Male songbirds that sing in a low, "sexy" voice are most likely to snag mates, but noisy human environments are cramping their style, forcing some species to sing shrilly to pierce the auditory urban blight. Now, Dutch scientists have shown that male great tits can overcome this dilemma by pulling out their Barry White impressions at just the right moment.
Previous research had already established that lower-frequency mating songs were perceived to be sexier. In many creatures — including, perhaps, humans — a deep voice correlates with size, fitness and overall masculinity.
Studies had also indicated, however, that some male birds would be forced to sing at a higher-than-ideal frequency to stand out against the low rumble of highways, construction zones and other hallmarks of urbanization. Noise pollution also affects some songbirds' nesting rates, research shows, which means fewer lovebirds shacking up.
For the study, published online Monday by Proceedings of the National Academy of Sciences, researchers tested how the attractiveness of a male bird's voice affected its reproductive success.
The Netherlands-based team tracked the romantic liaisons of great tits, a common songbird in Europe that picks a single mate to build a nest with. The males rise before dawn and sing to their respective females, who — if they like their mates' songs — will leave their nests to copulate.
But the females may canoodle with another male on the sly if their own mates fail to impress.
"Early in the morning, she'll slip out of her nest box, and if she likes another male nearby, she'll sneak off and steal a mating from him," said lead author Wouter Halfwerk, a behavioral ecologist at Leiden University in the Netherlands. "Bird sex is incredibly quick — she can do this and come back within a minute — and [her] male won't even have picked up on it."
The researchers guessed that the males that sang in higher registers were more likely to be cuckolded than the low-throated crooners.
Sure enough, when they conducted paternity tests on the chicks that hatched from the females' eggs, they discovered that about 30% of the males had been cheated on by their mates. By and large, these males sang at a higher frequency, particularly around the time that their females were about to lay eggs and were thus at their most fertile. None the wiser, the cuckolded mates would then help raise another bird's offspring.
In a second part of the experiment, the researchers outfitted nests with microphones and speakers, and then tracked the females as they listened to different recordings of their mates' calls. They also piped in traffic-like noise to see how it affected the females.
With urban noise in the mix, the females responded to their mates' high-pitched calls more often than the lower, sexier ones — probably because they could hear them better.
"So there's this conundrum that the birds face — I'll do this so I can get heard, but I'm not showing the best of myself,'" said Jeffrey Podos, a behavioral ecologist at the University of Massachusetts Amherst who was not involved in the study.
The key, Halfwerk said, is the timing. Males don't have to sing low all the time; they simply have to sing low when their mate is feeling friskiest — which might be a learned behavior.
There was some evidence that the cuckolded males tended to be younger, which could mean that they hadn't yet learned from years of experience — and possible heartbreak — how to figure out when their females wanted them to crank up the bass.