Abstract
BackgroundThe extent to which seasonal changes in food availability affect small-scale movements in free-ranging populations of birds of prey is relatively little studied. Here we describe a seasonal “micro-migration” of a farm-island population of striated caracaras (Phalcoboenus australis) in the Falkland Islands in response to seasonal changes in the availability of seabird carcasses. We banded more than 450 individuals on Saunders Island, deployed archival and satellite GPS data loggers on 17 individuals, and monitored movements within and between two feeding areas on Saunders Island, a “marine-subsidized” site near seabird colonies and an anthropogenic “human-subsidized” farm site 16 km to the southeast.ResultsDuring 67 observation days between 2010 and 2015, resightings of 312 banded caracaras were greater at the marine-subsidized site during austral summer than winter, and the total daily resightings varied significantly between spring versus summer, summer versus winter, autumn versus spring, and autumn versus winter. Resightings were higher at the human-subsidized site in austral winter than summer and the total daily resightings varied significantly across all bi-seasonal comparisons. Resightings indicated that at least 12 of 197 birds (6.1%) moved between the human- and marine-subsidized sites at least once during the same winter, 15 of 335 birds (4.5%) did so in spring, none of 164 birds did so in summer, and 16 of 297 birds (5.4%) did so in autumn. Individuals fitted with archival GPS data loggers at the marine-subsidized site in summer maintained highly localized 95% kernel core areas (0.55 ± 0.12 km2 [mean ± SD]), whereas those at the human-subsidized site in winter maintained larger 95% kernel core areas (3.8 ± 4.6 km2). Two of 6 satellite-tagged individuals that summered at known caracara breeding colonies 80 km WNW of Saunders Island were subsequently resighted in winter at the human-subsidized site.ConclusionOur results suggest that seasonal shifts in food resource availability drive seasonal micro-migrations in a farm-island population of striated caracaras, and that farm sites can be critical in providing nutritional resources for caracaras when naturally occurring marine-subsidized resources become less available. Our results have important implications for striated caracara spatial ecology and conservation, as increased winter survival could improve the status of this globally Near-Threatened population.
Highlights
The extent to which seasonal changes in food availability affect small-scale movements in freeranging populations of birds of prey is relatively little studied
Migration is a response to changes in resource availability over time that involves an animal moving between habitats with seasonally shifting resources [1, 3]
Our objective was to study the behavior of alphanumerically color-marked, mainly juvenile and sub-adult striated caracaras on Saunders Island, a non-breeding “nursery island” for caracaras, to assess the extent of their intra-island movements as individuals switched from feeding on mainly seabirds and tidal invertebrates (i.e., a “marine subsidy” [9]) in summer to feeding on nutritional resources associated with farming (i.e., a “human subsidy” [12]) in winter
Summary
The extent to which seasonal changes in food availability affect small-scale movements in freeranging populations of birds of prey is relatively little studied. Many animals live in seasonally dynamic environments and must adapt to seasonal changes in their environments. Movement behavior enables such animals to adapt to changes in resource availability in both space and time [1]. Recent research on the European roe deer (Capreolus capreolus) [5] supports the notion of a migratory continuum, often over relatively short distances in which individuals commute (sensu [3]) or engage in multiple, brief residencies in seasonal ranges, a movement pattern that includes use of a primary range and a secondary “winter refuge” [5]
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