Most people will agree that a nicely performed song by a guy hoping to impress a girl will go a long way. This is particularly true in the world of birds, where singing is one of the main ways males try to seduce females. Interestingly, many songbirds focus their serenading efforts at dawn, which is usually the coldest time of day. But why would they want to do this? Singing at dawn represents a thermal challenge: metabolic rate increases in the cold and therefore the task of singing becomes more energetically costly. But perhaps that is exactly why they do it, to impress females with their cold tolerance and dazzling performance. If so, then female birds would find songs sung in the cold sexier than songs sung in warm weather. To test this hypothesis, Michaël Beaulieu and Keith W. Sockman from the University of North Carolina, USA, studied the songs of Lincoln's sparrows at different temperatures.

Lincoln's sparrows are common throughout Canada and parts of the USA. The average temperature during their song chorus is 7.8°C, well below the temperature (23°C) beneath which metabolic rates rise above basal levels. To determine whether female Lincoln's sparrows are more attracted to songs performed at cold temperatures than songs performed in warm conditions, Beaulieu and Sockman divided females into two groups, and played them songs performed by two males. First, they exposed one group of females to one male's songs at 16°C and played the second male's songs at 1°C, recording how long the females spent next to the loudspeaker. However, they had to be sure that the females weren't simply hanging out by the speaker because they preferred that male's recital, so they reversed the temperatures when they played the songs to the second group of females, playing the first set at 1°C and the second set at 16°C. The researchers also tested whether the females would remember if a particular song had been sung in the cold and if they preferred it over a song that they had heard at warmer temperatures.

Having analysed the females' preferences, the team found that the female sparrows spent 40% more time close to the speaker when they were serenaded in the cold than when they were listening to love songs in mild conditions. Furthermore, even at 16°C, females preferred the song they had initially heard in the cold, so songs performed in the cold are clearly sexier than songs that are warbled when it's warm.

For songbirds, singing in the cold seems to be a way of advertising the quality of a male in the form of cold tolerance. The preferred choice of females for males singing at low temperatures might represent their choice of a male that may be energetically fitter and may be able to endure energetic challenges better than soft males who sing in the warm. The fact that females remembered which songs were performed in the cold might allow females to remain in the roost or perform other activities or while listening and then reciprocate the male's advances later in the day – when timing and temperature may be more convenient. Unfortunately, females' preferences for males with the stamina to serenade on chilly mornings may be severely affected by climate change as morning temperatures rise, although one thing is clear: cool singing is hot for birds.

K. W.
Song in the cold is ‘hot’: memory of and preference for sexual signals perceived under thermal challenge
Biol. Lett.