Male-male associations in spotted bowerbirds (Ptilonorhynchus maculatus) exhibit attributes of courtship coalitions

Abstract

Abstract: Despite strong selective pressures inherent in competition for mates, in species with non-resource-based mating systems males commonly engage in non-agonistic interactions with same-sex visitors at display arenas. Bowerbirds perform courtship dances on elaborate display structures - known as bowers - that are built and defended by one resident male. Several reports have suggested that bower owners tolerate the presence of specific male visitors at their display arenas, referred to here as 'subordinates'. Subordinate males may learn the skills required for successful sexual signalling via prolonged social interactions at adults' arenas, but little is known about whether courtship proficiency changes with experience and/or whether subordinates actively contribute to enhancing the resident male's mating success. In this study, we investigated male-male associations in wild spotted bowerbirds (Ptilonorhynchus maculatus). We first sought to determine whether courtship behaviour differs based on bower ownership status. We then examined whether social interactions between bower owners and subordinate males may qualify as courtship coalitions. Our analysis of courtship postural components did not reveal differences in timing or relative occurrence of postural components between subordinate males and bower owners, whereas we found evidence that male-male associations in spotted bowerbirds may provide an example of rudimentary courtship coalitions. In particular, higher subordinate attendance is associated with lower destruction rates by neighbouring rivals and with overall higher mating success, and male pairs are stable in subsequent years. This study provides novel information about social dynamics among male bowerbirds, and further insights into the evolution of coalitionary behaviour in male displays.

Significance statement: Same-sex associations between established males and subordinate visitors on display arenas are common in birds, yet poorly understood. Using video recordings from a population of wild spotted bowerbirds, Ptilonorhynchus maculatus, we performed a quantitative analysis on motor courtship components across males, and on their social interactions on display arenas to investigate the nature of male-male partnerships. Our results showed that motor courtship performance in subordinate visitors is not suggestive of an early ontogenetic stage, as previously speculated. Moreover, though bower 'owners' and subordinate males do not coordinate their behaviour during courtship or bower building, male-male partnerships may qualify as a rudimentary or incipient form of courtship coalitions. Subordinate males are tolerated at bowers, the magnitude of subordinate attendance correlates with owner males' mating success, and repeated interactions between individuals reveal consistent partner associations.

Supplementary information: The online version contains supplementary material available at 10.1007/s00265-022-03200-x.

Keywords: Coalitions; Courtship behaviour; Courtship learning; Ptilonorhynchidae; Same-sex associations.

Fig. 1

a Display arena of spotted bowerbird with pile of sun-bleached snail shells in the foreground and avenue-shaped bower in the background. Males arrange various decorations within the avenue walls and at the bower entrances. b A visitor positioning itself parallel to the bower walls while watching a displaying male. c Schematic depiction of a typical display sequence. Display bouts are defined here as courtship sequences separated by less than 10 s (top). We differentiate between different display segments depending on the position of the receiver during a display bout (middle). See text for definitions. During a display bout, males perform a repertoire of up to 19 stereotyped display elements (Supplementary Table S1) separated by intervals of variable length (bottom)

Fig. 2

Six visual display elements exhibited by male spotted bowerbirds during courtship. a Leap. b Crest presentation. c Wing flick, in the two-wings variant (left) and single-wing variant (right). d Rising. e Nodding. f ‘Wings drooping’ strut. Other visual display elements are depicted in Warham (1962) and Frith and Frith (2004)

Fig. 3

Comparisons between males with different ownership status (white = subordinate males; grey = bower owners) and in different display segments. a Comparison between the proportions of peripheral, central static and central dynamic display elements calculate for each male in different display segments; N = 66 observation, 22 birds. b Comparison between the durations of intervals calculated for each male in different display segments; N = 27,705 observations, 22 birds. c Comparison between proportions of display elements performed with a decoration held in the beak by each male; N = 138 observations, 22 birds. Box plots show median (black horizontal line), 25% and 75% quartiles, upper and lower values within 1.5 inter-quartile range (whiskers), and extreme values above 1.5 inter-quartile range (outliers); stars on top of horizontal bars depict significance levels: ***p < 0.001)

Fig. 4

Proportions of attendance of bower owners, subordinate males and unbanded birds as a function of date. Proportions of attendance are total proportions of time per total video recording time per bower, calculated for each category of bird per day. Data are shown separately for 2018 and 2019. Regression lines are shown with 95% confidence intervals (grey shading). N = 3120 observations, 1226 days-bower

Fig. 5

Effect of subordinate attendance on number of marauding events (left; N = 28 bowers-year) and on the number of copulations per bower (right; N = 14 bowers-year). Red lines depict the fitted model and grey areas depict 95% confidence intervals. Camera recording time is controlled for in both models by including this variable as an offset term (see ‘Methods’)