Molt in the annual cycle of canvasbacks

-We studied the intensity of molt of capital, down, and contour plumages of male and female Canvasbacks (Aythya valisineria) from late fall through winter, spring migration, and nesting. Molt intensities in both juvenile and adult males were highest among arriving fall migrants and during prewinter fat deposition, and molt in juvenile males increased during early spring migration. Among females, capital molt was high among arriving migrants in fall, and molt of all plumage types increased during spring migration and courtship. Paired females tended to molt more heavily than unpaired females during spring migration, but many females initiated intensive prebasic molt before pairing. Molt in females peaked in spring migration and early breeding, concurrently with accelerated fat storage and ovarian growth. Capital and contour molt in females decreased during laying and essentially ceased during incubation, perhaps to mitigate energetic constraints during these periods. Prewinter nutrient storage appeared independent of molt intensity, except for body protein among juveniles. During spring migration, molt intensities were greater in adult females with higher levels of body fat and protein, indicating that molt did not necessarily precede or interfere with nutrient storage. We suggest that outside the laying, incubation, and wing molt periods, the energetic costs of molt in Canvasbacks are not sufficient to mandate temporal separation of molt from nutrient storage, migration, courtship, or early stages of rapid ovarian growth. Received 14 September 1987; accepted 29 March 1988. THE costs of avian molt include nutrient demands for feather components, increased amino acid metabolism, changes in water balance, increases in blood volume, and enhanced heat loss (King 1980). These and other adjustments logically seem to require changes in a bird's allocation of time and energy. However, the energetic and nutritional costs of molt have proven difficult to partition from those of other physiological or ecological processes (Payne 1972, King 1980, Walsberg 1983, Ankney 1984), so that inference as to the energetic significance of molt has been largely theoretical rather than empirical (e.g. Walsberg 1983, Heitmeyer 1985: 289-298). Consequently, despite recent insights into molt physiology (e.g. Murphy and King 1984, 1986), there is disagreement over whether the energy demands of molt constrain other events in the annual cycle (Payne 1972, Walsberg 1983). That annual cycles of birds have evolved to minimize overlap of energetically demanding events has been repeatedly asserted (Payne 1972, King 1974, Wingfield and Farner 1980, Heitmeyer 1987). This "staggered costs" hypothesis 3Present address: International Crane Foundation, E-11376 Shady Lane Road, Baraboo, Wisconsin 53913 USA. has in some cases been extended to a "sequential events" hypothesis, by which the initiation of presumably expensive events such as molt, nutrient storage, migration, courtship, and breeding is contingent on completion of preceding events (Heitmeyer 1985: 153; see Farner et al. 1980). These ideas imply that overlap of such processes would at times be prohibitively stressful (King and Murphy 1985). Perhaps because the costs of molt are so uncertain (Payne 1972, Walsberg 1983), workers have often speculated that seasonal molt patterns reflect a staggered costs strategy (Payne 1972; Owen and Krohn 1973; Chilgren 1977; Heitmeyer 1985, 1987; Miller 1986), despite many examples of molt coinciding with other demanding processes (Erskine 1971, Billard and Humphrey 1972, Payne 1972, Foster 1974, Page 1974, Young and Boag 1982, Paulus 1984, DuBowy 1985, Wishart 1985, Austin and Fredrickson 1986, Gruenhagen 1987). The energetic significance of molt is also at issue in its use as an indicator of the physiological status, habitat requirements, and probable fitness of migrants in nonbreeding birds. For example, in winters of low habitat availability molt is delayed in Mallards (Anas platyrhynchos) and Northern Pintails (Anas acuta) (Heitmeyer 1987, Miller 1986). Shortage of food, 543 The Auk 105: 543-552. July 1988 This content downloaded from 157.55.39.104 on Sat, 18 Jun 2016 06:29:28 UTC All use subject to http://about.jstor.org/terms 544 LOVVORN AND BARZEN [Auk, Vol. 105 or perhaps of particular food types (Petersen 1981), may hinder timely completion of molt. This also implies that individuals more capable of acquiring resources may exhibit advanced molt chronology, so that analyses of patterns of nutrient reserves might be improved by including molt status as a covariate (Heitmeyer 1985). The value of molt descriptions in such analyses hinges on whether there are important consequences to retarded molt. Possible disadvantages include delays in nutrient storage, pairing, or migration (Paulus 1984, Heitmeyer 1985, Wishart 1985, Miller 1986), in which molt supposedly slows the progress of sequential events, or forces birds to incur otherwise staggered costs simultaneously. Hence, the energetic significance of molt in the annual cycle should be reflected in the degree to which overlap of molt with other costly events is avoided. We describe the intensity of molt of capital, down, and contour plumages in free-ranging Canvasbacks (Aythya valisineria) throughout winter, spring migration, and breeding. We use these data to examine the applicability of the staggered costs concept to molt in Canvasbacks. STUDY AREAS AND METHODS Canvasbacks were collected by shooting at major concentration areas from late fall through winter, spring migration, and nesting (Table 1; see Bellrose 1980). Canvasbacks begin arriving at Lake Mattamuskeet, North Carolina, during the first week of November, and move from the lake to Pamlico Sound during the first 2 weeks of December. Northward movement begins in late February, and migrants apparently stage in upper Chesapeake Bay until open water allows further migration. Canvasbacks then migrate west across the Great Lakes region to the upper Mississippi River near La Crosse, Wisconsin, where they mix with Canvasbacks migrating north from wintering areas along the Gulf Coast and Mississippi valley. They then move to nesting areas located primarily in the pothole region of North Dakota and the