Influence of Age and Time on Reproductive Performance of Female Lesser Scaup

-I examined the influence of age and time on female reproductive performance in a marked population of Lesser Scaup (Aythya affinis) breeding in southwestern Manitoba from 1977 to 1980. Arrival date and length of prelaying period varied among years but were not related to female age. Rate of nonbreeding and nest-initiation date varied among years and by female age. Age variation in nonbreeding was due to the failure of yearlings and a few 2-yr-olds to breed. Yearlings began laying an average of 5-8 days later than older females. Clutch size increased with female age but showed no significant annual variation despite extreme changes in water conditions. Reproductive performance generally increased with female age and with improving water conditions. These results were consistent with theoretical models that predict that reproductive effort (RE) increases with age or with declining residual reproductive value. My data support Williams' (1966) prediction that organisms adapted to living in variable environments adjust their RE in relation to probability of success. Female Lesser Scaup exhibited a temporally dynamic reproductive strategy, and natural selection seemingly has favored "yes-if" genes (Williams 1966) that effect breeding only when conditions are favorable for the individual. Received 27 April 1983, accepted 3 October 1983. PARENTAL age and breeding experience influence the reproductive performance of many avian species (Lack 1966, Klomp 1970, Ryder 1980). Among waterfowl, several breeding parameters may vary with age, including spring arrival date, nesting chronology, and clutch size (e.g. Sowls 1955, Finney and Cooke 1978, Krapu and Doty 1979, Baillie and Milne 1982, Rockwell et al. 1983). Reproductive performance of waterfowl also varies annually in response to fluctuating environmental conditions in breeding, wintering, and migration areas (e.g. Crissey 1969, Bengtson 1971, Heitmeyer and Fredrickson 1981, Davies and Cooke 1983, Krapu et al. 1983). Female age and prairie drought are important factors influencing reproduction of Lesser Scaup (Aythya affinis; Rogers 1959, 1964; Trauger 1971). Nonbreeding and age at first breeding in this species have been discussed and speculated upon (see Trauger 1971: 87), but specific factors affecting these breeding parameters re1 Present address: Wetland Wildlife Populations and Research Group, Minnesota Department of Natural Resources, 102 23rd Street, Bemidji, Minnesota 56601 USA. main obscure due to insufficient information on known-age individuals. Trauger (1971) reported that productivity, measured by the proportion of returning marked hens that were later observed with broods, increased with female age from 1 to 4 yr. The consistent low productivity of yearling females was attributed to poor nesting success and nonbreeding (Trauger 1971: 78). I collected reproductive information on a marked population of Lesser Scaup breeding in southwestern Manitoba from 1977 to 1980, a period during which wetland conditions varied dramatically. In this paper, I examine the influence of age and time on the reproductive performance of females. Breeding parameters estimated were: (1) arrival date, (2) rate of nonbreeding, (3) length of prelaying period, (4) nest-initiation date, (5) clutch size, (6) nest success, (7) rate of renesting, and (8) brood survival. Results are discussed in relation to predictions of theoretical models concerning agespecific and temporal variation in reproductive effort. STUDY AREA AND METHODS The 777-ha study area was located 3.3 km southeast of Erickson, Manitoba (50'30'N, 99?55'W) and was de255 The Auk 101: 255-265. April 1984 This content downloaded from 207.46.13.131 on Sat, 15 Oct 2016 04:33:03 UTC All use subject to http://about.jstor.org/terms 256 ALAN D. AFTON [Auk, Vol. 101 TABLE 1. Comparisons of selected environmental parameters and spring arrival dates of Lesser Scaup, 19771980.