Vocal cooperation between the sexes in Little Spotted Kiwi Apteryx owenii

Abstract

Sexual call dimorphism in birds is usually associated with sexual size dimorphism. Departures from this relationship can be used to infer call function, but research into inter‐sexual call differences, as with song function in general, has been restricted by a bias towards male passerines. The nocturnal and flightless New Zealand kiwi (Apterygidae) are acoustically similar but taxonomically and ecologically very different from other birds, so provide a contrast in exploring avian call function and evolution. However, kiwi acoustic ecology is poorly understood, with the calls of only one of the five kiwi species spectrally described, and acoustic differences between the sexes virtually unknown. We conducted the first bioacoustic study of Little Spotted Kiwi Apteryx owenii, and assessed sexual call dimorphism in this species. There were significant inter‐sexual differences in call temporal and frequency characteristics that were not related to size dimorphism. Contribution to duets and variation in temporal structure with call context also differed between the sexes. We suggest that these differences indicate divergent call function, with male calls more suited for territory defence, and female calls for pair contact. There was a striking lack of overlap in the frequency spectrum distributions of male and female calls, which was also unrelated to size and was further emphasized by the presence of formants in female calls. We propose that this provides evidence for inter‐sexual acoustic cooperation in call frequency, of a type which to our knowledge has not previously been described in birds. This may result from selection for enhanced joint resource defence in kiwi.

Temporal and environmental influences on the vocal behaviour of a nocturnal bird

“…Combined with inter‐sexual differences in call frequency (Digby et al ), duet contribution (Digby et al ), and response to playback (Robertson and Fraser ), the contrast in call rates and first call times support the hypothesis that males LSK take a greater role in territory defence than females. This is counter to hypotheses that song function may be convergent between the sexes and that territory defence is an important driver of female song in monogamous birds holding year‐round territories (Kirschel et al , Tobias et al ).…”

“…However, these field‐based counts were conducted for different populations, and comprise a much smaller sample and considerably less monitoring time. That more males than females were detected may be partly due to male calls being better suited for longer range transmission (Digby et al ). But this cannot explain such a large sex bias, and hence males clearly have a greater requirement to vocalise than females.…”

Gender identification using acoustic analysis in birds without external sexual dimorphism

“…In the Little Spotted Kiwi (Apteryx owenii), loud whistle calls differ substantially between the sexes (Digby et al 2013). The average fundamental frequency of males was 1.5 times higher than those of females (2800 and 1800 Hz, respectively).…”

Kiwis to pewees: the value of studying bird calls

“…In an article published in this issue of Ibis , Digby et al . () describe the calls of the Little Spotted Kiwi Apteryx owenii , providing an overview of call usage patterns and call structure. This article is one of only a handful describing kiwi vocalizations and it is the first to present data on the Little Spotted Kiwi.…”