Effects of West Nile Virus Mortality on Social Structure of an American Crow (Corvus brachyrhynchos) Population in Upstate New York

Abstract

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Reproductive partitioning and the assumptions of reproductive skew models in the cooperatively breeding American crow

“…Overall, male auxiliaries were significantly older than female auxiliaries (Mann-Whitney U test: U = -2.24, N 1 = 22, N 2 = 39, P = 0.025; mean + SE = 2.28 + 0.25 years for males and 1.45 + 0.13 years for females). This latter result is congruent with a previous report showing that males are older than females in the same social class (Clark et al 2006). From 2004-2007, five female breeders and only one male breeder died or disappeared from the 21 focal groups.…”

“…For example, reproductive skew in a population of cooperatively breeding white-winged choughs ( Corcorax melanorhamphos ) decreased after a drought disrupted previously stable nuclear family groups (Heinsohn et al 2000). American crows have also experienced a recent change in environmental conditions in the form of West Nile virus, which elevated breeder mortality in 2002-2003 in the Ithaca population (Clark et al 2006). Although we have not yet examined patterns of reproductive skew before and after the epidemic, it might have led to an increase in population level reproductive skew, as predicted by the restraint model (Johnstone & Cant 1999), if it lowered ecological constraints by creating more opportunities for independent breeding.…”

“…From 2004-2007, we examined mating strategies and genetic group structure in a population of American crows in Ithaca, New York, which has been monitored continuously since 1989 (McGowan 1995, 2001; Clark et al 2006). Auxiliaries in this population are both male and female, and individuals of both sexes help with antipredator vigilance, territory and nest defence (Serrell 2003; Wilson 2008), as well as provisioning the incubating females, nestlings and fledglings.…”

“…Although male breeders appeared behaviourally distinct in our sample, and American crows generally have obvious social pair bonds (e.g. Kilham 1984; Caffrey 1992; Clark et al 2006), we tested the repeatability of our social role assignments by determining whether or not the behaviours of male breeders were quantifiably distinct from the behaviours of male auxiliaries. In 2007, we conducted focal observations on 19 family groups during the nest building, egg laying and early incubation periods (ending observations by the second day of incubation).…”

“…obs.). Also, although long term monitoring in our population of banded birds suggests that most group members are first or second order kin, auxiliary individuals do sometimes immigrate into non-natal groups (Clark et al 2006; pers. obs.…”