Age‐specific breeding probability in black brant: effects of population density

Abstract

Summary We estimated age‐specific probability of breeding for black brant ( Branta bernicla nigricans Lawrence) (hereafter brant) at the Tutakoke River Colony, Alaska, during a series of years in which local breeding density and juvenile mortality both increased. We use these analyses to examine demographic responses to increased population density. Estimates of breeding probability were based on Kendall & Nichols (1995) full‐likelihood modification of Pollock’s (1982) robust design applied to observations of uniquely colour‐banded brant. Breeding probability varied with female age but not across years, indicating lack of response to increased population density. Annual probability of breeding increased from 0·67 ± 0·042 for 2‐year‐old females to 0·90 ± 0·024 for females 5 years old or older. Annual breeding probability of males ≥ 5 years old was 0·78 ± 0·03. Lower breeding probability of males compared to females probably reflects dispersal of males whose mates died. Lower juvenile survival appears to be a general response to increased density in large herbivorous birds and mammals, while changes in adult survival and reproductive investment may be more variable.