Molting patterns vary amongst different types of birds; some transition from their juvenile plumage to their adult feathers in one single molting cycle, while others may take years to reach their fully matured plumage.3 In the West Palearctic, encompassing Europe and parts of the Middle East and north Africa, songbirds typically lean towards the latter option, molting some of their feathers in one cycle and molting the rest at later points in time.3 As it is energetically costly to grow feathers, spreading molts out over longer periods of time can help offset energetic costs. However, there is a reproductive tradeoff: the presence of juvenile feathers signal that the bird is young and not as competitively fit as an older bird with a whole set of adult feathers. Interestingly, however, climate change may be changing up these molt patterns.3
The relationship between climate change and migration patterns has been well-studied; for short-distance migrants like many of the West Palearctic passerines, warming temperatures have led to them leaving for their wintering grounds later.2 Kiat et al. (2019) looked at the plumages of juvenile birds over 212 years and saw that their molting grew more extensive with climate change. As they molt right after the breeding season, before migrating to their winter range, this change in migration timing may be giving birds more time to molt.3 They also saw that female birds have been molting more extensively than males in recent years, a reversal of past trends.3
Figure 1. Diagram representing the increase in molt over time in two West Palearctic songbirds. Adapted from Kiat et al. (2019)
For birds, these changes in molting may affect their reproductive and social interactions. A bird that molts its nest-grown feathers faster may look mature sooner than they did in past years. On one hand, as Kiat et al. note, this may give younger birds an advantage in the reproductive scene, as quickly replacing their juvenile feathers allows them to compete with the older birds with regards to the attractiveness of their plumage. On the other hand, looking more mature may also attract attention in the form of aggression from older birds, something that the juveniles may not be prepared to handle.3
The effects of this trend are in many ways still unstudied, and the effect of climate change may operate differently depending on a bird’s molt pattern and the climate conditions in which it lives. For instance, birds like gulls and eagles may need as many as five years to reach their adult plumage, while smaller songbirds reach their mature plumage much sooner and may feel the effects of climate change more strongly.1 Either way, this phenomenon shows just how all-encompassing climate change can be, and how there are even more effects that researchers have yet to discover.
- All About Birds (2008) The Basics: Feather Molt. https://www.allaboutbirds.org/news/the-basics-feather-molt/ (last accessed 23 April 2022).
- Jenni L and Kéry M (2003) Timing of autumn bird migration under climate change: advances in long–distance migrants, delays in short–distance migrants. R. Soc. Lond. B. 270:1467–1471. doi:10.1098/rspb.2003.23942.
- Kiat Y, Vortman Y and Sapir N (2019) Feather moult and bird appearance are correlated with global warming over the last 200 years. Commun. 10, 2540(2019). doi: 10.1038/s41467-019-10452-1.