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.

1. All About Birds (2008) The Basics: Feather Molt. https://www.allaboutbirds.org/news/the-basics-feather-molt/ (last accessed 23 April 2022).
2. 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.
3. 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.

Many hummingbird species will soon be making their way back up to the United States to breed after a winter spent in Central America and Mexico, and their iridescent feathers and insect-like movement makes them hard to miss once they arrive.  But perhaps the most impressive thing about these tiny birds is their metabolism: some species show maximum heart rates of over 1,200 beats per minute, and their oxygen consumption outpaces even the best human athletes (Hargrove, 2005).  Having to feed every 15 minutes or so, life as a hummingbird certainly seems difficult, but the biggest struggle of such an active metabolism is sleeping for extended periods of time without their four meals per hour.

While they are not the only bird with this adaptation, hummingbirds actually have a strategy that allows them to sleep through the night without starving: overnight, hummingbirds enter torpor, a low-activity resting state that saves energy by allowing their body temperature and metabolic rate to fall far below their daytime measurements (Shankar et al., 2020).  More recently, studies have shown that hummingbirds are capable of entering both deep and shallow states of torpor, with deeper torpor leading to lower body temperatures and greater energy savings.  Shankar et al. (2022) looked at hummingbirds of different sizes and their body temperatures overnight.  The smallest species they studied, the Black-chinned Hummingbird, spent about half the night every night in a state of deep torpor that dropped their body temperatures from 30°C to 20°C.  Larger species like the Blue-throated Mountain-gems preferred to remain in shallow torpor for most of the night, only occasionally dipping into deeper torpidity.  The researchers hypothesized that entering deep torpor only when absolutely necessary gives the birds an advantage.  They may be more aware of predators, for example, and it takes less energy to bring their body temperature back up in the morning if the temperature doesn’t fall so low to begin with.  Because small animals have higher relative metabolisms, the tinier hummingbirds like the Black-chinned Hummingbird have less flexibility in choosing whether to use shallow or deep torpor.

(T) “Archilochus alexandri” by Mdf is licensed under CC BY-SA 3.0

(B) “Blue-throated Mountain-gem” by Alan Schmierer is licensed under CCO 1.0

     Torpor is a crucial ability for hummingbirds, allowing them to balance their constant need for food with the need to sleep.  Fortunately, studies show that climate change does not appear to affect a hummingbird’s ability to enter torpor (Shankar et al., 2020), but warming environments could still be a threat to its efficiency.  Warmer ambient temperatures could prevent hummingbirds from entering the deepest stages of torpor, meaning they would be using more energy just to get through the night.  This is a particularly important concern for the smallest hummingbirds with the highest metabolisms, as well as migratory hummingbirds that require the energy to fly thousands of miles every year.  To bring energy expenditures down, they can use a variety of strategies such as reducing their activity or eating more food, but that is often easier said than done.  Many hummingbird species have no choice but to migrate long distances, as their summer breeding grounds become cold and barren in winter.  And simply “eating more” can be a tall task for a bird that has to feed so often already (Hargrove, 2005).  Even a temperature change of a degree or two will affect the delicate biological processes these birds have developed to exist in their fast-paced worlds, but only time will tell if they can adapt to these new environments.

References

1. Hargrove JL (2005) Adipose energy stores, physical work, and the metabolic syndrome: lessons from hummingbirds. Nutr J 4(36). doi:10.1186/1475-2891-4-36.

2. Shankar A, Cisneros INH, Thompson S, Graham CH, Powers DR (2022) A heterothermic spectrum in hummingbirds. J Exp Biol 225(2):jeb243208. doi: 10.1242/jeb.243208.

3. Shankar A, Schroeder RJ, Wethington SM, Graham CH, Powers DR (2020) Hummingbird torpor in context: duration, more than temperature, is the key to nighttime energy savings. J Avian Biol 2020:e02305. doi:10.1111/jav.02305.

4. Spence AR and Tingley MW (2021) Body size and environment influence both intraspecific and interspecific variation in daily torpor use across hummingbirds. Funct Ecol 35(4):870-883. doi: 10.1111/1365-2435.13782.

5. Mdf. (2006, June 19). Archilochus alexandri / [Male Black-chinned Hummingbird (Archilochus alexandri) in flight, photographed in Moab, Utah, USA] [image/jpeg]. Wikimedia Commons. https://commons.wikimedia.org/wiki/File:Archilochus-alexandri-003.jpg

6.  Alan Schmierer. (2013, April 26). Blue-throated Mountain-gem / [Male Blue-throated Mountain-gem (Lampornis clemenciae) in flight, photographed in Cochise Co., Arizona, USA] [image/jpg]. Flickr. https://www.flickr.com/photos/sloalan/8690505859