A Multi-Scale Examination of Stopover Habitat Use by Birds

Abstract

Most of our understanding of habitat use by migrating land birds comes from studies conducted at single, small spatial scales, which may overemphasize the importance of intrinsic habitat factors, such as food availability, in shaping migrant distributions. We believe that a multi-scale approach is essential to assess the influence of factors that control en route habitat use. We determined the relative importance of eight variables, each operating at a habitat-patch, landscape, or regional spatial scale, in explaining the differential use of hardwood forests by Nearctic-Neotropical land birds during migration. We estimated bird densities through transect surveys at sites near the Mississippi coast during spring and autumn migration within landscapes with variable amounts of hardwood forest cover. At a regional scale, migrant density increased with proximity to the coast, which was of moderate importance in explaining bird densities, probably due to constraints imposed on migrants when negotiating the Gulf of Mexico. The amount of hardwood forest cover at a landscape scale was positively correlated with arthropod abundance and had the greatest importance in explaining densities of all migrants, as a group, during spring, and of insectivorous migrants during autumn. Among landscape scales ranging from 500 m to 10 km radius, the densities of migrants were, on average, most strongly and positively related to the amount of hardwood forest cover within a 5 km radius. We suggest that hardwood forest cover at this scale may be an indicator of habitat quality that migrants use as a cue when landing at the end of a migratory flight. At the patch scale, direct measures of arthropod abundance and plant community composition were also important in explaining migrant densities, whereas habitat structure was of little importance. The relative amount of fleshy-fruited trees was positively related and was the most important variable explaining frugivorous migrant density during autumn. Although constraints extrinsic to habitat had a moderate role in explaining migrant distributions, our results are consistent with the view that food availability is the ultimate factor shaping the distributions of birds during stopover.

Prioritizing tropical habitats for long-distance migratory songbirds: an assessment of habitat quality at a stopover site in Colombia

“…Given that migratory birds are expected to seek out habitats supporting the highest rates of fuel deposition, density has been considered to be a reliable indicator of stopover habitat quality under a wide range of conditions because birds making multiday stopovers respond numerically to resource availability (Buler et al 2007, Buler andMoore 2011). However, density can also be misleading if there are associated density-dependence effects on habitat quality, such as diminished resources or active exclusion by dominant individuals (Van Horne 1983).…”

Recent advances in understanding migration systems of New World land birds

“…The combination of these factors creates a complex and shifting environment within which migrants must trade off safety and foraging. To date, the behavior of birds confronted by this dynamic situation has received limited attention (but see Cimprich et al 2005, Cimprich and Moore 2006, Buler et al 2007). …”

Avian Resources of the Northern Gulf of Mexico

“…For migrant Passerines using the northern Gulf Coast, suitable stopover habitat is a critical feature. Migrant densities were most strongly related to forest cover within a 5 km radius; this feature influenced where migrants made landfall (Buler et al 2007). Indeed along the coast of Mississippi, northbound songbirds made landfall in resource-rich habitats within 18 km (11 mile) of the coastline (Buler and Moore 2011).…”

“…The main families in the Gulf are ducks (Anatidae, 46 species), gulls, terns and skimmers (Laridae, N ¼ 41), herons and egrets (Ardeidae, N ¼ 17), rails (Rallidae, N ¼ 16), warblers (Parulidae, N ¼ 36), and flycatchers (Tyrannidae, N ¼ 17). The latter two groups are Passerines, but they frequently occur on coastal islands, on marshes, and in coastal forest habitats either as migrants or during the breeding season (Moore et al 1990;Buler et al 2007;Buler and Moore 2011). For a full list of the species, see Gallardo et al (2009)…”