Happy New Year! In honor of brand-shiny-new 2013, I have… a continuation of the last post. I left a few things out of that post, since it was starting to get quite long; and then in the course of researching to answer some comments, I found some more things; so here are a few more ways that birds keep warm.
Shelter: Wind is cold. Wind whisks away the tiny layer of air next to you that you had slightly warmed up, and replaces it with unwarmed air, and then does that again and again and again. Brrr.
Verdins exposed to 3 meters/second winds (about 6.7 miles/hour, or a “light breeze” on the Beaufort scale) increased their metabolic rate 14% (Wolf & Walsberg 1996). That’s not a huge number, but it’s a considerable cost for just a “light breeze.” Over the course of a ten-hour winter night, for a small bird already barely making it to morning on scant fat reserves, that could be fatal.
So birds seek shelter – even if it doesn’t always look like shelter to us. The House Sparrows here spend much of their time inside bushes with many thin branches. They don’t look like a human idea of “shelter,” but Walsberg (1986) found that similar brushy roosting sites cut 2.4 meters/second winds to just 0.4 meters/second, effectively making the temperature inside 8°C (14°F) warmer.
A few birds seek refuge in cavities. Mountain Chickadees and Juniper Titmice will roost overnight in cavities – holes in trees, nestboxes placed by curious ornithologists – and, due to the reduced wind inside the cavity, enjoy up to 37% energy savings. These energy savings translate to an ability to survive while fasting for up to an additional seven hours in winter (Cooper 1999). That’s one more blizzard safely waited out; one more cold spell endured without starving.
Before I looked this up, I thought that birds did not sleep in cavities except while breeding. Now it’s clear that for some small birds in particularly bad weather, finding a cavity can be a matter of life or death.
Bananaquits don’t just hope to find a cavity: they make one. Sleeping in their big, grassy nests cuts their rate of heat loss by a third (Merola-Zwartjes 1998).
Bananaquits live in tropical climates, so keeping warm, for them, is more of an issue of saving energy than of not dying. Lucky birds.
Facultative hypothermia: Isn’t that a lovely phrase? It really rolls around on the tongue. If I ever have a daughter I shall name her Facultative Hypothermia. (A son would, obviously, be named Bananaquit.) Anyway.
Hypothermia means “body temperature below normal.” Facultative hypothermia is hypothermia driven and regulated by the animal: hypothermia-on-purpose, as opposed to hypothermia-because-you-are-freezing-to-death. The benefit of facultative hypothermia is that it reduces the bird’s metabolic rate, allowing it to survive longer on its energy stores. The cost is that, at a lower metabolic rate, everything goes slower – digestion, etc. – but if the bird is forced by circumstances to hunker down and wait out the bad weather or food shortage anyway, this isn’t much of a loss.
Many birds exhibit some degree of facultative hypothermia. The most intense form, called “torpor,” involves the bird slowing down so much that it can be difficult to rouse. This leaves them highly vulnerable to predators; but if the options are A) maybe get eaten, or B) definitely starve to death, it’s easy to see why they go with A.
Torpor is widely used in the Caprimulgiformes, an order of birds that contains such great characters as the Tawny Frogmouth and the Oilbird, as well as more familiar folk like the Common Nighthawk. The latest genetic work has placed hummingbirds within Caprimulgiformes, so take a moment to consider the family resemblance:
Both hummingbirds and the more traditional wide-eyed, wide-gaped Camprimulgids use torpor a lot, so that does make some evolutionary sense, even if their appearances don’t.
Combining heat-preserving strategies can give birds flexibility in choosing how to cope with the cold. Australian Owlet-nightjars, another Caprimulgiform, both shelter in cavities and use torpor to get through the night. Rock crevices are warmer than tree hollows at night, but the owlet-nightjars prefer to roost in tree hollows anyway, apparently because they are safer from predators there and because the tree hollows warm up more in the morning, allowing the birds to wake up from torpor more easily by passively warming up. (Doucette et al. 2011).
Tiny Puerto Rican Todies – they weight just six grams, slightly less than two Saltine crackers – somehow get around the difficult-to-rouse part of torpor: they can lower their body temperatures up to 11°C (19°F) at night while still remaining active and alert. This is especially surprising because the todies already start out cold, by bird standards, with normal body temperatures around 36.7°C (98°F) instead of the usual for birds of around 40°C (104°F) (Merola-Zwartjes & Ligon 2000). It’s not clear how they manage to get so cold and still be alert, but it is impressive! Normal human body temperature is also about 98°F, like the tody’s. If my body temperature dropped to 79°F, nobody would be describing me as “alert.”
Cooper SJ. 1999. The thermal and energetic significance of cavity roosting in Mountain Chickadees and Juniper Titmice. Condor 101:863-866.
Doucette LI, Brigham RM, Pavey CR, Geiser F. 2011. Roost type influences torpor use by Australian owlet-nightjars. Naturwissenschaften 98:845-854.
Merola-Zwartjes M. 1998. Metabolic rate, temperature regulation, and the energetic implications of roost nests in the Bananaquit (Coereba flaveola). Auk 115:780-786.
Merola-Zwartjes M, Ligon JD. 2000. Ecological energetics of the Puerto Rican Tody: heterothermy, torpor, and intra-island variation. Ecology 81:990-1003.
Walsberg GE. 1986. Thermal consequences of roost-site selection: the relative importance of three modes of heat conservation. Auk 103:1-7.
Wolf BO, Walsberg GE. 1996. Thermal effects of radiation and wind on a small bird and implications for microsite selection. Ecology 77:2228-2236.