In late summer, the birds and birdsong of spring seem to have departed from our gardens. Listen carefully however, and cryptic calls and furtive rustlings from the shrubbery have replaced the upfront songs. Look closely and you may spot some very scruffy-looking birds lurking as they conserve energy and avoid predators while they moult.
Why moult? Feathers take a battering during the processes of courtship, breeding and rearing young to independence. Damage by encounters with competitors and predators, brushing against vegetation, attacks by mites and other parasites all mean that there comes a time when new feathers are needed. As each new feather grows from the follicle of the previous one, the old feathers must be shed before its replacement is in working order. Replacing its feathers, being between 5% and 10% of the bird’s total body mass, requires considerable effort. To minimise other energy use, keep out of the way of predators, deal with limitations to flight, insulation and water resistance as a result of feather loss and to forage for specific nutrients for feather production demands careful timing.
Most garden birds moult in late summer, when food, cover and weather remain favourable, breeding is largely completed before the demands of migration and winter. Exact timings and the length of the process vary with factors such as latitude, size, the extent of the replacements and migration strategies. Few achieve even a partial moult in less than a month, with some larger birds taking more than three months.
When fully feathered again, look out for smartly dressed nuthatches and coal tits visiting your feeders. And marvel at the dowdy chaffinches, greenfinches and house sparrows whose feathers will gradually abrade over the winter to reveal their exciting breeding plumage when rivalry hots up in the spring.