Is it too simplistic to assume there is a causal link between declines in Afro-Palaearctic migrants and widespread land-use change in Africa?
LINKED PAPER
Apparent resilience of a declining Afro-Palaearctic migrant to forest loss on the wintering grounds. Mallord, J.W., Orsman, C.J., Roberts, J.T., Boafo, K., Skeen, R.Q., Sheehan, D.K. & Vickery, J.A. 2018. IBIS. DOI: 10.1111/ibi.12572. VIEW
Land-use change and subsequent loss of natural habitats is the greatest cause of population declines and extinctions globally (Pimm & Raven 2000). It is, therefore, easy to jump to apparently simple conclusions about its role in the decline of long-distance migrants, a group of species that have undergone some of the greatest reductions in population size over the last few decades (Robbins et al. 1989, Sanderson et al. 2006). In North America, although the finger of blame for the decline of Neotropical migrants was initially pointed at deforestation in Central and South America (Terborgh 1980), subsequent research highlighted how complex the picture was in reality (Faaborg et al. 2010).
In Europe, too, we are facing large-scale declines of our long-distance migrants, and this, too, against a backdrop of widespread land-use change and forest loss in their wintering areas, sub-Saharan Africa (Vickery et al. 2014). For one species, especially, the Wood Warbler Phylloscopus sibilatrix, there are reasons to believe that there could be a causal link. Several years of research in the UK (Mallord et al. 2012, 2016a, 2017a) has failed to turn up any evidence that the drivers of the decline are operating on the breeding grounds. They are also one of the few Afro-Palaearctic migrants that appear forest dependent in the non-breeding season (Morel & Morel 1992), overwintering in the humid zone, and further south than many other migrants. Therefore, habitat loss on the wintering grounds seems a plausible hypothesis for the cause of the decline of Wood Warbler populations in Europe.
Our previous work in Ghana (Mallord et al. 2016b) has shown that, although Wood Warblers are obviously dependent on trees, they tend not to occur in the most densely forested areas, favouring more open woodland, especially if their favoured tree Albizia zygia was present. Our study site near the village of Pepease in the Eastern Region of Ghana, a mosaic of secondary forest and cultivation of various crops, undergoes regular land use change, as new areas are cleared for cultivation, and others are left fallow. The aim of our recent work, just published online in Ibis (Mallord et al. 2018), was to assess what effect such changes, and especially the continued loss of forests, would have on the number of Wood Warblers recorded there in winter.
Figure 1 Radio tracking Wood Warblers in secondary forest habitat in Ghana © J. Mallord
Our study was composed of three parts:
- identify the preferred habitats of Wood Warblers;
- calculate how the extent of this habitat had changed during the three-year study; and
- assess what effect these changes had on the number of Wood Warblers recorded in winter.
Firstly, by mapping different habitats across the study area and following birds fitted with radio tags, we identified (perhaps not surprisingly) that forest (both open and dense) was the preferred habitat. Secondly, although we had noticed how areas of forest were disappearing while walking around the study site, it still came as quite a surprise when we calculated that over a quarter of the forest had been lost over the course of just three years. Initially cleared or burned, in subsequent years these once-forested areas are then planted with various crops, especially Cassava, or Plantain and Palm plantations. Surely, this level of habitat loss would have an impact on the number of Wood Warblers using the site. Finally, we also conducted monthly surveys using playback of the Wood Warbler’s call and song. Although they are not territorial on the wintering grounds, they respond readily and from quite a distance, which is fortunate as they tend to forage unobtrusively in the canopy of tall trees, making it very difficult to estimate how many birds there are. Although the numbers of birds recorded varied across the season, reflecting the itinerant nature of the Wood Warbler’s migration south through Africa (Mallord et al. 2017b), the numbers were similar in all three years.
Figure 2 Wood Warbler fitted with a radio tag © C. Orsman
Figure 3 Surveying Wood Warblers in January 2017 © J. Mallord
Although Wood Warblers are known as a forest bird on the breeding grounds in Europe, and as mentioned earlier are one of the few migrants wintering in the forest zone, in reality, in Africa at least, they are perhaps best considered not a forest species at all, but yet another species dependent on the human-modified farmed landscapes of West Africa. When forest was lost from our study site, it was not replaced by a tree-less landscape. In fact, there were often similar numbers of trees in recently ‘cleared’ land as there was in some of the patches of open forest. Often, the only distinguishing feature was what was growing under the trees, natural vegetation or crops. And these more open wooded landscapes are just what Wood Warblers liked.
