Recent population declines in Afro‐Palaearctic migratory birds: the influence of breeding and non‐breeding seasons

Abstract

Aim Recent, rapid population declines in many Afro‐Palaearctic migratory bird species have focussed attention on changing conditions within Africa. However, processes influencing population change can operate throughout the annual cycle and throughout migratory ranges. Here, we explore the evidence for impacts of breeding and non‐breeding conditions on population trends of British breeding birds of varying migratory status and wintering ecology. Location Great Britain (England & Scotland). Methods Within‐ and between‐species variation in population trends is quantified for 46 bird species with differing migration strategies. Results Between 1994 and 2007, rates of population change in Scotland and England differed significantly for 19 resident and 15 long‐distance migrant species, but were similar for 12 short‐distance migrant species. Of the six long‐distance migrant species that winter in the arid zone of Africa, five are increasing in abundance throughout Britain. In contrast, the seven species wintering in the humid zone of Africa are all declining in England, but five of these are increasing in Scotland. Consequently, populations of both arid and humid zone species are increasing significantly faster in Scotland than England, and only the English breeding populations of species wintering in the humid zone are declining. Main conclusions Population declines in long‐distance migrants, especially those wintering in the humid zone, but not residents or short‐distance migrants suggest an influence of non‐breeding season conditions on population trends. However, the consistently less favourable population trends in England than Scotland of long‐distance migrant and resident species strongly suggest that variation in the quality of breeding grounds is influencing recent population changes. The declines in humid zone species in England, but not Scotland, may result from poorer breeding conditions in England exacerbating the impacts of non‐breeding conditions or the costs associated with a longer migration, while better conditions in Scotland may be buffering these impacts.

Quantifying drivers of population dynamics for a migratory bird throughout the annual cycle

“…However, changes in population size are most responsive to habitat loss during the period in which populations are more severely limited [3]. Unfortunately, relatively few empirical studies provide quantitative evidence indicating whether population size of migratory species is more sensitive to the amount of breeding habitat or non-breeding habitat [7,34,35] and we are unaware of any study done at range-wide scales.…”

Demographic drivers of decline and recovery in an Afro-Palaearctic migratory bird population

“…In Britain, population declines in willow warblers were initiated by a few consecutive years of poor survival but, in the northwest, these declines were quickly reversed by a recovery of survival rates alongside consistently higher productivity. The consistency of the regional differences in population trends across a suite of long-distance migrants [9,10] suggests that similar demographic processes could be mirrored across these species and, while regional IPMs cannot yet be constructed for these species, they show similar regional differences in productivity (northwest: 3.71 (3.65, 3.79), southeast: 3.44 (3.37, 3.50), n = 15 species) but not survival (northwest: 0.44 (0.22, 0.61), southeast: 0.46 (0.34, 0.58), n = 5 species) as willow warblers (see the electronic supplementary material, table S5 for species). Identifying and reducing the frequency of conditions associated with low survival is likely to be very difficult to achieve and, on its own, is unlikely to recover populations.…”

“…This has led to suggestions that migrant declines are being driven by changes on wintering and passage sites [5,7]. However, declines are not occurring everywhere; for example, within Britain, examination of within-species variation in population trends has revealed substantially greater declines in English than Scottish breeding populations [9,10]. These regional differences are also evident in populations of resident species, strongly suggesting that breeding season conditions are contributing to the observed population trends [10].…”

Survival of Afro‐Palaearctic passerine migrants in western Europe and the impacts of seasonal weather variables

“…, Morrison et al . ). The complex life cycle of migrants can make it particularly difficult to determine the causes of population declines (Robbins et al .…”