WHY DO MALE BIRDS STOP COPULATING WHILE THEIR PARTNERS ARE STILL FERTILE

In most monogamous birds copulation frequency peaks a few days before the start of egg laying and then either ceases or drops markedly. This decline occurs despite the fact that females remain fertile until the day their penultimate egg is laid. Several hypotheses to explain the decline in copulation frequency following the onset of egg laying were tested, using data from different species. There was no evidence that the decline was a consequence of (1) one or other partner being involved in incubation and hence unavailable to copulate, (2) male partners being unavailable because they were seeking extra-pair copulations, (3) there being fewer eggs to fertilize as egg laying proceeds, (4) offspring from late hatched eggs being of lower value, or (5) copulations being costly in terms of the risks of damage to unlaid eggs. There were insufficient data to test (6) the egg-predation hypothesis: That for species that copulate at or near their nest those vulnerable to egg predation should cease copulation relatively early in their cycle. The hypothesis (7) that copulations were less efficient once egg laying has started was supported, as was (8) the sperm competition hypothesis across different mating systems. Where there is little sperm competition, as in lekking species, copulation ceases very early, but with intense sperm competition, as in simultaneously polyandrous species, copulations continue until the end of the female's fertile period. In polyandrous species continued copulation may benefit both sexes: Males because it increases their probability of fathering offspring, and females because by continuing to copulate they obtain material benefits (paternal care) from their male partners. With intermediate sperm competition (monogamy) copulation shows an intermediate pattern, but within monogamous birds factors, as yet unidentified, affect the temporal pattern of copulation. In most birds copulation frequency is controlled by the female: A female requires only a single insemination to fertilize her eggs. Male interests differ: Because of last male sperm precedence, males benefit from more frequent copulation and over a longer period to ensure their paternity. Within species this conflict between the sexes is evident from the temporal pattern of female- and male-initiated copulations. © 1993 Academic Press, Inc.