Bioenergetics and food use of wintering and staging Atlantic brant
Date
2010
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
University of Delaware
Abstract
I conducted behavioral and food sampling of Atlantic brant (Branta bernicla hrota) across their winter range on the northeastern coast of the USA. I used time-activity budgets to estimate daily energy expenditure (DEE) of brant. Sampling occurred 1 Dec -- 31 May 2006–2008 in eleven 225 km2 sites between Rhode Island and Virginia containing important estuarine and upland habitat. Overall DEE for brant was 1,530±64 kJ/day. There was considerable variation in time-activity budgets between years, months, regions, habitat, tide, temperature, and time-of-day, and I detected no significant difference in DEE of brant between years or regions. However, DEE was significantly different between months and ranged from 2,018±173 kJ/day in January to 1,048±137 kJ/day in May. Brant spent most of their time feeding (32%), swimming (26%), resting (16%), and flying (15%). I also quantified diet of brant and energy density of food plants. Brant mostly ate macroalgae (Ulva sp., Enteromorpha sp.) in early-mid winter and salt marsh cordgrass (Spartina alterniflora) and upland grass and clover (Poa sp. and Trifollium sp.) in late winter and spring. Energy density differed by vegetation type: macroalgae (12.6±0.1 kJ/g), eelgrass (Zostera marina) (14.1±0.1 kJ/g), salt marsh cordgrass (16.9±0.2 kJ/g), and upland grass and clover (17.7±0.1 kJ/g). I explored variation in the diet of brant between 1 November – 31 May 2007-2008. I determined dietary trends using stable δ13C and δ15N isotopes in four known food sources that brant eat. I predicted relative contributions from food sources would differ between months as food source availability shifted over time. I sampled liver and leg muscle tissues from 525 individuals over a seven month winter period in four geographic regions. I measured δ13C and δ15N in each of the tissues and food sources collected within each region. I detected significant differences between regional and monthly δ13C and δ15N ratios in both liver and leg tissues. Dietary trends were determined using Bayesian mixing-models to estimate relative contributions of food sources with the software package SIAR. I detected variation between regions and months in brant tissues and diet, reflecting population level responses to variation in food resource availability over the winter range.