Mapping bald eagle activity shadows around communal roosts

Abstract

We assessed diurnal activity patterns associated with communal roosts (n ¼ 26) by tracking nonbreeding bald eagles (Haliaeetus leucocephalus; n ¼ 58) within the upper Chesapeake Bay, USA, 2008-2013. We used daytime locations (n ¼ 54,165) to map activity shadows (using home range analytics, 90% kernel) around communal roosts, to evaluate the spatial structure and to delineate diurnal activity centers. We overlaid a range (100-3,200 m) of buffers around the perimeter of each roost to estimate the benefits of management scenarios in extending protection to daytime activities. Activity shadows around roosts varied from 1.5 km 2 to 116 km 2 ( x ¼ 30.3 AE 5.48 [SE]), reflecting landscape context. Roosts with small (<10 km 2 ) activity shadows tended to have simple shapes with roosts centrally located and positioned along primary shorelines. Roosts supporting large (>50 km 2 ) activity shadows tended to have complex shapes with roosts not centrally located and set back from primary shorelines. Daytime locations were highly concentrated in areas near communal roosts (76% of locations within 2 km of roost perimeters). Diurnal activity centers (n ¼ 38) included areas surrounding roosts and secondary activity centers that were primarily located along prominent shorelines. Communal roosts play a more significant and multi-faceted role in the eagle life cycle than we previously understood. Many of the roosts positioned along the shoreline provided resting places during the night and day, served as social gathering places during the day, and functioned as feeding locations. Evaluation of management buffers supports current management guidelines that recommend the establishment of 800-m buffers. Establishment of 800-m buffers within the study area would enclose 54% of all daytime locations, 66.7% of the area enclosed within activity centers associated with roosts, and 12.1% of the area enclosed in secondary activity centers. Ó 2017 The Wildlife Society.

Patterns of roost site use by Asian hornbills and implications for seed dispersal

“…We determined the diurnal activity ranges of individual birds by using the kernel density estimation method using the library "adehabitatHR" in R (Calenge 2006) to generate the utilization distributions of the individual birds. We used the default "href" function as the smoothing parameter (Worton 1995, Watts andTurrin 2017). Utilization distribution is a representation of the relative space use by an individual bird within its entire activity range (Worton 1989).…”

“…Often the choice of the roosts by birds is positively influenced by the position of the foraging sites (Watts andTurrin 2017, Johnston-González andAbril 2019). The choice of roost sites may also be influenced by factors like thermoregulation (Williams and Du Plessis 2013) and predation pressures (Townsend et al 2009, Johnston-González andAbril 2019).…”

“…Certain species roost communally, which may accord foraging benefits through information transfer (Ward and Zahavi 1973) apart from enabling energy efficiency (Williams and Du Plessis 2013) and reduced per-capita predation pressure (Eiserer 1984). Most information on roosting comes from raptors (Bock et al 2013, Watts andTurrin 2017), water birds (Peters andOtis 2007, Jankowiak et al 2015, Johnston-González and Abril 2019) and birds in temperate regions (Zabala et al 2012, Jamieson et al 2016 with relatively little information from tropical birds (Jirinec et al 2018).…”

Roost site use by Great (Buceros bicornis) and Wreathed (Rhyticeros undulatus) Hornbill and its implications for seed dispersal

“…We used the kernel density estimation method using the library “adehabitatHR” in R to determine the foraging activity range of the individual birds (Calenge, 2006). We used the default “href” function as the smoothing parameter rather than the least square cross validation (LSCV) method (Watts & Turrin, 2017; Worton, 1995). LSCV method tends to estimate smaller home ranges than the href method.…”

Predation risk and resource availability explain roost locations of Whimbrel Numenius phaeopus in a tropical mangrove delta

“…Our results partially align with the hypothesis of proximity to foraging-patch (Caccamise & Morrison 1988), such that roosts can be understood as the result of birds aggregating near rich sources of food, rather than or in addition to social aggregations per se. Raptors (Josens et al 2013, Watts & Turrin 2017, colonial waterbirds (Sparling & Krapu 1994), corvids (Gorenzel & Salmon 1995, Sonerud et al 2001) and other birds and mammals (Beauchamp 2014) form aggregations around food-rich patches. In such cases, roosts are thought to work as 'information centres' and individuals participating could alter their daily behaviour to follow successful foragers.…”