The Relationship Between Body Mass and Survival of Wintering Canvasbacks

Abstract

Mass and recapture histories of 6,000 Canvasbacks (Aythya valisineria) banded in upper Chesapeake Bay were used to test two hypotheses: (1) early-winter body mass is associated with the probability of surviving the winter, and (2) early-winter body mass is associated with annual survival probability. Data were analyzed by a binary regression method that treated mass as a continuous variable and estimated parameters to describe a general relationship between body mass and survival probability. Results for adult males, which provided our largest data sets, presented strong evidence that birds with high relative early-winter masses had both greater overwinter and annual survival probabilities. Results of overwinter analyses necessarily are qualified by the alternative explanation of mass-dependent emigration, i.e. the possibility that lighter birds move south in response to cold weather and leave only heavy birds for recapture. Such a phenomenon remains to be documented. Results concerning annual survival probabilities are not vulnerable to this alternative explanation because of the strong fidelity of Canvasbacks at the banding site. Because of small sample size, data were inadequate to permit mass/survival inferences for adult females. Sample sizes were adequate for young Canvasbacks, but the results were less consistent than for adult males. Although early-winter body mass was associated positively with overwinter as well as annual survival for young Canvasbacks in some years, we suspect that the lack of established wintering patterns among these birds may underlie the less consistent result.

The Role of Energy Intake Rate in Prey and Habitat Selection of Common Eiders Somateria mollissima in Winter: A Risk-Sensitive Interpretation

“…In the case of the eider, these include (i) selecting a diet that permits a higher energy intake, (ii) minimizing the energy expended, and (iii) increasing the time spent foraging. Recent evidence supports the hypothesis that body mass (and reserves) in ducks in winter is positively correlated with the probability of survival (Haramis et al 1986;Hepp et al 1986). Eiders may in some way minimize energy expenditure to decrease the energy gain required for energy balance.…”

The Relationship between Body Mass and Annual Survival in American Black Ducks

“…A reviewer raised the question whether our results were biased because of differential migration of light birds out of the study area vs heavy birds remaining there (see also Haramis et al 1986). This notion, at first, seems difficult to resolve except that if it were true, the analysis should have reflected such a differential migration, i.e., only heavy birds would have been accessible to trapping operations and thus the z scores would have indicated that heavy birds had higher (i than lighter birds.…”

“…Weatherhead and Ankney (1984) suggested that p is different for each bird dependent on the body mass of that individual. For an in-depth description of these models and for the testing procedures used, refer to Haramis et al (1986). If this pattern were true for Black Ducks, the separation of Oi and pi would be difficult and thus, the relationship between 0i and 4D would be unclear.…”

“…Evidence supporting this hypothesis includes: (1) that preseason-banded Mallards Anas platyrhynchos have a lower probability of being shot that autumn if they are in relatively good condition (Hepp et al 1986), (2) that Mallards shot over decoys have relatively lower body mass than Mallards shot at a roost, i.e., Mallards with higher body mass are not as exposed to hunter mortality as Mallards with lower body mass (Greenwood et al 1986), (3) that Canvasbacks Aythya valisineria have a higher probability of surviving the winter and/or the year if they are in relatively high body mass during the early winter (Haramis et al 1986), and (4) that adult American Black Ducks Anas rubripes have a higher probability of surviving the winter if they are in relatively good condition at the beginning of the winter (Pollock et al in press). Evidence supporting this hypothesis includes: (1) that preseason-banded Mallards Anas platyrhynchos have a lower probability of being shot that autumn if they are in relatively good condition (Hepp et al 1986), (2) that Mallards shot over decoys have relatively lower body mass than Mallards shot at a roost, i.e., Mallards with higher body mass are not as exposed to hunter mortality as Mallards with lower body mass (Greenwood et al 1986), (3) that Canvasbacks Aythya valisineria have a higher probability of surviving the winter and/or the year if they are in relatively high body mass during the early winter (Haramis et al 1986), and (4) that adult American Black Ducks Anas rubripes have a higher probability of surviving the winter if they are in relatively good condition at the beginning of the winter (Pollock et al in press).…”

TESTS FOR SENESCENT DECLINE IN ANNUAL SURVIVAL PROBABILITIES OF COMMON POCHARDS,AYTHYA FERINA

“…For example, if animals of a particular age at some time represent a heterogeneous mix of survival probabilities, then at some later time (age), we would expect the cohort to be composed of more individuals with higher survival probabilities (Vaupel and Yashin 1985, Johnson et al 1986, Burnham and Rexstad 1993. Of course, if the heterogeneity in survival probability is associated with an observable covariate (e.g., body size or mass; Haramis et al 1986), then separate estimation of age effects should be possible. Because the high-survival individuals would constitute increasing proportions of the surviving birds as age increased, this scenario could produce the appearance of no age-specific decline in survival probability, despite the existence of such decline within each individual.…”