Inter- and intraspecific relationships between egg size and clutch size in waterfowl

--Lack (1967, 1968a) proposed that clutch size of waterfowl and other birds with self-feeding young was limited by females' ability to produce eggs. Lack supported this eggproduction hypothesis by showing a strong inverse relationship between egg size and clutch size within and among species of waterfowl. A reanalysis using updated data and more appropriate statistics failed to confirm Lack's results. Grouping all the waterfowl produced a weak (r = = 0.13) inverse relationship between relative egg size and relative clutch size. This relationship was due mainly to a handful of ducks that nest on oceanic islands. Analyses by tribes showed that relative egg size and relative clutch size were inversely related in only 2 of the 8 major tribes of waterfowl. Finally, intraspecific analyses failed to reveal a trade-off between egg size and clutch size in Blue-winged Teal (Anas discors) and Northern Shovelers (A. clypeata). Similar intraspecific analyses for 12 other waterfowl have failed to show the predicted inverse relationship between egg size and clutch size. These results suggest hat the widely accepted egg-production hypothesis may be considerably overemphasized. Received 19 December 1986, accepted 13 July 1987. A CENTRAL question concerning life-history adaptation is how many young to have in any breeding event. Field studies of this problem have largely dealt with birds. Much of this literature was inspired by the work of David Lack (1947, 1948, 1954a, 1968a). Lack's thesis was that clutch size in most birds has evolved to correspond to the maximum number of young the parents can feed. Lack suggested that females that laid larger than normal clutches would leave fewer descendants because the brood would be undernourished and suffer greater nestling or fledgling mortality. Lack's (1954a, 1968a) conclusions that parents' ability to feed young is more likely to constrain clutch size than their ability to lay or incubate eggs are widely accepted (Klomp 1970, Ricklefs 1977, HiSgstedt 1980). Waterfowl (Anatidae) have highly precocious young that leave the nest shortly after hatching and secure their own food. Parental duties consist of leading the brood to feeding areas, warming chilled young, watching for predators, and, in the larger species, defending the brood from predators. With such forms of parental care, it seems unlikely that survival of young would be affected by brood size. Manipulations of brood size in Blue-winged Teal (Anas discors) • Present address: Appalachian Environmental Laboratory, Gunter Hall, University of Maryland, Frostburg, Maryland 21532 USA. and Canada Geese (Branta canadensis) have shown no relationship between survival of young and brood size (Rohwer 1985, Lessells 1986). Brood size alterations brought about by intraspecific nest parasitism also failed to affect duckling survival in Wood Ducks (Aix sponsa) and Common Goldeneyes (Bucephala clangula) (Heusmann 1972, Clawson et al. 1979, Rothbart 1979, Dow and Fredga 1984; but see Andersson and Eriksson 1982). Likewise, clutches enlarged either experimentally or through intraspecific nest parasitism have shown ducks to be capable of hatching greatly enlarged clutches with little or no reduction in the percentage of eggs that hatch (Leopold 1951, Hori 1969, Morse and Wight 1969, Heusmann 1972, Clawson et al. 1979, Eriksson 1979, Dow and Fredga 1984, Rohwer 1985). Waterfowl lay large eggs relative to their body size (Lack 1968a, King 1973, Rahn et al. 1975), and they lay large clutches (Johnsgard 1978, Bellrose 1980). In many species the total clutch mass approaches the mass of the female (Appendix). Such a large commitment to egg nutrients suggests that the production of eggs could constrain reproductive output. Lack (1967) proposed that "the average clutch of each species (of waterfowl) has been evolved in relation to the average availability of food for the female around the time of laying, modified by the relative size of the egg. " Lack suggested that species laying eggs that were small relative to their 161 The Auk 105: 161-176. January 1988 162 FRANK C. ROHWER [Auk, Vol. 105 body size would be able to lay many eggs, whereas species laying large eggs would lay fewer eggs. As a test of this hypothesis Lack (1967, 1968a) related egg size to clutch size, and concluded that the two were inversely related. Lack's description of a trade-off between egg size and clutch size has been widely accepted as strong support for the hypothesis that clutch size in waterfowl is limited by egg production. Lack, however, employed an inappropriate correction for the allometry of egg size to body size, made relatively arbitrary categories of egg sizes in his analyses, and was forced to use questionable data for some species (Lack 1968a: appendix 15). I re-examined the relationship between egg size and clutch size on interand intraspecific levels in an effort to reassess the hypothesis that egg production limits clutch size in waterfowl.