Thomas W. Custer

Egg size and laying order of snowy egrets, great egrets, and black-crowned night-herons

The nesting biology of the family Ardeidae (bitterns, herons, and egrets) has been intensively studied (e.g., Owen 1960, Milstein et al. 1970, Werschkul 1979), but egg size in relation to laying order has not received attention. The last egg laid in gull and tern clutches is generally smaller than preceding eggs (e.g., Parsons 1970, Nisbet 1978). The relative size of the final egg in a clutch decreases with increased body size among bird species and this relationship may be correlated with an increased brood-reduction strategy (Slagsvold et al. 1984). Relative egg size could be an important component to brood reduction, because egg size can affect subsequent survival of young (Parsons 1970, Nisbet 1978, Lundberg and Vaiisdinen 1979). Asynchronous hatching can facilitate size differences among nestlings and assist in brood reduction (Lack 1954, Ricklefs 1965). In herons and egrets, asynchronous hatching results in senior broodmates who are superior to their younger siblings in both food handling and aggressive interactions (Fujioka 1985, Inoue 1985, Mock and Parker 1986). Our objective was to describe egg size in relation to laying order for Great Egrets (Casmerodius albus), Snowy Egrets (Egretta thula), and Black-crowned NightHerons (Nycticorax nycticorax) in a southern Texas colony and Great Egrets in a southern Florida colony. Based on egg-size patterns in other colonial waterbirds and the occurrence of brood reduction in egrets and herons (see above), we predicted that the final egg laid in a clutch would be smaller than those laid earlier.