Intruders and Defenders on Avian Breeding Territories: The Effect of Sperm Competition

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Abstract
Trespassing and defence on bird breeding territories can be due to males seeking extra-pair copulations with fertile females (the sperm competition hypothesis), to non-territory holders prospecting for potential vacancies (the non-territory holder hypothesis) or to conspecifics unaware of territory boundaries or seeking superior territories during the territory establishment phase of the breeding season (the territory establishment hypothesis). Nine predictions on the variation in trespassing on territories in relation to season, time of day, participating sex, spatial origin of intruders, social dispersion, number of broods, nest predation rate, breeding synchrony, and breeding habitat together fully separate the three alternative hypotheses. Existing evidence to a large extent supports the sperm competition hypothesis.