Reduced activity in middle-aged thick-billed murres: evidence for age related trends in fine-scale foraging behaviour

Several biological functions, such as reproductive success, peak during middle age in long-lived vertebrates. One possible mechanism for that peak is improved foraging performance during middle age, after a period of youthful inexperience and before senescence. Age may be particularly important in relating foraging behaviour to fitness for diving animals as altered skeletal muscle contraction efficiency or cardiovascular function could play a strong role at the scale of a single dive. Miniaturized accelerometers provide the opportunity to measure fine-scale foraging behaviour at the scale of a single wing beat. To examine the relationship between age and foraging behaviour in a diving seabird, whose reproductive success peaks during middle age, we equipped 115 thick-billed murres, Uria lomvia, with accelerometers and depth loggers. Murre activity (i.e. dynamic body acceleration) decreased with increasing dive depth during descent but increased with increasing maximum depth achieved. These patterns were inversed during ascent. After accounting for depth and maximum depth, middle-aged birds had low activity levels during the dive, compared with old and young birds, suggesting that old and young birds were less efficient at diving and foraging than middle-aged birds. Young individuals appeared to struggle at depths above 20 m during descent and throughout ascent when compared to middle-aged birds, while old individuals struggled at the end of deep dives during descent and during the initial parts of ascent when compared to the same middle-age class. We conclude that there is a link between foraging behaviour and age, contrary to previous findings. By examining foraging behaviour at a fine-scale, such as a dive, different energy expenditures across age groups were revealed, giving us insight on how seabirds are affected by senescence in the wild.