Call rate, fundamental frequency, and syntax determine male-call attractiveness in blue petrels Halobaena caerulea

In blue petrels (Halobaena caerulea), females are supposed to be particularly choosy and mate choice can take a couple of years. In these lifelong monogamous seabirds, choosing a good mate is crucial and has a strong influence on their fitness. Due to their nocturnal habits, the absence of sexual dimorphism, and the physical barrier between males calling from their burrow and females flying above the colony, vocal signals seem to be one of the main channels for males to communicate with potential mates. In a previous study, we investigated whether acoustic parameters of male calls carry information about morphological characteristics that might be indicators of males' qualities. Here, we experimentally test whether these acoustic parameters linked to male characteristics are actually attractive to females. To do so, we played back modified calls of males to females in a colony of blue petrels of the Kerguelen archipelago. We found that flying females were more attracted by high-pitched calls, and by calls broadcasted at a high call rate. Previous studies showed a relationship between pitch and bill depth and length. In filter-feeding birds, such as blue petrels, bill morphology influences feeding efficiency. A high call rate is an indicator of sexual motivation and makes the caller easier to locate by potential mates and predators in the hubbub of the colony. We thus hypothesized that producing frequent high-pitched calls appeared to be preferable for a conspicuous sexual signaling although it may increase predation risks. Significance statement Mate selection process is largely unknown in burrowing petrels due to their cryptic life at the colony. Here, we examined the implication of vocal signals in mate choice in the blue petrel Halobaena caerulea. We used an experimental setting based on a two-choice test to show that male calls are sexual signals attracting females. As expected, broadcasting male calls attracted females. Despite the apparent stereotypy of male calls, their acoustic parameters transmit pieces of information that may influence females' preference. We found that females are more attracted by high call rate and high-pitched calls. This is the first evidence of the implication and influence of vocal signals in mate choice in burrowing petrels.

Automated summary

Female blue petrels show a preference for high-pitched calls and calls with a high call rate, possibly related to male bill morphology and sexual motivation. Despite the potential increase in predation risks, producing frequent high-pitched calls seems to be favorable for conspicuous sexual signaling in mate selection.