supernormal

African Birdlife|May - June 2020

supernormal
Clutches in Hartlaub’s Gulls
PETER RYAN

Most gulls usually lay two or three eggs, but clutches of four to six eggs have been recorded in many species, including the Kelp Gull (Ryan et al. 2013, Ostrich 84: 157–160). Such ‘supernormal’ clutches typically occur as a result of female-female pairs, although they might also result from two females mating with one male. Experimental removal of males from populations of California Larus californicus and Ring-billed L. delawarensis gulls confirm that supernormal clutches occur when there is a shortage of adult males.

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May - June 2020