The relationship between body mass and survival of wintering canvasbacks

-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 earlywinter 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. Received 11 February 1985, accepted 9 January 1986. FOR several species of waterfowl, evidence now exists for a relationship between the physiological state or "condition" of an individual and its subsequent reproductive performance (e.g. Ankney and MacInnes 1978; Raveling 1979; Krapu 1979, 1981). It is also reasonable to expect that the "physiological condition" of an individual at some time t is related to its probability of surviving some subsequent period (t, t + A), especially if the period is one in which food resources are limited or environmental conditions are energetically stressful, or both. This expectation has been accepted in the literature not only for waterfowl, but for birds and mammals in general, and it is now a tacit assumption of most published studies dealing with physiological condition during nonbreeding periods. We are aware of few efforts that use birds, however, to test the hypothesis that particular physiological characteristics are related to survival probability. There is considerable speculation and some evidence for several bird species that food supplies may be limited during winter and that both food availability and winter weather are related to survival probability (Lack 1966; Fretwell 1968, 1972; Goss-Custard 1979; North and Morgan 1979; Pulliam and Parker 1979; van Balen 1980; Ekman et al. 1981; Jansson et al. 1981). Winter is thus a logical time to look for a relationship between physiological condition and survival within many Temperate Zone birds. Two general approaches have been used to draw inferences about the relationship between condition and survival in wintering birds. The first involves comparisons of variables believed to reflect condition (e.g. body mass, percentage of fat or lipid in the body, percentage of protein in the body) between two groups: (1) birds found dead during periods of severe winter weather, and (2) birds collected during "normal" or mild winter weather and believed to be representative of live wintering birds. There are large differences between these two groups for Eurasian Coots (Fulica atra; Visser 1978), Common Redshanks (Tringa totanus; Davidson and Evans 1982), and Eurasian Oystercatchers (Haematopus ostralegus; Davidson and Evans 1982, Swennen and Duiven 1983). Thus, these results provided evidence that lipid and 506 The Auk 103: 506-514. July 1986 This content downloaded from 157.55.39.100 on Thu, 25 Aug 2016 04:57:15 UTC All use subject to http://about.jstor.org/terms July 1986] Canvasback Mass and Survival 507 protein reserves are mobilized during periods of severe weather and may be important to winter survival. Nevertheless, these studies did not demonstrate that variation in survival probability is associated with variation in body mass or nutrient reserves. The second general approach permits stronger inferences about this relationship. Birds are captured and weighed (or assigned to a condition category by some other means) at the beginning of a winter period. Recaptures or resightings of the birds at the end of the period are then used to draw inferences about variation in survival probabilities for birds of different masses or condition categories. We are aware of two such studies on wintering birds. In a 2-yr study, Fretwell (1968) captured and banded 85 Field Sparrows (Spizella pusilla) in early winter and assigned them to two fat categories (above and below average). The proportion of birds seen later in winter was significantly greater for the above-average category in 1 of 2 years. In another study, Kikkawa (1980) weighed and color-banded 388 first-year Gray-backed White-eyes (Zosterops lateralis) at the beginning of winter over a 3-yr period. Resightings at the end of each winter showed no significant difference between the proportion of birds above vs. below average in initial mass. We use an approach similar to the second described above with Canvasbacks (Aythya valisineria) wintering on Chesapeake Bay. Winter weather conditions on Chesapeake Bay are frequently subfreezing, and prolonged ice cover may limit food availability for Canvasbacks. In addition, resource-related differences in winter mortality associated with behavioral dominance or fasting endurance may be important selective forces in the evolution of winter distribution patterns in this species (Nichols and Haramis 1980, Alexander 1983, Haramis et al. 1985). In ducks, there is generally a good correlation between body mass and both fat (Owen and Cook 1977, Bailey 1979) and total nutrient (lipid + protein) reserves (Wishart 1979). Therefore, we expect high body masses to reflect good reserves and possibly confer a greater probability of surviving the winter. In our study, masses and recapture histories of Canvasbacks were used to test two hypotheses: (1) Canvasback body mass in early winter is associated with the probability of surviving that winter, and (2) Canvasback body mass in early winter is associated with subsequent annual survival probability.