Relationship between the annual percentage change in population size and trend in climate suitability (CST) across the breeding ground ranges of 61 short‐distance migrant species and 39 long‐distance migrant species.

Lines indicate the mean non‐standardised effect size from across 100 separate generalised linear mixed models, while shaded areas indicate the 95% credible intervals. Data from Howard et al. (2020).

Afro-Palaearctic long-distance migratory species are declining more rapidly than short-distance migratory or resident species (Cresswell 2014, Gregory et al. 2019, Sanderson et al. 2006, Vickery et al., 2014). It has been suggested that these declines may be partially explained by the effects of climate change and land use change (Both et al. 2006, Jiguet et al. 2010, Kirby et al. 2008, Stephens et al. 2016, Studds et al. 2017, Vickery et al. 2014). Howard et al. (2020) looked at the relative importance of climate change and land cover change at breeding and non-breeding grounds on long-term population trends of migratory species. Annual abundance data from the Pan-European Common Bird Monitoring Scheme (PECBMS) was used to study population trends of 39 long-distance migrants and 61 short-distance migrants over a 36-year period (1980–2016). Populations of short-distance migrants have remained relatively stable since 1980, but long-distance migrants have slowly declined. Howard et al. (2020) analysed these population trends in relation to trends in land cover and climate suitability across breeding and non-breeding grounds.

Population trends of migrant birds tended to be more closely related to changes in climate than changes in land cover on their breeding grounds, with an overall positive relationship between climate suitability and population trends. However, this relationship differed between migratory strategies—long-distance migrants had not significantly benefitted from an increase in climate suitability—whereas short-distance migrants had. Their dependence on multiple widely dispersed areas at different times of year renders long-distance migrants more vulnerable to environmental change than their resident counterparts. For example, climate change may lead to a mismatch between the breeding period of insectivorous migratory birds and the peak abundance of their insect prey at breeding sites (Both et al. 2006). Milder winters may also boost the overwinter survival of resident species, creating stronger competition for breeding resources to the detriment of those that arrive at the breeding grounds last (Berthold et al. 1998, Pearce-Higgins et al. 2015).

In contrast, on migratory species’ non-breeding grounds, land cover change tended to be of greater importance than climate change in explaining population trends. Anthropogenic land use change such as the intensification of agriculture may cause population declines by reducing the quantity and/or quality of suitable habitat at overwintering sites. There has been an approximately 5% decrease in the extent of suitable land cover at the non-breeding grounds of long-distance migrants (Howard et al. 2020), reflecting extensive changes in land-management across sub-Saharan Africa, particularly in the Sahel region (Adams et al. 2014). The relationship between land cover suitability and population trends suggests that this may be one of the factors driving the decline in long-distance migrants.

It is important to note that the combined effects of climate and land-cover change only accounted for around 15% of the variation in population trends of long-distance migratory species and 31% in short-distance migrants. This suggests that a number of other factors are involved in the observed population declines. The dependence of long-distance migrants on multiple landscapes throughout the year exposes them to a wide range of site-specific threats such as hunting, habitat loss, and invasive species. More information is needed on the locations of stopover sites and their associated threats before we fully understand the reasons behind the ongoing decline of migratory species.