The further away from the equator, the quicker tempo animals' lives tend to follow. Birds at more northerly latitudes mature faster, start reproducing younger and don't live as long, probably as a way of dealing with seasonal variation in resources. A new study from The Auk: Ornithological Advances shows for the first time that this pattern also plays out in feather development, with northern birds completing their annual moult faster to keep up with the demands of life far from the tropics.
Louisiana State University's Ryan Terrill looked at museum specimens of four species with ranges that span a wide swathe of latitude in both the Northern and Southern Hemispheres. Slight differences in feather growth between day and night during a bird's annual moult produce visible pairs of light-coloured bars, each pair representing 24 hours' growth. Terrill could determine the rates at which individual feathers grew by measuring their spacing. He found that in all four species, individuals collected at higher latitudes had grown their feathers faster.
Terrill sees two potential explanations for this pattern, which aren't mutually exclusive. First, where the availability of food changes with the seasons, birds may need to moult faster so that they have the necessary resources. Second, because birds at higher latitudes tend to be more invested in producing offspring than in extending their own survival, faster production of lower-quality feathers may be an acceptable trade-off.