Avian population consequences of climate change are most severe for long-distance migrants in seasonal habitats

Abstract

One consequence of climate change is an increasing mismatch between timing of food requirements and food availability. Such a mismatch is primarily expected in avian long-distance migrants because of their complex annual cycle, and in habitats with a seasonal food peak. Here we show that insectivorous long-distance migrant species in The Netherlands declined strongly (1984-2004) in forests, a habitat characterized by a short spring food peak, but that they did not decline in less seasonal marshes. Also, within generalist long-distance migrant species, populations declined more strongly in forests than in marshes. Forest-inhabiting migrant species arriving latest in spring declined most sharply, probably because their mismatch with the peak in food supply is greatest. Residents and short-distance migrants had non-declining populations in both habitats, suggesting that habitat quality did not deteriorate. Habitat-related differences in trends were most probably caused by climate change because at a European scale, long-distance migrants in forests declined more severely in western Europe, where springs have become considerably warmer, when compared with northern Europe, where temperatures during spring arrival and breeding have increased less. Our results suggest that trophic mismatches may have become a major cause for population declines in long-distance migrants in highly seasonal habitats.

Figure 1.

(a) Population trends of passerines in Dutch forests and marshlands between 1984 and 2004 for species with different migration behaviour. Results GLM: interaction habitat * migration status: F_2,51 = 6.16, p = 0.004 (shaded boxes, forest; open boxes, marsh). (b) Within-species comparison of population trends in forests and marshes, showing that within species long-distance migrants decline stronger in forests than in marshes (open triangles, residents; open circles, short-distance migrants; filled circles, long-distance migrants). GLM: dependent: forest growth rate, explanatory variables: marsh growth rate: F_1,11 = 7.08, p = 0.022, migration status: F_2,11 = 18.49, p < 0.001, interaction: F_2,9 = 0.82, p = 0.47. (c) Population trends of migratory passerines living in forests and their spring arrival date on the breeding grounds. Later arriving species declined most (GLM: mean arrival date: F_1,10 = 12.41, p = 0.006). Population trends are expressed as the ratio of the densities present in 2004 relative to 1984, which is based on the annual population growth rates (1 = stable, 0.1 is a 90% decline, 10 is a 10-fold increase). Population trends are from the Dutch Breeding BMP (see the electronic supplementary material for details). Arrival data are based on the first three males arriving annually in a study site in Drenthe (northern Netherlands).

Figure 2.

Population trends (1989–2005) of 15 species of forest-breeding passerines in northern and western Europe, separated for long-distance migrants (filled circles) and residents and short-distance migrants (open circles). Only species are used for which we had trends in both regions, and each dot is a pair of species’ population trends. The x = y line is dotted and species that fall below this line fare worse in western compared with northern Europe. Population trends are from the PECBM (see the electronic supplementary material for details). For migrants, common species names are given in abbreviations: TP, tree pipit; CR, common redstart; WW, wood warbler; GW, garden warbler; PF, pied flycatcher; SF, spotted flycatcher.