Effects of sex and birth date on premigration biology, migration schedules, return rates and natal dispersal in the Mountain white-crowned sparrow

A long term mark-recapture study was conducted on a montane population of migratory sparrows to determine interannual variations in numbers of young produced, the behavior and physiology of young birds once independent from parental care, and their return in subsequent years as a function of sex, calendar date, birth date, and birth location. Measures of natal dispersal distance and subsequent involvement in breeding were also obtained. Sex ratio of nestlings was 1:1 but 28% more independent juvenile males than females were trapped. Return rates were 6.8% for nestlings and 12.2% for independent juveniles and were higher in males than in females in both groups. Numbers of young trapped on the study area showed a three-fold interannual variation which was attributed to the impact of snowpack on reproductive success. Following completion of molt juveniles fattened quickly and departed on migration in late September but not before most had traveled some distance from their birth site. Date of arrival on the breeding grounds from wintering quarters was not appreciably affected by age (1-year olds vs. 2-year olds +) and although males tended to arrive ahead of females, the sexes overlapped greatly in their schedules. Return rates of yearlings, among those handled as independent juveniles, were about 8 to 15%; no matter the sex or the duration they were known to linger as juveniles on a study area meadow, except for a group of males that stayed for more than four weeks. Members of this group were highly philopatric, having a return rates of 28.5%. Time spent on the same meadow as a juvenile did not affect the ability of either sex subsequently to attain breeding status although more yearling females than males were known breeders, 62.1% vs. 43.6%. The majority of yearlings (recruits) appearing on the study area each year were captured for the first time as adults. This group, called adult recruits, comprised 68.7% of the yearling males and 82.5% of the yearling females. In females, adult recruits were as successful at attaining breeding status as were juvenile recruits (birds captured and banded on the study area previously as juveniles). In males, however, previous familiarity with the area may have enhanced success because the number of known breeders was 10% higher in juvenile recruits than in adult recruits. Return rates peaked at about 10% in both sexes born 10-20 July and decreased thereafter. This was attributed to increased mortality in late-born young due to inadequate preparation time for migration. Returns of females, but not males, also decreased in those born before 10 July. This was attributed to a propensity for postfledging dispersal distances of females to increase with available time. Natal dispersal (distance from birth nest to first nest as a yearling) was greater in females than in males but sample sizes were small and the difference was not significant. Close inbreeding was rare. It occurred once in 473 pairings. Finally, the data are incorporated into a descriptive model of natal dispersal which suggests that natal dispersal is temporally divided, one part occurring in juveniles during the postfledging period and the other in yearlings soon after arrival at potential breeding areas. Natal dispersal distances tend to be greater and apparent philopatry lower in females than in males because females are more vagile as juveniles and they establish larger home ranges which they may enlarge even further as yearlings while in the process of choosing breeding sites. Current hypotheses concerning sexual asymmetries in dispersal are considered and expanded upon, a key addition being the proposal that intrasexual competition in females may contribute importantly to dispersal patterns in migratory passerines.