Energy Expenditure in Reproductive Effort of Male and Female Killdeer (Charadrius vociferus)

-The reproductive investment strategies of the sexes during the breeding season are detailed for Killdeer (Charadrius vociferus), a monogamous plover. I measured the energy investments of the sexes in reproductive, mating, and parental effort. As predicted, males expend more mating effort than females; however, the sexes expend equal amounts of parental effort. Total energy expenditure in reproductive effort (mating and parental effort) during a successful nesting attempt was also equal for the sexes. However, early parental effort expenditures by females, early mating effort expenditures by males, and high rates of nest failure combine to result in female reproductive energy expenditures being significantly higher over the breeding season. This suggests that energy expenditure alone is not adequate for accurate comparisons of the relative investments of the sexes. Studies investigating male and female investments need to consider the degree and pattern of nest failures along with patterns of energy expenditure. The advantages to male and female Killdeer of sharing parental care is demonstrated using adult removal experiments. In general, a deserted parent expends more energy in parental effort than a bi-parental parent and has significantly lower reproductive success. However, males are able to hatch chicks, whereas females lose or abandon their nests within a few days of mate removal. Thus, monogamy in Killdeer appears to result from high nest failure rates, the necessity of two parents for any reproductive success, and the generalizable nature of Killdeer parental care. Received 30 October 1987, accepted 11 April 1988. ALL activities performed by an individual can be classified into either somatic effort, i.e. the time, energy, and risks involved in growth and maintenance activities, or reproductive effort, i.e. the time and energy (Fisher 1958), and risks (Williams 1966, Trivers 1972, Hirschfield and Tinkle 1975, Brunton 1986) incurred in reproduction. Reproductive effort can be divided further into mating effort, activities used to secure copulations, and parental effort, the sum of parental investments in each offspring (Trivers 1972) for a given period (Low 1978, Alexander and Borgia 1979). An important difference between mating and parental effort is that parental effort is tied to specific offspring whereas mating effort is more general (Low 1978, Alexander and Borgia 1979). Females are able to maintain greater control over their expenditures in parental effort (e.g. egg size, clutch size and number of clutches produced) and benefits from parental effort tend I Present address: Department of Biology, Yale University, OML, P.O. Box 6666, New Haven, Connecticut 06511-7444 USA. to be greater for females. Males have less control and greater uncertainty of paternity, thus, potentially lower benefits from parental effort. Also, sperm are smaller than eggs and represent a much smaller proportion of an individual's total reproductive potential. The resulting predictions are that females will tend to invest more than males in the parental effort, including egg production, for any given clutch and males will tend to invest more than females in mating effort. I compared the energy expenditure strategies of male and female Killdeer (Charadrius vociferus) in relation to the adaptive value of bi-parental care and monogamy for shorebirds. Killdeer are common North American shorebirds that inhabit a wide variety of open-land habitats. Killdeer are monomorphic, and both sexes are equally capable of providing care for the precocial young (Furniss 1933, Bunni 1959, Phillips 1972, Lenington 1980, Mundahl 1982, Brunton 1988). Chicks leave the nest within 24 h of hatching and are never fed by their parents. Parents lead chicks to feeding areas where both feed on small invertebrates (Furniss 1933, Bunni 1959). Killdeer are multiple brooding; pairs 553 The Auk 105: 553-564. July 1988 This content downloaded from 157.55.39.18 on Thu, 12 May 2016 05:44:12 UTC All use subject to http://about.jstor.org/terms 554 DIANNE H. BRUNTON [Auk, Vol. 105 remain together throughout the breeding season, and renest after chicks fledge or a nest fails (Brunton 1987). Desertion of second clutches by females has been reported among some populations (Bunni 1959, Lenington 1980, Mundahl 1982), but is absent in others (Brunton 1987). Few studies have measured the advantages of monogamy by rigorous examination and comparison of male and female reproductive strategies. To measure reproductive effort over the wide range of reproductive activities and the problem of expressing these activities in a common currency is difficult. However, for the different activities involved in reproduction, comparisons can be made between males and females. I tested 4 predictions: First, the pattern of energy expenditure over a nesting attempt will be different for males and females, with females investing early in egg production. Second, males expend more energy in mating effort and females expend more energy in parental effort. Third, males and females spend similar amounts of energy in reproductive effort (Alexander and Borgia 1979). Fourth, mate removal will result in increased energy expenditure by the "deserted" parent. Predictions were tested by detailing the daily activities of Killdeer and comparing the relative energy expenditures of the sexes during each stage of the nesting cycle. Cumulative energy expenditures of the sexes were compared during a single nesting attempt and over the entire breeding season. To assess the possible advantages of bi-parental care, I removed adults and compared the energy expenditures of deserted birds with those of bi-parental birds.