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.
Rufous-collared Sparrow, one of the species studied, is common and widespread across much of Central and South America (Keith Dover).
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.
"Working with museum specimens was a lot of fun," said Terrill. "One of my favourite things about museum specimens is using them in ways that other folks might not consider, and especially using them in ways for which the original collector couldn't have known they might be useful. It wasn't until recently that many people considered that how feathers grow might be important for birds or realised that you could measure feather growth rates on specimens, and I hope this study will publicise yet another way that museum specimens are useful for understanding birds."
Each of the four widespread Neotropical species surveyed showed a correlation between feather growth rate and and latitude.
Institute for Bird Population's Peter Pyle, an expert on bird moult patterns, added: "Most aspects of avian moult, with the exception of feather-replacement sequence, are thought to be rather flexible. The timing, location and extent of moults appear to respond quickly to environmental constraints, even within populations of the same species occurring at different latitudes, as either permanent or winter residents.
"Yet moult strategies remain vastly understudied compared to other avian topics such as breeding, migration and behavioural responses. This paper shows that a fourth component of moult, feather growth rate, also appears to vary, with equatorial populations showing slower moult intensity than those of higher latitudes. The author ties this nicely into other studies suggesting a decelerated pace of other life history traits in less seasonal environments, perhaps as a function of slower basal metabolic rates."
Reference
Terrill, R S. 2018. Feather growth rate increases with latitude in four species of widespread resident Neotropical birds. The Auk 135(4):1055-1063. DOI: https://doi.org/10.1642/AUK-17-176.1
