Nonbreeder birds at colonies display qualitatively similar seasonal mass change patterns as breeders

Abstract

The difficulty in studying nonbreeding birds means that little is known about them or their resource requirements, despite forming a large and significant component of a population. One way to assess food requirements is to examine changes in body mass, because it indicates the amount of food acquired. In terms of body mass changes, our expectation is that nonbreeders will either (a) be in poorer condition than the breeders which potentially explains why they do not breed or (b) remain at a stable higher mass as they are unconstrained by the physiological costs associated with rearing chicks. Here, we interrogate body mass datasets of breeding and nonbreeding birds of two penguin species to assess these predictions and determine whether differences in mass exist between these two groups throughout the breeding season. The first dataset is from a wild Adélie penguin population, where bird mass was recorded automatically and breeding status determined from a resighting program. A second population of captive gentoo penguins were weighed regularly each breeding season. We demonstrate that although there were times in each year when breeders were heavier than their nonbreeding counterparts for both populations, the mass changes showed qualitatively similar patterns throughout the breeding season irrespective of breeding status. Heavier breeders at times during the breeding season are not unexpected but the overall similar pattern of mass change irrespective of breeding status is in contrast to expectations. It appears that breeding status per se and the constraints that breeding places on birds are not the only driver of changes in mass throughout the breeding season and, although not explicitly studied here, the role of hormones in driving changes in appetite could be key to explain these results. These results present a significant step toward understanding food requirements of nonbreeders in avian populations.

Behavioural and heart rate responses to stressors in two populations of Little Penguins that differ in levels of human disturbance and predation risk

“…It should be noted, however, that body condition in birds is a dynamic attribute that is strongly influenced by other factors, such as breeding stage, time of capture, year or age (e.g. Robinson et al 2005, Norte et al 2009, Emmerson et al 2019, which could also explain differences in body condition between the colonies. In addition, our study only compared responses between two colonies, and further differences between the colonies, such as parasites or genetic predisposition, may have impacted the observed differences.…”

The foraging behavior of nonbreeding Adélie penguins in the western Antarctic Peninsula during the breeding season

“…Other studies have also described this behavior, with adult nonbreeding birds present at the breeding colony during egg laying, and again when chicks were hatching and being guarded (Ainley, 1978). Hormone cycles and nutritional status (body condition) can influence nonbreeders' foraging decisions; for example, they may return to the breeding colony to shed surplus fat as an adaptive advantage (rather than a constraint) for swimming performance (Emmerson et al, 2019). Once ashore, there is no transfer of energy from parent to offspring in nonbreeders, meaning that more assimilated energy is available for self‐maintenance, thus reducing their need to forage.…”

Density dependence forces divergent population growth rates and alters occupancy patterns of a central place foraging Antarctic seabird

“…Our use of primary productivity as an indirect proxy for food abundance is common practice in studies of marine predators, but empirical quantitative validation of the robustness of the proxy is limited. While polynyas are generally considered to be important features for foraging Adélie penguins, optimal foraging habitat may only occur in the marginal ice zone at the edge of polynyas (Lescroël, Ballard, Grémillet, Authier, & Ainley, ), or alternatively polynyas may provide easier access to more distant foraging grounds because their reduced sea‐ice allows more efficient travel (Emmerson, Walsh, & Southwell, ). Finally, although we accounted for potential intra‐ and interspecific competition from cohabiting breeding seabirds, other potential competitors that were not accounted for in the indices include nonbreeding individuals which can be as abundant as breeders (Southwell et al, ), seals that breed and forage in the Southern Ocean (Southwell et al, ), other seabird and marine mammal species that breed in more temperate locations but feed close to the Antarctic continent (Branch, ; DeLord et al, , ; Raymond et al, ), and fish or squid species that predate on the same prey as penguins (Lyver et al, ).…”