Abstract
Reproductive investment is typically considered in terms of size and number of propagules produced. Compared with a thorough understanding of the overall patterns and ecological correlates of avian clutch size, egg size has received less attention and the total effort invested in laying a clutch of eggs is rarely considered. We used clutch volume as an alternative estimate of reproductive investment and present the first class-level analysis of clutch volume in birds using 1,364 randomly-selected species in 204 families. The relationship between body mass and egg volume was very strong (r2 = 0.946), validating previous studies identifying four families (Apterygidae, Pelecanoidiididae, Sternidae and Dromadidiae) with disproportionately large eggs. Clutch volume was also closely related to body mass (r2 = 0.909) and all but one of the taxa with disproportionately large eggs conformed to the overall relationship, their greater egg dimensions compensated by diminished clutch size. The only family which departed significantly from the relationship between body mass and clutch volume was the mound builders (Megapodiidae)—the only group of birds that do not rely on body heat for incubation. Although previously known for laying large clutches of large eggs containing disproportionately large yolks, the remarkable investment of megapodes in reproduction (more than seven times greater than other birds of comparable mass) has been hitherto overlooked. We consider the evolutionary basis and ecological implications of this finding, suggesting that energetic costs associated with incubation act as an upper limit on reproductive output of other birds. We recommend clutch volume as a sensitive, fine-grained measure of reproductive effort for research at a wide range of scales and advocate further analysis of ecological correlates of clutch volume in birds and amniotes generally.
Highlights
Having evaluated the strength and shape of the relationship between body mass and clutch volume, we identify those groups that diverge from the overall allometric relationship and compare them with those taxa deemed exceptional in previous work based on egg dimensions
Body mass and clutch volume ranged over four orders of magnitude, from the 2.6 g Doricha enicura with a clutch volume of 0.74 ml to the 83,500 g Struthio camelus with a clutch volume of 9,700 ml
We confirmed the very close relationship between egg volume and body mass across the class Aves, with body mass explaining about 95% of variability in egg volume at the family scale (79–90.6% after correcting for phylogeny)
Summary
Several studies have used this variable for research on single species or within a family [19], [20] but few higher-level comparisons of clutch volume have been made (e.g., [17], [21]) and, to-date, the nature and extent of variation in clutch volume across all birds has escaped attention. We stress the importance of random taxon sampling when conducting higher-level comparisons and urge ornithologists to consider reproductive investment in a more holistic manner than the number of eggs laid. Using these data, we have quantified the relationship between clutch volume and various life history traits, comparing emergent relationships with previous findings regarding correlates of clutch size, but these findings will be presented elsewhere
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