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Secret sexual liaisons explain mystery of night-singing birds

By Karl Gruber

8 July 2016

Dimly lit shot of field sparrow with beak open

Singing for its supper? Perhaps not

Glenn Bartley/Getty

For most birds the night brings a well-deserved rest. But for some, it is time for more risqué activities.

Nocturnal birds sing at night – no surprises there – mainly to attract mates or repel rivals, the same reasons other birds sing at daytime.

But a small number of species active by day also occasionally sing at night. Why they invest time and energy in such behaviour has been something of a mystery.

Now Antonio Celis-Murillo at the Illinois Natural History Survey in Champaign and his colleagues think they have an answer – and it wasn’t what they expected.

The team spent two years studying field sparrows, Spizella pusilla, a common bird across eastern North America. Active during the day, these birds are territorial and largely monogamous, though they engage in occasional infidelity.

The researchers observed 28 pairs in the wild, recording the songs of territorial males, as well as those of intruder and neighbouring males. They then conducted playback experiments at night, studying the responses of the pairs.

“I was surprised to see what these birds were up to,” says Celis-Murillo. The males sing to attract other male’s partners, and these females are all too willing to wake up for a night-time rendezvous.

The team also found that males sang more during periods when females were reproductively receptive, and that the females responded to such song more often when they were fertile.

The female’s mate didn’t appear to kick up a fuss and counter-sing – which would be expected if nocturnal songs served to repel rivals.

In fact, says Celis-Murillo, if the male wasn’t already out singing at some other female, it remained asleep throughout the playback experiment, while the female responded earnestly.

Short and sweet

Night-time songs were also different from the daytime repertoire, when males perform a simple one to attract a mate or a complex one to deter rivals. They sing these songs over and over during the day. But at night, males sing infrequently and very briefly.

“These short songs are used by mated males and appear to be a way to attract additional females,” says Celis-Murillo. “Instead of loudly calling for females, they say, very discreetly, ‘Hey, I’m here… Do you want to mate with me?’ They sing perhaps hoping that there is a female just around the corner.

“It seems like some birds, despite being active primarily during the day and being mated, have a busy nightlife and waste no time in trying to find additional females willing to cheat on their partners.”

“We’ve known for some time that in many songbirds females are going off of their mate’s territory to seek copulations with neighbours,” says Scott MacDougall-Shackleton of the Advanced Facility for Avian Research at the University of Western Ontario, Canada. “The exciting finding of this new study is that it suggests that nocturnal song may be a signal to coordinate these extra-pair matings.”

Celis-Murillo now plans to test just how much cheating females and males do, by looking at the genetic make-up of their offspring. This will help disentangle just who is mating with who.

Read more: How birds of paradise find mates, by David Attenborough

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