Recruitment and the timing of reproduction in Lesser Snow Geese (Chen caerulescens caerulescens)

Recruitment of offspring into a breeding population of Lesser Snow Geese (Chen caerulescens caerulescens) at La Pérouse Bay, Manitoba was used as a measure of reproductive success to assess the relative fitness of females who hatched their clutches early, middle, and late in the breeding season. In three of seven seasons investigated, goslings from early-hatching clutches showed significantly greater recruitment rates than their middle- or late-hatching counterparts. No significant differences in recruitment rates were detected in the other four seasons, although early-hatching clutches showed numerically higher recruitment rates in three of these seasons. There is, therefore, some indication of directional selection for early breeding. This conclusion contrasts with that drawn by Cooke and Findlay (1982), who, using fledging success as a measure of reproductive fitness, showed that females whose clutches hatched in the middle period had the highest fitness and concluded that the population was being exposed to stabilizing selection for synchronization. The discrepancy between these results and those presented in this paper indicates that conclusions concerning the action of selection in natural populations depend heavily upon the stage of the life cycle during which reproductive success is estimated. As such, evolutionary biologists must be cautious of relying too heavily on measures taken too early in the life of the organism. Received 28 June 1983, accepted 12 January 1984

Evidence of former allopatry of the two colour phases of Lesser Snow Goose (Chen caerulescens caerulescens).

Plumage color is distributed clinally in the Gulf Coast population of the dimorphic Lesser Snow Goose (Chen caerulescens caerulescens); the white phase predominates in the west, and the blue phase in the east. A similar distribution occurs in the breeding colonies of this population. Historical evidence, stretching back to the mid-18th century, shows that the two phases were almost allopatric until the third decade of the 20th century. Allozyme variants at 6 loci also support the conclusion that the morphs were until recently two distinct taxa. The recent merging of the taxa probably is due to a change in winter feeding habits that allowed birds of both morphs to meet in the rice-growing areas of inland Texas and Louisiana. Because pair formation occurs at this time, this change permitted gene flow to occur between the morphs. There is no evidence of reduced fitness among mixed pairs, and interbreeding among the morphs is common. We know of no other case of a historically documented merging of two formerly allopatric taxa of birds where interbreeding is so widespread and where there is no evidence of reduced fitness of the hybrids. Received 30 January 1987, accepted 26 February 1988

Changes in Survival Rates of Lesser Snow Geese with Age and Breeding Status

Survival rates of Lesser Snow Geese (Chen caerulescens caerulescens) were examined based on recoveries and recaptures of about 350,000 geese banded at breeding colonies in northern Canada, at migration stopover points in the Dakotas and Missouri, and on the wintering grounds in Louisiana and Texas. First-year survival rates for goslings banded on the breeding grounds varied from 10 to 70% of adult survival rates. Much of the juvenile mortality occurred on the breeding grounds or early on the first migration. Young geese that reached migration stopovers or the wintering grounds were more vulnerable to hunters than adults, but had only slightly lower survival rates than adults. Greater vulnerability and lower survival continued through the second year of life, even though yearlings do not breed. In contrast, older birds that did not breed, or failed early in a nesting attempt, were much less vulnerable to hunters in the following hunting season than successful breeding adults, but did not appear to have higher survival as a result. Geese captured for the first time as breeding adults had slightly lower survival rates than geese that had been recaptured at the colony at least once, suggesting experienced breeders have higher survival. Although there was some evidence that older birds were slightly more vulnerable to hunters, there were no signs of any changes in survival rate with age in older geese, indicating that senescence, if it affects survival, does not do so for at least the first 10 to 15 years of age. With current hunting levels, less than 5% of Lesser Snow Geese are likely to live beyond this age. Our study demonstrates a variety of statistical methods for testing hypotheses about age-specific survival using both recovery and recapture data, even when the data do not permit estimation of the exact survival rates

Body size variation and fitness components in lesser snow geese Chen caerulescens caerulescens.

We examined the potential action of selection on body size in a population of Lesser Snow Geese (Chen caerulescens caerulescens) breeding in the Canadian subarctic. We evaluated the genetic basis of phenotypic variation in body size and examined the association of body size and components of fitness related to fecundity and viability. There was a heritable component to body size in this population derived in part from the action of additive genes. There was no relation between adult body size and the number of eggs laid, the number of eggs surviving predation, the number of goslings that left the nest, or the number of goslings fledged. Small birds entered the breeding population at a younger age. They did so with no reduction in viability and may actually live longer than large birds. The heritable variation in body size combined with the directional selection gradient should lead to a gradual reduction in adult body size in this population. We found no evidence for such a change over 5 generations. We discuss this in terms of additional fitness components, the retarding effects of age structure on the response to selection, and the interaction of selection and gene flow. Received 6 October 1987, accepted 8 May 1988

The colour of fitness: plumage coloration and lifetime reproductive success in the tawny owl

We studied variation in plumage colour and life history in a population of tawny owls (Strix aluco) in southern Finland, using 26 years of data on individually marked male and female owls. Colour was scored on a semi-continuous scale from pale grey to reddish brown. Colour scoring was repeatable and showed a bimodal distribution (grey and brown morph) in both sexes. During the study period, colour composition was stable in the study population in both sexes. The sexes did not mate assortatively with respect to their colour. Colour was a highly heritable trait and was under selection. Grey-coloured male and female owls had a higher lifetime production of fledglings, and grey-coloured male (but not female) owls produced more recruits during their lifetime than brown individuals. Selection on colour was mediated through viability selection and not through fecundity selection. Our results reveal remarkably strong selection on a genetically determined phenotypic trait