Non-breeding season habitat quality mediates the strength of density-dependence for a migratory bird

Abstract

Our understanding of when natural populations are regulated during their annual cycle is limited, particularly for migratory species. This information is needed for parametrizing models that can inform management and conservation. Here, we use 14 years of data on colour-marked birds to investigate how conspecific density and habitat quality during the tropical non-breeding period interact to affect body condition and apparent annual survival of a long-distance migratory songbird, the American redstart (Setophaga ruticilla). Body condition and survival of birds in high-quality mangrove habitat declined as density increased. By contrast, body condition improved and survival did not vary as density increased in adjacent, lower quality scrub habitat, although mean condition and survival were almost always lower than in mangrove. High rainfall enhanced body condition in scrub but not in mangrove, suggesting factors such as food availability outweighed consequences of crowding in lower quality habitat. Thus, survival of overwintering redstarts in mangrove habitat, disproportionately males, appears to be regulated by a crowding mechanism based on densitydependent resource competition. Survival of individuals in scrub, mostly females, appears to be limited by density-independent environmental factors but not regulated by crowding. The contrasting effects of density and food limitation on individuals overwintering in adjacent habitats illustrate the complexity of processes operating during the non-breeding period for migratory animals, and emphasize the need for long-term studies of animals in multiple habitats and throughout their annual cycles.

Winter temperatures limit population growth rate of a migratory songbird

“…Whereas density-dependent processes have been shown to regulate other bird populations within and across seasons111718192930, we only found clear evidence for breeding density-dependence regulating population growth. The fact that our study was of a highly philopatric island population could have influenced the strength of density-dependence we observed at the breeding grounds.…”

“…Although non-breeding population density has been found to influence vital rates of other migratory birds during the non-breeding season111718 and subsequent periods of the annual cycle (for example, sequential density-dependence)19, none of the vital rates were found to be strongly correlated with wintering Savannah sparrow abundance in our study (Fig. 4a; Supplementary Fig.…”

“…For migratory species, quantifying seasonal effects of density-dependent and density-independent factors on population dynamics is particularly challenging due to the difficulty of following individuals between their distinct and often distant breeding and non-breeding grounds34. In birds, variation in the non-breeding environment has been shown to influence individual condition5, migration timing67 and subsequent reproductive success6, as well as some components of population growth rate, such as survival and fecundity891011. However, the extent to which density-dependent and -independent effects during the non-breeding period scale up from the individual to affect population growth rate, and the magnitude of these effects relative to those that occur during the breeding period, are unknown.…”

Dietary opportunism, resource partitioning, and consumption of coffee berry borers by five species of migratory wood warblers (Parulidae) wintering in Jamaican shade coffee plantations

“…), and evidence for density‐dependence or food limitation in two Jamaican habitats (Marra et al. ). Evidence for food limitation, and for resource limitation generally (e.g., Winker , Dhondt ), also comes from the presence of numerous floater individuals in wintering areas (e.g., Peele et al.…”

Survivorship across the annual cycle of a migratory passerine, the willow flycatcher

“…Yet, most estimates of survivorship in migratory passerines are for 12‐month annual periods, which average survival probabilities across different periods of the year. Moreover, the majority of annual survivorship estimates are based on breeding ground studies (Marra et al ), while the estimates of winter survival are few (but see Conway et al , Sillett and Holmes , Dugger et al , Marra et al , Blackburn and Cresswell ), despite the fact that migrants spend the majority of their annual cycle on their wintering grounds. Only one published study to date has estimated survival during the migratory period for a small passerine (Sillett and Holmes ), even though the migratory period is considered a time of high mortality (Newton ).…”