The relationship between the presence of a radio-tagged bird and the number of trees suggested there is an optimal number, not too few, but not too many either. This quadratic, or ‘humped’, relationship also suggests that the large scale implications of forest loss for Wood Warblers will depend on the balance between loss of trees from suitable open woodland and well-wooded farmland (bad) and degradation of more pristine forests (good – at least for Wood Warblers!). We have conducted surveys across several West African countries, and future work will attempt to determine the status of suitable habitat across the Wood Warbler’s wintering range, and estimate how its coverage has changed. Then, we may be a step closer in determining whether loss of wintering habitat genuinely is a plausible cause of this species’ decline.
References
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Mallord, J.W., Orsman, C.J., Cristinacce, A., Butcher, N., Stowe, T.J. & Charman, E.C. 2012. Mortality of Wood Warbler Phylloscopus sibilatrix nests in Welsh oakwoods: predation rates and the identification of nest predators using miniature nest cameras. Bird Study 59: 286-295. VIEW
Mallord, J.W., Smith, K.W., Bellamy, P.E., Charman, E.C. & Gregory, R.D. 2016a. Are changes in breeding habitat responsible for recent population changes of long-distance migrant birds? Bird Study 63: 250-261. VIEW
Mallord, J.W., Orsman, C.J., Roberts, J.T., Skeen, R., Sheehan, D.K. & Vickery, J.A. 2016b. Habitat use and tree selection of a declining Afro-Palaearctic migrant at sub-Saharan staging and wintering sites. Bird Study 63: 459-469. VIEW
Mallord, J.W., Orsman, C.J., Cristinacce, A., Stowe, T.J., Charman, E.C. & Gregory, R.D. 2017a. Diet flexibility of a declining long-distance migrant may allow it to escape the consequences of phonological mismatch with its caterpillar food supply. IBIS 159: 76-90. VIEW
Mallord, J.W., Orsman, C.J., Roberts, J.T., Skeen, R.Q., Maiga, A.I. & Issa, O.B. 2017b. First evidence for recurrence of Wood Warblers Phylloscopus sibilatrix in sub-Saharan Africa. Ring. Migr. 32: 25-27. VIEW
Mallord, J.W., Orsman, C.J., Roberts, J.T., Boafo, K., Skeen, R.Q., Sheehan, D.K. & Vickery, J.A. 2018. Apparent resilience of a declining Afro-Palaearctic migrant to forest loss on the wintering grounds. IBIS. DOI: 10.1111/ibi.12572. VIEW
Morel, G.J. & Morel, M-Y. 1992. Habitat use by Palearctic migrant passerine birds in West Africa. IBIS 134: 83-88. VIEW
Pimm, S.L. & Raven, P. 2000. Extinction by numbers. Nature 403: 843-845. VIEW
Robbins, C.S., Sauer, J.R., Greenberg, R.S. & Droege, S. 1989. Population declines in North American birds that migrate to the neotropics. Proc. Natl. Acad. Sci. USA. 86: 7658-7662. VIEW
Sanderson, F.J., Donald, P.F., Pain, D.J., Burfield, I.J. & van Bommel, F.P.J. 2006. Long-term population declines in Afro-Palearctic migrant birds. Biol. Cons. 131: 93-105. VIEW
Terborgh, J.W. 1980. The conservation status of neotropical migrants: present and future. P.21-30 in A. Keast and E.S. Morton (eds). Migrant birds in the neotropics: ecology, behavior, distribution, and conservation. Smithsonian Institution Press, Washington D.C., USA.
Vickery, J.A., Ewing, S.R., Smith, K.W., Pain, D.J., Bairlein, F., Škorpilova, J. & Gregory, R.D. 2014. The decline of Afro-Palearctic migrants and an assessment of potential causes. IBIS 156: 1-22. VIEW
Image credit
Featured image: Wood Warbler, Phylloscopus sibilatrix, Kakum National Park, Ghana, 1 February 2017 © J. Mallord
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