Nutrient Reserves, Probability of Cold Spells and the Question of Reserve Regulation in Wintering Canvasbacks

1. Interpreting body mass and composition of wintering birds is often confounded by the inability to discriminate endogenous regulation of reserves from effects of proximate weather and food conditions. Endogenous regulation is thought to act through genetically controlled set-points that change throughout the year, due to evolutionary adaptation to long-term probabilities of needing reserves at different times. 2. For canvasbacks (Aythya valisineria) wintering in upper Chesapeake Bay, coastal North Carolina, and Louisiana, I calculated over many years the probability of cold spells when canvasbacks likely depend on reserves. I then analysed whether the timing of such cold spells is predictable enough to form the basis for monthly schedules of endogenous reserve regulation in free-ranging canvasbacks. 3. Based on 41 years of data, probabilities of cold spells are higher at more northern sites. However, the timing of cold spells of different durations is not predictable at any latitude, providing little selective basis for genetic, time-dependent regulation of reserves on a monthly basis. 4. In North Carolina in 1983-84, the probability of surviving periods of fasting, as calculated from energy reserves and respirometry, decreased more than the probability of cold spells in midwinter, and less than the cold-spell probability in late winter. This suggests that reserves were not successfully regulated to maintain relatively constant mortality risk. Widely varying body mass patterns among years, and higher reserves at some southern sites than northern sites, indicate that canvasbacks maintain greater reserves if proximate conditions allow. 5. Mating and migration systems of many diving ducks (Aythya spp.) may disrupt genetic adaptation to specific wintering conditions because (i) pairbonds are mostly formed during spring migration so that mates have often wintered in different areas, (ii) offspring disperse to diverse wintering sites often different from those of their parents, and (iii) ducks often move long distances during winter in response to weather. 6. This analysis provides little evidence that endogenous schedules of nutrient reserves can be effectively tuned to monthly probabilities of cold spells at particular latitudes. Despite evidence for endogenous regulation in captive birds, it appears that free-ranging canvasbacks attempt to maintain high reserves throughout winter within constraints of proximate conditions.