Polygyny, parentage and parental investment in the corn bunting, Miliaria calandra.

1) This study investigated the costs and benefits of a polygynous mating system, and its effects on reproductive success and parental care in male and female com buntings (Miliaria calandra), on North Uist, Outer Hebrides. In 1989 and 1990, 41.2% of males were monogamously paired, 33.3% polygynous and 23.5% unpaired; the sex ratio was 1:1. Polygynous males usually paired with two females; occasionally three. The distribution of females among males was not significantly different from a binomial distribution. 2) DNA fingerprinting showed that there were only two possible cases of extra-pair paternity (EPP) due to extra-pair copulation (EPC) (4.5% of 44 offspring; 6.7% of broods), both from the same brood. There were no cases of intra-specific brood parasitism (IBP) (0% of 50 offspring; 0% of 16 broods). EPP was probably rare because of mate guarding by the male, asynchrony between broods for polygynous males and limited opportunities for EPC. Polygynous males fledged more offspring from their territories than did monogamous males because they paired with more females; unpaired males fledged no offspring. 3) Unlike most other polygynous birds, primary and secondary females of polygynous male com buntings had similar reproductive success and both had greater reproductive success than monogamous females. Monogamous females had lower reproductive success because their chicks starved more often. Although monogamous females provisioned nestlings at a similar rate to females of polygynous males, monogamous females delivered smaller food loads, which may have led to the reduced success of those nests. 4) Males rarely fed nestlings before they were four days old. Males provided less food for nestlings than did females at all chick ages and, on average, provided a maximum of only 22.0% of all feeds. Unusually for a polygynous species, males provisioned nestlings of monogamous, primary and secondary females with similar sized food loads and at similar rates. 5) Broods belonging to primary and secondary females were apparently equally valuable to their males because EPP was low in this population and brood sizes were similar between nest classes. Males may have been able to feed both nests because they were temporally separate, although at least one male was able to feed overlapping nests simultaneously. Males provisioned proportionately less than females at early stages of the nestling period, possibly because of sex differences in other available reproductive opportunities. Females had no better available option than to provide parental care to the current brood, whereas males could potentially increase their fitness better by defending a territory in which females could breed. 6) Males appeared to defend areas in which females chose to nest rather than to forage. Females often foraged outside male territories. The territories of unpaired, monogamous and polygynous males did not differ significantly in habitat composition. 7) Female corn buntings neither suffered costs to polygyny, nor did they compete for resources, such as male parental care, nest sites or food. Nests were under-dispersed in space, because of habitat aggregation, and were randomly dispersed in time within territories. Primary and secondary females of polygynous males did not choose territories in the same order, and the first settling females of polygynous males did not settle significantly earlier than monogamous females; this suggests that males were chosen randomly, rather than by the quality of their resources. I suggest that low variance of male territory quality facilitated random female choice of males. A no-cost, no-benefit model, with females choosing males randomly is suggested as the best explanation for the maintenance of polygyny in the com bunting.