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Polygamy in the Marsh Harrier, Circus Aeruginosus: Individual Variation in Hunting Performance and Number of Mates

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1. Theories postulating that sexual task differentiation may lead to polygamy such that the sex investing the least effort in raising the offspring, engages in simultaneous matings, contrast with polygyny in raptors where the male provides most of the food for its females and nestlings. A field study was undertaken to describe parental effort and success in marsh harriers of different mating status to elucidate this controversy. 2. Data on clutch size and laying date were collected on 421 nests in two Dutch land reclamations, Flevoland and Lauwersmeer. 156 nests were known to have monogamous parents, 30 males had two females and nests. Bigamous males raised on average twice as many fledglings (5.7) than monogamous males (3.0). However, their primary females had more success (3.5) than secondary females (2.3), related to increased nestling mortality in secondary nests (Table 7). Male fledglings were significantly heavier in primary than in secondary nests (Fig. 16). 3. Nest observations made on 22 nests (5 of monogamous, 17 of polygamous males) revealed that daily prey deliveries by males were fewer in mono- than in bigamous males (Fig. 12). The latter delivered prey by preference to their primary nests (Fig. 14). The prey delivered by a trigamous male were consistently larger than those of a bigamous and monogamous male in the same area (Table 4). 4. Time budget observations revealed that hunting effort was maximal in the nestling phase (ca 8 hrs foraging per day for all three males observed (Fig. 10); at other times of year foraging was reduced in early morning and late afternoon (Fig. 11). Net hunting yield (prey brought to nests per hour of hunting) increased in three males with their number (1, 2, or 3) of mates (Fig. 13). With progress of the breeding season, male hunting ranges extended further outside the breeding territories (Figs. 7, 8, 9) and had a great measure of overlap, suggesting that territory quality was not a major factor in male hunting yield. 5. Secondary females participated in provisioning for the nestlings more than primary or monogamy-females (Fig. 15), thus compensating for reduced male prey deliveries. 6. Classical polygyny theory addresses the question of female choice: which benefits compensate a secondary female for reduced breeding success by mating with an already paired male? Several hypotheses (enhanced offspring survival, offspring genetic quality, parent chances of future reproduction) are discussed, but evidence is nearly completely lacking (ch. 6.1-3). 7. An alternative approach stresses the male's role in the decision process. Males may have more interindividual variation in their capacity to bring food than females in their capacity to lay and incubate eggs. Optimal strategies for males would then range with increasing quality from non-breeding via polyandry and monogamy to polygyny (Fig. 17). In species like harriers, non-breeding may be optimal for yearling males with submaximal hunting skills, thus creating a skewed sex ratio forcing some females to accept secondary status as mate of older, high quality males. Polygyny is then associated with slower male than female maturation. The evolution of polyandrous traits in species living isolated in poor environments is likewise explained by this model.

Affiliations: 1: Zoölogisch Laboratorium, Rijksuniversiteit Groningen, Haren; 2: Rijksdienst voor de IJsselmeerpolders, Lelystad, The Netherlands

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