In conditions of deteriorating food availability, seabirds may maximize their breeding success by increasing reproductive effort, which compromises body condition, stress level, survival, and future reproductive success. We studied a small planktivorous alcid, the little auk Alle alle, in 2 breeding colonies in west Spitsbergen, Norway (Hornsund and Magdalenefjorden) with contrasting oceanographic conditions (Arctic and Atlantic environments, respectively). We hypothesized that the chick diet composition and parental foraging effort differs between these colonies, which results in intercolony variation in body mass and stress level of both adults and chicks. We found differences in sea temperature and salinity (both lower in Hornsund) and the abundance of Atlantic copepod Calanus finmarchicus (3.6 times less abundant in Hornsund) between foraging areas. Birds from both colonies foraged selectively on Calanus glacialis CV. Composition, total biomass and energy content of food loads were similar in both colonies, though food delivered to nestlings in Magdalenefjorden was more diverse. The frequency of feeds was also similar in both colonies. Parent little auks in Magdalenefjorden, however, performed longer foraging trips than in Hornsund (medians 183 vs. 124 min). Longer foraging trips suggest that these birds traveled longer distances to find abundant prey at the marginal sea ice zone and/or spent more time foraging close to the colony but in poorer foraging areas. Despite increased parental efforts in Magdalenefjorden, body mass and stress levels of adults and chicks were similar in both colonies. This suggests that little auks from northwest Spitsbergen did not reach a threshold requiring prioritization of self-maintenance over chick provisioning.
Using GpS-tracked individuals, we compared foraging ecology and reproductive output of a High-Arctic zooplanktivorous seabird, the little auk Alle alle, between three years differing in environmental conditions (sea surface temperature). Despite contrasting environmental conditions, average foraging fights distance and duration were generally similar in all studied years. Also, in all years foraging locations visited by the little auk parents during short trips (St, for chick provisioning) were significantly closer to the colony compared to those visited during long trips (LTs, mainly for adults' self-maintenance). Nevertheless, we also found some differences in the little auk foraging behaviour: duration of Lts was the longest in the coldest year suggesting more time for resting for adults compared to warmer years. Besides, birds foraged closer to the colony and in significantly colder water in the coldest year. Interestingly, these differences did not affect chick diet: in all the years, the energy content of food loads was similar, with the Arctic copepod, Calanus glacialis copepodite stage V being the most preferred prey item (>73% of items by number and >67% by energy content). Also chick survival was similar in all the study years. However, when examining chicks growth rate we found that their peak body mass was lower in warmer years suggesting that overall conditions in the two warm years were less favourable. While our results, demonstrate a great foraging flexibility by little auks, they also point out their vulnerability to changing environmental conditions. Optimal foraging theory predicts that animals should adjust their foraging behaviour to environmental condition the way to maximise net energy gain and to increase their survival and/or reproductive success 1. The ability to efficiently adjust foraging behaviour should be especially apparent in central place foragers, such as seabirds that forage at distant locations from which they need to regularly return to the colony to feed their young 2. Prey distribution and abundance vary spatially and temporally in response to macroscale fluctuations of oceanographic conditions (e.g., North Atlantic Oscillation, El Niño Southern Oscillation; 3,4 as well as to mesoscale processes [e.g., sea ice dynamics, sea current distribution, upwelling strength and spread, atmospheric blocking; e.g. 5-9 ]. Seabirds are generally well adapted to the foraging in such the heterogeneous marine habitat. However, the recent climate change increases additional, often unpredictable fluctuations in prey abundance 10,11. It is specially visible in the Arctic where both marine and terrestrial ecosystems are rapidly changing as climate warming affects these regions faster than the global average 12-15. Moreover, high-latitude nesting seabirds have a limited time window to reproduce, which may limit their ability to adjust the timing of breeding to shifts in peaks of food abundance 7. Thus, Arctic endemic species are extremely susceptible to climate change. In the high Arctic marine ...
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