The Southern Ocean is in an era of significant change. Historic overharvesting of marine mammals and recent climatic warming have cascading impacts on resource availability and, in turn, ecosystem structure and function. We examined trophic responses of sympatric chinstrap (Pygoscelis antarctica) and gentoo (Pygoscelis papua) penguins to nearly 100 y of shared environmental change in the Antarctic Peninsula region using compound-specific stable isotope analyses of museum specimens. A century ago, gentoo penguins fed almost exclusively on low-trophic level prey, such as krill, during the peak of historic overexploitation of marine mammals, which was hypothesized to have resulted in a krill surplus. In the last 40 y, gentoo penguin trophic position has increased a full level as krill declined in response to recent climate change, increased competition from recovering marine mammal populations, and the development of a commercial krill fishery. A shifting isotopic baseline supporting gentoo penguins suggests a concurrent increase in coastal productivity over this time. In contrast, chinstrap penguins exhibited no change in trophic position, despite variation in krill availability over the past century. The specialized foraging niche of chinstrap penguins likely renders them more sensitive to changes in krill availability, relative to gentoo penguins, as evinced by their declining population trends in the Antarctic Peninsula over the past 40 y. Over the next century, similarly divergent trophic and population responses are likely to occur among Antarctic krill predators if climate change and other anthropogenic impacts continue to favor generalist over specialist species.

Our understanding of the annual life-cycle movements of small migratory birds has 1 advanced rapidly with the advent of light-weight geographical positioning devices (i.e., 2 geolocators), yet the effects of geolocators on reproduction and survival have not been 3 adequately quantified. We tested for impacts of attaching a 1g geolocator (using a harness 4 around the legs and back, anterior to the tail) to adult Tree Swallows (Tachycineta bicolor) on 5 parental feeding behaviour, nestling growth and size, fledging success, and return rates 6 between 2011-2012. At one breeding site, we compared feeding visits, nestling growth, and 7 nestling size between paired nest boxes where one parent was marked at the 'geolocator' box 8 with a 'control' nest box where neither parent was marked. We detected no differences 9 between geolocator and control nests in either the frequency of feeding visits to nestlings or 10 the amount of time spent at nests. Birds marked with geolocators fed nestlings as frequently 11 as their unmarked mates. Likewise, nestlings raised at geolocator nests grew at similar rates 12 to those at control nests, and had similar structural size and body mass at fledging. At three 13 widely-separated sites across the Tree Swallow breeding range in Canada, we also found that 14 fledging success was similar for geolocator and control nests. Although we found no 15 evidence for short-term negative impacts of geolocators, the return rates of geolocator-16 marked swallows tended to be significantly lower than those of unmarked control birds. 17Thus, we found little evidence for short-term impacts of geolocators on reproduction but our 18 study does suggest that long-term impacts of geolocators could be manifested in terms of 19 lower survival, higher emigration rates, or lower breeding propensity.

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