Advancing Our Understanding of Cerulean Warbler Space Use Through Radio Telemetry

Abstract

Space use information can be integral in the creation of effective conservation and management strategies. However, avian territories (defended areas) are studied far more frequently than home ranges (entire use areas), and few studies have compared the two areas. This is the case for the cerulean warbler Setophaga cerulea , a declining Neotropical migrant songbird. There is an extensive record of cerulean warbler territory estimates, while the home range has only recently been explored. Studies of these space use areas differ in their sampling techniques, estimation techniques, and location. Consequently, comparison of both space use areas is difficult. We used radio telemetry to delineate cerulean warbler diurnal space use areas in southern Indiana. The primary objective of this study was to describe the relationship between the home range and territory. Kernel density home range estimates of sampled adult male cerulean warblers ( n = 14, mean ± SE = 2.33 ± 0.29 hectares) were significantly larger (0.54 ± 0.18 ha, P = 0.006) than territory estimates of the same group of individuals ( n = 14, mean ± SE = 1.79 ± 0.39 hectares; P = 0.006). Minimum convex polygon home range estimates of the same group of birds ( n = 14, mean ± SE = 3.45 ± 0.55 hectares) were also significantly larger (1.38 ± 0.19 ha, P < 0.001) than territory estimates ( n = 14, mean ± SE = 2.07 ± 0.58 hectares). Additionally, territory estimates described here are considerably larger than other published estimates for this species, which were delineated using spot mapping methods. Cerulean warbler home ranges contain territorial and extra-territorial space, and the latter has not been studied in detail. Area and habitat requirements likely vary throughout this species’ range, and regional conservation management might benefit from study in peripheral space use areas.

Winter GPS tagging reveals home ranges during the breeding season for a boreal-nesting migrant songbird, the Golden-crowned Sparrow

“…However, further studies could improve the accuracy of home range estimates and determine how space use changes throughout the breeding season by collecting more points on the breeding grounds as tracking technology improves and/or through individual tag calibration to assess tag-specific error that can be incorporated into modeling [ 50 , 71 ]. More generally, our data align with other studies that show how visual and auditory observations of breeding birds have the potential to miss more subtle movements outside of territories [ 15 , 16 , 19 ]. Likewise, researchers may even intentionally restrict the data used to create estimated territory boundaries, as such methods (e.g., spot mapping; [ 10 ]) are typically used to estimate the core breeding territory and not the broader home range, and territory estimates based on singing may intentionally exclude areas where other birds sing even if home range overlaps may be occurring.…”

“…We documented much larger space use with miniaturized GPS tags carried by the birds than from territory estimates based on field observations, indicating that Golden-crowned Sparrows move relatively large distances on the breeding grounds that are not being detected in the field by spot-mapping. Similar space use dynamics may be occurring for other passerine species for which there have only been territory estimates in the field (as also supported by recent radio telemetry studies [ 15 , 16 ]), and GPS tracking offers a chance to discover larger and more secretive movements—including, as in this study, by tagging birds on their wintering grounds and passively tracking their home range movements on their breeding grounds.…”

“…Previous studies suggest that home ranges are generally much larger than territories. For example, radio telemetry has shown that birds often use resources outside of territories, including Cerulean Warblers ( Setophaga cerulea ) with home ranges significantly larger than territories [ 15 ], and Golden-winged Warblers ( Vermivora chrysoptera ) with home ranges 2–4 times as large as spot-mapped territories and 40% of telemetry locations outside these territories [ 16 ]. Similarly, territories of male Nightingales ( Luscinia megarhynchos ) made up only 50% of their total activity area, although they spent 90% of their time within the territory [ 17 ].…”