Surprising variation in singing behavior, breeding territory and home range size and shape in a cryptic migratory thrush

Background/Question/Methods

For migratory songbirds, the term territory is usually defined as the area an individual or breeding pair defends against conspecifics, while home range refers to the total area that an individual uses, but does not defend, during the breeding season.  Since one of the primary functions of passerine song is territory defense, singing behavior has often been assumed to occur only inside territories or near disputed boundaries.  In fact, many studies have delineated territory boundaries by observing singing behavior.  Only recently have advanced methods such as radio-telemetry and GIS (Geographical Information Systems) allowed researchers to accurately document territory and home range sizes and shapes for species with cryptic plumage, behavior and/or habitat.  Here, we report the results of a telemetry and GIS study of singing and space-use behavior in a cryptic thrush species, the veery (Catharus fuscescens).

Results/Conclusions

In May and June of 2011, we attached radio-tags to 20 male veeries, and we then attempted to track each male until we recorded a total of 30 independent GPS points for that male.  We also noted any singing behavior present at each point, and monitored nests.  We tracked 17 of the males to 27 or more points (mean±SE: 49±2.1).  We used kernel analysis to determine territory (50% isopleth) and home range (95% isopleth) sizes and overlap.  Veery territories (1.8±0.9ha) were smaller than home ranges (8.1±0.8ha), and overlapped with fewer other males (territories 1.4±0.3, home ranges 5.2±0.6).  While it is unusual to find overlap in territories, this might be due to the high rate of nest predation in this species, which causes territory borders to change rapidly as nests are lost and females re-build in new locations.  Surprisingly, we also found that males veeries often vocalize off territory (vocal points on territory: 6.5±0.6, vocal points off-territory: 15.2±1.5), and most of the vocalizations were songs (pattern holds for both songs and calls).  This result might indicate that veeries are singing in the far reaches of their ranges in order to solicit extra-pair copulations or to test out alternative breeding locations.