Ardea
Official journal of the Netherlands Ornithologists' Union

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van Rhijn J. & Groothuis T. (1985) Biparental care and the basis for alternative bond-types among gulls, with special reference to Black-headed Gulls. ARDEA 73 (2): 159-174
Unusual breeding bond-types, such as polygyny and male-male pairing were very common among Black-headed Gulls held in aviaries. Such associations seemed to be rare among Black-headed Gulls in the field (3.9). To investigate the factors responsible for the occurrence of the different bond-types, we analysed all variants of parental care which could be observed. Most individuals participating in unusual bond-types also appeared to participate in the normal monogamous bond-type during other seasons (3.1). Yet, the usual bond-type was not very successful amongst the experimental gulls. Forced extra-pair copulations occurred between birds breeding in each other's vicinity, and were directed to females which were in the phase of egg-laying (3.3). The only case of a female helping a monogamous pair resulted in a polygynous association in the following season (3.4). The strongly competing females in polygynous mating units laid in the same nest or in different, but adjacent ones. Polygynous males only cared for one nest (3.5). Female-female pairing was not observed. Male-male pairing could lead to nest-building and even to incubation and care for chicks when the nest was supplied with fertile eggs (3.6). Brood-parasitism was shown by one female, which also appeared to lay 'attractive' eggs for incubation (3.7). Adoption of eggs was easily accomplished. Adoption of chicks was feasible too, but only when they were still young. It was repeatedly observed between families on neighbouring nests (3.8). These different phenomena and those observed among various species of wild gulls, cannot sufficiently be explained by assuming that all bond-types are adaptive strategies which have been perfected by natural selection. Neither the idea that particular bond-types are bound to particular genotypes, nor that each individual has the disposal of a set of adaptive responses to various ecological conditions, offers a satisfactory solution. Consequently, the explanation must mainly bear upon constraints of proximate mechanisms for the formation of monogamous pair bonds.


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