Molt and bioenergetics of Pacific black brant (Branta bernicula nigricans) on the arctic coastal plain, Alaska

Abstract

I examined feather molt and bioenergetics of nonbreeding and failed-nesting Pacific Black Brant near Teshekpuk Lake, Alaska. Molt initiation primarily occurred during the first week in July but extended from 26 June through 29 July. Total molt score, a measure of wing and body contour feathers in blood quills, increased from arrival to early molt, but changes thereafter were generally nonsignificant. Age and sex classes showed little variation in total molt score. Primary feather growth averaged 7.2 mm/day for 249 individuals banded and recaptured and growth rate showed no significant difference when analyzed across year, age and sex class, stage of molt or date. Primary feather growth rate of captive birds equaled 7.0 mm/day but significantly declined during molt. Some Pacific Black Brant flew when the 9th primary averaged 61-64% of mean final length, but most regained flight when the 9th primary reached 70%. Flightless Pacific Black Brant occurred in the Teshekpuk Lake molting area from 26 June through 19 August. Pacific Black Brant significantly lost weight during molt. Adults molted at heavier body weights than subadults. Body weight - primary length relationship did not change during molt except for failed-nesting adult females. Lipid reserves declined 71-88% and comprised only 2-4% of corrected fresh body weight at late molt. Protein reserves significantly declined except in subadult males. Ether-extracted lipid was accurately estimated by internal fat deposits and percent carcass water. Carcass protein was predicted by a multiple regression using corrected fresh body weight, breast muscle weight and leg muscle weight. Grasses (Poaceae) comprised 75% of the diet and occurred in 100% of 24 birds examined; mosses averaged less than 1% of the diet. With an estimated net caloric intake of 815 kJ/d and an average daily energy expenditure of 994 kJ/d, adult female Pacific Black Brant required 139 g of endogenous lipid over the 31 d flightless period. Diet and endogenous lipid met 82% and 18% of daily energy expenditure, respectively. Pacific Black Brant may select this molting area more because of protection and seclusion than the quality of food resources.