Which factors shape animals' migration movements across large geographical scales, how different migratory strategies emerge between populations, and how these may affect population dynamics are central questions in the field of animal migration [1] that only large-scale studies of migration patterns across a species' range can answer [2]. To address these questions, we track the migration of 270 Atlantic puffins Fratercula arctica, a red-listed, declining seabird, across their entire breeding range. We investigate the role of demographic, geographical, and environmental variables in driving spatial and behavioral differences on an ocean-basin scale by measuring puffins' among-colony differences in migratory routes and day-to-day behavior (estimated with individual daily activity budgets and energy expenditure). We show that competition and local winter resource availability are important drivers of migratory movements, with birds from larger colonies or with poorer local winter conditions migrating further and visiting less-productive waters; this in turn led to differences in flight activity and energy expenditure. Other behavioral differences emerge with latitude, with foraging effort and energy expenditure increasing when birds winter further north in colder waters. Importantly, these ocean-wide migration patterns can ultimately be linked with breeding performance: colony productivity is negatively associated with wintering latitude, population size, and migration distance, which demonstrates the cost of competition and migration on future breeding and the link between non-breeding and breeding periods. Our results help us to understand the drivers of animal migration and have important implications for population dynamics and the conservation of migratory species.

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Seabirds consume forage fish, which are keystone species in many marine ecosystems. The Junk Food Hypothesis proposes that high-lipid prey should produce better reproductive performance by seabirds. In the Gulf of Maine, changes in the forage fish community followed rapid warming post-2005 and included a decline in high-lipid Atlantic herring Clupea harengus. We studied 3 species of alcid (Atlantic puffin Fratercula arctica, razorbill Alca torda, common murre Uria aalge) over 23 yr at 3 colonies to assess changes in chick diet and its relationships with reproductive success. Puffin and razorbill chick diet changed over time; puffin diet was highly variable taxonomically, whereas razorbill diets were more consistent, showing proportional changes within fewer taxa. For puffins and razorbills, herring was replaced by sand lance Ammo dytes spp. and other taxa with lower energy density. Puffins did not require high-lipid fish to breed successfully, but diet -reproduction relationships became unpredictable following ex tremely warm winters (2013 and 2016). Razorbills and murres provisioning with low-lipid fish showed reduced chick condition and breeding success. We concluded that razorbills and murres need higher-quality diets than puffins, which more frequently exploited lower-lipid food during food shortages. However, puffin reproductive output was much more vulnerable to ocean warming owing to their longer breeding season and more varied diet. Different responses of closely-related species to changes in prey are driven by differences in chick-development strategies with clear implications for using seabirds as environmental indicators.

We examined the diet of Atlantic puffin (Fratercula arctica) chicks at three midcoast Maine, USA, colonies during the years 2005-2014 and found that the puffins at each island have a distinct diet that has changed in recent years. White hake (Urophycis tenuis) is by far the most frequently delivered prey at each island. Atlantic herring (Clupea harengus) is the second most frequently delivered food, but has declined in recent years on two islands. In contrast, butterfish (Poronotus triacanthus), haddock (Melanogrammus aeglefinus), and redfish (Sebastes spp.) have increased in the puffin diet on all islands. Chick condition declined significantly from 1993 to 2009. We demonstrate that puffin chicks with greater body weight experience a higher chance of postfledging survival as compared to chicks with lower body weight. The years 2012-2013 were a period of extreme sea surface warming, in which puffin hatching success and productivity sharply declined. This study provides new insight into changes in marine communities, examining changes in chick diet. We discuss our findings in relation to warming sea surface temperatures, recent climate-related decline in puffin productivity in the Gulf of Maine, and the impact of commercial fisheries on forage fish.

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