationship between TH concentrations and basal metabolic rate (BMR) has been demonstrated whereas evidence from species in the wild is scarce. Even though basal and field metabolic rates (FMR) are often thought to be intrinsically linked it is still unknown whether a relationship between TH and FMR exists. Here we determine the relationship between the primary thyroid hormone triiodothyronine (T3) with both BMR and FMR in a wild bird species, the black-legged kittiwake (Rissa tridactyla). As predicted we found a strong and positive relationship between plasma concentrations of T3 and both BMR and mass-independent BMR with coefficients of determination ranging from 0.36 to 0.60. In contrast there was no association of T3 levels with either whole-body or mass-independent FMR (R2 = 0.06 and 0.02, respectively). In accordance with in vitro studies our data suggests that TH play an important role in modulating BMR and may serve as a proxy for basal metabolism in wild birds. However, the lack of a relationship between TH and FMR indicates that levels of physical activity in kittiwakes are largely independent of TH concentrations and support recent studies that cast doubt on a direct linkage between BMR and FMR.