Effects of body weight and age on the time and pairing of American black ducks

-I used captive young and adult American Black Ducks (Anas rubripes) during October-February 1984-1985 to test whether body weight and age affected time of pair-bond formation. Eighty ducks were marked individually, and 10 ducks (6 males and 4 females, half of each age class) were assigned to each of 8 experimental pens. Ducks in 4 pens received an ad libitum diet of commercial duck food, and ducks in the other 4 pens received a restricted ration of the same food. During early winter ducks in both groups gained weight, but ducks on the restricted diet gained less than birds on the ad libitum diet; peak winter weight of ducks on the ad libitum diet averaged 22% greater than initial body weight compared with 6.5% for ducks on the restricted diet. In late winter ducks on the restricted diet lost 28.7% of peak winter weight, and ducks on the ad libitum diet lost 19.3%. Weight loss of ducks on the ad libitum diet began before weather conditions became severe and coincided with a reduction in food consumption. This result supports the idea that weight loss of waterfowl in late winter is controlled endogenously. Individuals on the ad libitum diet paired earlier than those on the restricted diet, and pair bonds were stronger. Adults of both sexes paired earlier than young ducks, but differences for females were not significant statistically. Age and energy constraints are factors that can affect intraspecific variation in pairing chronology. Received 8 July 1985, accepted 22 December 1985. FORMATION of pair bonds by North American ducks (Anatidae) generally occurs before birds arrive at spring breeding areas (Palmer 1976, Bellrose 1978). Courtship behavior and pairing chronology in fall and winter have been reported for many species (e.g. Weller 1965, Smith 1968, Soutiere et al. 1972, Armbruster 1982, Hepp and Hair 1983, Paulus 1983). These data indicate that time of pairing can vary both within and among species. Dabbling ducks (Anas spp.), for example, generally initiate courtship and establish pair bonds earlier than diving ducks (Aythya spp.). Many factors have been proposed to explain variation in time of pairing. Young individuals may pair later than adults (McKinney 1965), and age-related parameters such as time of alternate plumage development (Weller 1965, McKinney 1970) and intensity (Bossema and Kruijt 1982) or form (Dane and Van der Kloot 1964, Korschgen and Fredrickson 1976) of reproductive displays may affect time of pairing within species. Delays in courtship and pair' Present address: Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 29801 USA. bond formation also may be related to energetic constraints, resulting in intraspecific differences in pairing chronology (Wishart 1983, Brodsky and Weatherhead 1985). Temporal variation in pairing among species may be caused by differences in time of nesting (Weller 1965, Armbruster 1982), foraging strategies (Paulus 1983), and male-male competition (Spurr and Milne 1976). Compared with unpaired conspecifics, paired ducks are dominant (Paulus 1983, Hepp and Hair 1984) and thus occupy more favorable winter habitats in which thermoregulatory costs may be lower (Jorde et al. 1984). Further, paired females improve their foraging efficiency by having a male in attendance (Ashcroft 1976). Because these benefits may help in accumulating nutrient reserves, time of pairing may be related to subsequent reproductive success (Hepp 1984) and survival. If the benefits of pairing result in higher probabilities of reproducing or surviving, then it is not clear why some individuals and species delay pair-bond formation. Pairing involves costs and benefits that may vary intraand interspecifically. For example, high dominance rank may be important only if resources, such as food, are defen477 The Auk 103: 477-484. July 1986 This content downloaded from 207.46.13.51 on Sun, 19 Jun 2016 07:17:38 UTC All use subject to http://about.jstor.org/terms 478 GARY R. HEPP [Auk, Vol. 103 TABLE 1. Amount of food (g.bird-l -day-l) in the ad libitum and restricted diets and average daily temperature, 1984-1985. Values are means ? SE. Time periods are the intervals between consecutive dates on which American Black Ducks were weighed. Restricted diet Daily temperature Time period na Ad libitum diet (% of ad libitum diet) (OC)b 3-19 Oct 6 119.6 ? 12.0 116.4 ? 2.8 (97) 16.1 ? 0.5 20 Oct-1 Nov 11 111.5 ? 6.0 81.6 ? 3.1 (73) 18.6 ? 1.0 2-15 Nov 13 140.8 ? 6.3 110.8 ? 4.3 (79) 8.7 ? 0.9 16 Nov-5 Dec 18 132.5 ? 4.7 85.5 ? 7.3 (64) 4.6 ? 0.7 6-19 Dec 12 116.0 ? 9.6 40.4 ? 6.2 (35) 7.0 ? 1.4 20 Dec-14 Jan 25 97.6 ? 6.7 32.9 ? 1.4 (34) 4.2 ? 1.2 15-28 Jan 13 54.2 ? 6.0 71.8 ? 2.0 (132) -4.1 ? 1.3 29 Jan-11 Feb 13 80.6 ? 6.4 63.5 ? 1.2 (79) -1.1 ? 0.8 12-28 Feb 16 108.0 ? 7.7 54.7 ? 2.0 (51) 6.9 ? 1.4 a Number of days between weighings that were used in calculating mean ad libitum and restricted diets. bValues are means of the average daily temperatures [(maximum + minimum)/2] from nearby BaltimoreWashington International Airport (39011'N, 76?40'W) and include every day in the time period. sible. Variation in the economics of pairing, therefore, may produce temporal differences in pair-bond formation. An interesting area of research concerns the factors that affect trade-offs between costs and benefits of pairing, and that may ultimately determine the pairing chronology of waterfowl. In this study using young and adult American Black Ducks (Anas rubripes), I manipulated food availability, and therefore weight (i.e. nutrient reserves), during fall and winter to test the effects of age and body weight on time of pair-bond formation.