Habitat Use and Body Mass Regulation among Warblers in the Sahel Region during the Non-Breeding Season

<div><p>Migratory birds face significant challenges across their annual cycle, including occupying an appropriate non-breeding home range with sufficient foraging resources. This can affect demographic processes such as over-winter survival, migration mortality and subsequent breeding success. In the Sahel region of Africa, where millions of migratory songbirds attempt to survive the winter, some species of insectivorous warblers occupy both wetland and dry-scrubland habitats, whereas other species are wetland or dry-scrubland specialists. In this study we examine evidence for strategic regulation of body reserves and competition-driven habitat selection, by comparing invertebrate prey activity-density, warbler body size and extent of fat and pectoral muscle deposits, in each habitat type during the non-breeding season. Invertebrate activity-density was substantially higher in wetland habitats than in dry-scrubland. Eurasian reed warblers <i>Acrocephalus scirpaceus</i> occupying wetland habitats maintained lower body reserves than conspecifics occupying dry-scrub habitats, consistent with buffering of reserves against starvation in food-poor habitat. A similar, but smaller, difference in body reserves between wet and dry habitat was found among subalpine warblers <i>Sylvia cantillans</i> but not in chiffchaffs <i>Phylloscopus collybita</i> inhabiting dry-scrub and scrub fringing wetlands. Body reserves were relatively low among habitat specialist species; resident African reed warbler <i>A. baeticatus</i> and migratory sedge warbler <i>A. schoenobaenus</i> exclusively occupying wetland habitats, and Western olivaceous warblers <i>Iduna opaca</i> exclusively occupying dry habitats. These results suggest that specialists in preferred habitats and generalists occupying prey-rich habitats can reduce body reserves, whereas generalists occupying prey-poor habitats carry an increased level of body reserves as a strategic buffer against starvation.</p></div

Laying date data

Laying date dat

Nestling mass

Nestling mas

GLM predicted effects of supplementary feeding treatment, site and environmental variables on total first brood productivity in reed warblers.

<p>GLM predicted effects of supplementary feeding treatment, site and environmental variables on total first brood productivity in reed warblers.</p

Starting models of GLM and GLMMs relating breeding performance parameters of reed warblers monitored in Cardiff Bay Wetland Reserve and Cosmeston Lakes Country Park in South Wales, UK, between 2012 and 2014.

<p>Starting models of GLM and GLMMs relating breeding performance parameters of reed warblers monitored in Cardiff Bay Wetland Reserve and Cosmeston Lakes Country Park in South Wales, UK, between 2012 and 2014.</p

GLM Predicted effects of supplementary feeding treatment and environmental variables on incubation duration in reed warblers.

<p>GLM Predicted effects of supplementary feeding treatment and environmental variables on incubation duration in reed warblers.</p

Pairwise comparison of proportional odds of species in a particular habitat having a fat score of 1 unit higher than another species in a particular habitat, or the same species in a different habitat.

<p>Displayed odds values relate to species in the vertical column in reference to those in the horizontal row above the matrix. Positive odds values indicate a higher likelihood, and negative odds indicate a lower likelihood. The sign would be reversed to obtain the odds value for the species groups in the horizontal row in reference to those in the vertical column. * = P<0.05, **  = P<0.005, *** = P<0.0001.</p><p>Pairwise comparison of proportional odds of species in a particular habitat having a fat score of 1 unit higher than another species in a particular habitat, or the same species in a different habitat.</p

Generalised Additive Models of body mass (dependent variable) for each generalist species. Parameter estimates for levels of the factors “Habitat” and “Year”, are relative to the reference levels of “Wetland” and “2012” respectively.

<p>Effects shown in bold are significant (<i>P<0.05).</i></p><p>Generalised Additive Models of body mass (dependent variable) for each generalist species. Parameter estimates for levels of the factors “Habitat” and “Year”, are relative to the reference levels of “Wetland” and “2012” respectively.</p