Some birds moult twice a year, changing colours as well as actual feathers when they do – and now scientists think they know why.

Research by an international team suggests the trait evolved in species that are under pressure to be colourful for sexual attractiveness but also face high predation risk, when it is better to try to avoid detection.

“This trade-off is a classic problem in ecology and studying colour change in birds gives insight into how animal colours evolve,” says Anne Peters from Australia’s Monash University, which led the study.

“The results suggest that seasonal colour change is an adaptation that allows birds to have the best of both worlds: they can be sexually attractive and bright while breeding, but also dull coloured and difficult to detect by predators outside the breeding season.”

The colour change can sometimes be dramatic – the male red fody (Foudia madagascariensis), for example, moults its brown head and body feathers to become intense red with a black eye-stripe – and at other times relatively subtle.

The obvious question is why more birds don’t do it. Most remain the same colour year-round, replacing their feathers only once a year.

The short answer is that things need to be just right.

“We suggest seasonal plumage colours might be uncommon and frequently lost in evolutionary history because the trait requires (i) the right balance of strong sexual selection and high predation risk as well as (ii) seasonal changes in the environment that favour a defined breeding season and (iii) reliable cues that herald the arrival of ideal breeding conditions,” the authors write in the journal Ecology Letters.

It’s also not an inconsequential thing to do when compared with other mechanisms of avian colour change, such as feather abrasion or fading, and the use of cosmetics. It is associated with substantial changes in physiology and impacts on thermoregulation, activity levels and energetic reserves.

And it takes days or weeks to achieve and cannot easily be reversed. So it needs to be very accurately timed.

In their study, the research team – from Monash, the Max Planck Institute for Ornithology, Germany, New Zealand’s Massey University and the University of Windsor, Canada – assessed the occurrence of seasonal plumage colours for 5901 species of passerines.

It was found to be present in 4% of species (243 of the 5901) but in 22% of families

(21 of 79 families) – and these are likely to be underestimates, the researchers suggest.

It was found in males only for 143 species, in both sexes for 99 species and in females only for one – though in this case the method is not well understood.

Overall, the researchers found that colour change was more common in birds that live in highly seasonal environments, where they breed during periods of warmer weather or in response to seasonal rainfall.

Because this is relatively easy to detect in the field, it could also provide an indicator of how birds are responding to environmental change.

“Colour change may be a useful model to test whether birds adjust their breeding schedules in response to shifting seasonal patterns,” says lead author Alexandra McQueen, from Monash.

“Unfortunately, if birds do not flexibly adjust the timing of colour change, shifts in climate could lead to a mismatch between bright colours and the ideal conditions for breeding, with consequences for attracting mates.”