Abstract
Abstract Senescence has been widely documented in wild vertebrate populations, yet the proximate drivers of age‐related declines in breeding success, including allocation trade‐offs and links with foraging performance, are poorly understood. For long‐lived, migratory species, the non‐breeding period represents a critical time for investment in self‐maintenance and restoration of body condition, which in many species is linked to fitness. However, the relationships between age, non‐breeding foraging behaviour and fitness remain largely unexplored. We performed a cross‐sectional study, investigating age‐related variation in the foraging activity, distribution and diet of an extremely long‐lived seabird, the wandering albatross Diomedea exulans, during the non‐breeding period. Eighty‐two adults aged 8–33 years were tracked with geolocator‐immersion loggers, and body feathers were sampled for stable isotope analysis. We tested for variation in metrics of foraging behaviour and linked age‐related trends to subsequent reproductive performance. There was an age‐related decline in the number of landings (a proxy of foraging effort) during daylight hours, and a decrease in body feather δ13C values in older males but not females, yet this did not accompany an age‐related shift in distributions. Males conducted fewer landings than females, and the sexes showed some spatial segregation, with males foraging further south, likely due to their differential utilization of winds. Although younger (<20 years) birds had higher foraging effort, they all went on to breed successfully the following season. In contrast, among older (20+ years) birds, individuals that landed more often were more likely to defer breeding or fail during incubation, suggesting they have lower foraging success. As far as we are aware, this is the first demonstration of an age‐specific carry‐over effect of foraging behaviour in the non‐breeding period on subsequent reproductive performance. This link between foraging behaviour and fitness in late but not early adulthood indicates that the ability of individuals to forage efficiently outside the breeding period may be an important driver of fitness differences in old age. A plain language summary is available for this article.
Highlights
Age-related variation in demographic traits is well documented in long-lived vertebrates (Jones et al, 2008; Nussey, Froy, Lemaitre, Gaillard, & Austad, 2013)
Survival and reproductive performance generally increase throughout early life, stabilize during early- to mid-adulthood, and decline in old age due to senescence (Clutton- Brock, 1988; Froy, Phillips, Wood, Nussey, & Lewis, 2013), which is the process of progressive deterioration in physiological and molecular function (Monaghan, Charmantier, Nussey, & Ricklefs, 2008)
We ran a suite of models investigating changes in foraging behaviour by age and sex, with the following metrics as response variables in separate models: (1) δ13C and (2) δ15N values in feathers; (3) longitude, (4) latitude, (5) distance travelled per day, (6) distance from the colony, and (7) the size of 50% and (8) 95% utilization distribution (UD), generated from the tracking data; and the number of landings during (9) daylight and (10) darkness, flight bout duration during (11) daylight and (12) darkness, and the proportion of time spent in flight during (13) daylight and (14) darkness, from the activity data
Summary
Age-related variation in demographic traits is well documented in long-lived vertebrates (Jones et al, 2008; Nussey, Froy, Lemaitre, Gaillard, & Austad, 2013). Little is known about how age shapes the foraging behaviour of wild animals outside the breeding season (see Table 1), likely due to the challenges associated with tracking the movements of individuals of known ages for long periods of time (Phillips, Lewis, González-Solís, & Daunt, 2017). | Functional Ecolo gy 1835 throughout the Southern Ocean (Weimerskirch, Delord, Guitteaud, Phillips, & Pinet, 2015; Weimerskirch & Wilson, 2000) and appear to forage further south with age, as documented through changes in stable isotope values of feathers moulted on non-breeding grounds (Jaeger et al, 2014). Much less is known about age-related changes in the distributions and foraging behaviour of non-breeding birds from South Georgia. We made serial deployments of geolocator-immersion loggers and sampled feathers for stable isotope analysis to test for age-and sex-related variation in the non-breeding distribution, diet and activity patterns of wandering albatrosses from South Georgia. We predicted that (2) older birds would have a lower probability of breeding successfully in the following season (Froy et al, 2013), and (3) that age-related trends in foraging behaviour would be linked to lower subsequent breeding success in older birds
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