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

Abstract

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 Acrocephalus scirpaceus 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 Sylvia cantillans but not in chiffchaffs Phylloscopus collybita inhabiting dry-scrub and scrub fringing wetlands. Body reserves were relatively low among habitat specialist species; resident African reed warbler A. baeticatus and migratory sedge warbler A. schoenobaenus exclusively occupying wetland habitats, and Western olivaceous warblers Iduna opaca 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.

Contrasting sensitivity of nestling and fledgling Barn Swallow Hirundo rustica body mass to local weather conditions

“…Temperature may influence nestling mass indirectly, by affecting insect activity/availabilityand thus parental provisioning ratesover a daily timescale, or over the lifetime of a nestling, as discussed above. Overall, invertebrate activity and abundance tend to be reduced under cooler conditions (Bryant 1975, Turner 1983, Jenni-Eiermann et al 2008; a higher body mass under cool conditions is consistent with the use of strategic deposition of fat reserves as a buffer against starvation under conditions with low or unpredictable food availability (Witter et al 1994, 1995, Ratikainen & Wright 2013, Vafidis et al 2014.…”

Itinerant lifestyle and congregation of lesser kestrels in West Africa

“…coastal wetlands, Inner Niger delta, wetlands in eastern Mauritania), which dry out later and may suppress the adverse effects of the dry season for several months (Studds & Marra, 2005). Such least arid habitats may sustain vegetation and associated insect life for longer in the dry season, enabling kestrels to spend a large part of their non-breeding period at these sites (Zwartz et al, 2009; Vafidis et al, 2014). While the kestrels’ longitudinal itinerary movements differ from the predominantly latitudinal movements of other locust-specialists and aerial hunters (swifts, Akesson et al, 2012; swallows, Norevik et al, 2019, harriers, Trierweiler et al, 2013; Schlaich et al, 2016), east-west movements within the Sahel have also been reported for other species (e.g.…”

Moreau's Paradox Reversed, or Why Insectivorous Birds Reach High Densities in Savanna Trees

“…Tree species usually found on seasonal floodplains should attract more birds since there are many more insects in wetlands than in drylands (e.g. Vafidis et al 2014). 3.…”