Spacing and distribution of bowers in Macgregor's Bowerbird (Amblyornis macgregoriae)

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Bowerbirds (Ptilonorhynchidae) have been described as exhibiting exploded lek mating patterns. Quantitative documentation and analysis of spatial dispersion, however, is lacking for most species despite its importance to a fundamental understanding of social organization. We studied Macgregor's Bowerbird (Amblyornis macgregoriae) in eastern New Guinea in 1980 and 1981 to quantify spatial distribution and selection of bower sites in relation to ecological and social factors. Forty-two bower sites were located in approximately 750 ha of mid-montane forest. They were linearly and regularly spaced along ridges with a mean inter-bower distance of 169±64 m SD (range 75–348 m). Statistical analysis showed males to be regularly spaced within available habitat. Discriminant analysis of seven habitat variables measured along utilized ridge lines showed that bower sites differed significantly from potentially available sites with respect to slope and width of ridge, closure of canopy, and density of saplings. Placement of bowers with respect to these factors was not correlated with inter-bower distance. Most males maintained only one bower; four (10%) maintained two bowers simultaneously. Males spent an average of 54% of daylight hours within an activity center of 15–20 m radius surrounding the bower and aggressively defended this area against conspecific males. Marauding pressure on bowers was high, and 39% of all observed mating attempts (n=18) were disrupted by neighboring males. Macgregor's Bowerbird exhibits a social system that appears to be intermediate between lek behavior and territoriality, combining the social dynamics and opportunism of the former and the uniform field of display sites and nests of the latter. We hypothesize that dispersion in this species is partly due to males buffering their display space against intruder pressure. We suggest that strict adherence to the dispersion criterion in classifying promiscuous mating systems may be inappropriate.