Laying Dates, Clutch Size and Egg Weight of Captive Mallards

Factors affecting clutch or litter size have interested ecologists because they relate directly to reproductive rates and help to explain reproductive habits.Clutch size can be measured directly with little error, and the possible functional relationships with environmental or social variables can be examined.This has led to several major reviews on the possible determinants of clutch size in birds (Cody 1971, Klomp 1970, von Haartman 1971) .Waterfowl have posed particular problems because the chicks are precocial and do not require parental feeding, a factor that correlates with clutch size in many altricial species (Lack 1968a).Most hypotheses concerning the evolution of clutch size in waterfowl have implied that food supply for the female prior to laying and/or for the young is both a proximate and an ultimate determinant (Bengston 1971, Johnsgard 1973, Lack 1967, 1968a, 196813, Ryder 1970).We report the results of an experiment in which single pairs of Mallards (Anas p&yrhynchos) were held in identical breeding compartments and were thereby subjected to the same environmental cues for breeding.We determined laying dates, clutch sizes and egg weights for the same pairs of birds in 1973, 1974 and 1975.The responses of the birds in terms of the three parameters of nesting were considered to closely reflect interactions between genotype and environment.They should provide bases for evaluating proximate environmental effects in wild populations. METHODSStandard procedures for maintenance of captive birds were used throughout (Ward and Batt 1973).During the first week of April 1973, 1974 and 1975, up to 80 randomly assigned pairs of Mallards were randomlv placed in individual breeding nens at the Delta Waterfowl Research Station in Manitoba, Canada.The same members of a pair were kept together for the three years.The pens were 7 m' ; one-half swimming water and one-half dry concrete floor.Each contained a nest box and a feed dish.The birds were kept flightless by clipping or pinioning.Compartments were separated by 0.7-m walls that prevented physical and visual contact between pairs but allowed them to hear each other.Food was supplied freely.Birds used in the study were all reared from eggs taken from wild nests prior to 1973.In 1973, one-half were yearlings while the remainder were a mixture of two-to four-year-olds.An earlier analysis failed to detect significant age-related effects on the three nesting parameters under discussion here (Batt and Prince 1978), so all the birds are treated as "equals" for our analyses.During daily visits to each compartment, we recorded the occurrence of new eggs and egg weight.New eggs were numbered, returned to the nest and left until the the female had completed the clutch and incubated it for five days.Nests complete' d and incubated in a normal manner are designated as "good" nests.Deserted nests were also left until laying had not occurred for five consecutive days.In each case the eggs were then collected, all the nesting material was replaced and the females allowed to renest.This routine continued until all females ceased nesting and began their me-basic molt. Data analyses followedNie et al. (1975), Sokal and Rohlf ( 1969).and Becker ( 1967 ).For statistical I I , , significance, a 0.05 probability level was used unless otherwise stated.We estimated repeatability for the three reproductive parameters to evaluate the role of the individual genotypic breeding response.Repeatability is the ratio of variance among individuals to the total phenotypic variance (among and within individuals) and can take values from 0 to 1. Thus, it is not a measure of product-moment correlation but is analogous to the coefficient of determination (Kempthorne 1969:230). RESULTS AND DISCUSSION LAYING DATESThe earliest and latest dates of starting first nests were separated by 58, 50 and 43 days in 1973, 1974 and 1975, respectively (Fig. 1).The birds were held in the same breeding pen for the three years, diet was not changed, and the same birds were mated together each year.Therefore, annual variation was assumed to be primarily the result of climatic differences between years and the experience of the birds themselves.A one-way analysis of variance indicated significant differences between females for the three years.The repeatability of an individual' s nesting date was determined to be 0.57 (SE = 0.07, N of females = 60).This suggests that individual females are consistent and that within the population there is a wide range of individual phenotypic responses.In the wild, one would expect to find some pairs with a full commitment to nesting while others would be paired but show no breeding behavior.This may explain why some investigators have had dif-E351 Condor, 81