Bushtits are among the smallest songbirds in North America. The birds weigh just 4-8 grams, or roughly the same as two pennies. They’re up to 3.1 inches or so long, but much of their body length is in their tails.
The species’ range throughout Mexico and much of the south and Pacific west. So sure, they’re a bird of warm climates.
But this doesn’t mean Bushtits don’t have to deal with cold.
Here in Colorado, our winter days are relatively mild. But our nights are cold, with lows regularly drop below freezing. We also see cold snaps that can leave us in the single digits for days at a time. Lows below zero, sometimes well below zero, are not uncommon.
And snow?
Yeah, we’ve got snow.
Denver’ can see snow as late as mid-May. And we can see snow again as early as October — even late September. Between then, Colorado’s cities receive around the same amount of snow as northern cities like Milwaukee or Detroit.
Bushtits in Winter
For a little bird, this means trouble. Smaller animals lose heat faster because they have relatively little mass in proportion to their surface area.
According to the biologist Susan B. Chaplin, at 20 degrees Celsius ( a balmy 68 degrees Fahrenheit!) Bushtits need to eat about 80% of their body weight in food just to maintain themselves.
At lower temperatures, life is ever more challenging and risky. But Bushtits have a secret weapon: they tolerate (perhaps even enjoy?) each other’s company.
At night, some small songbirds, such as chickadees, shelter in tree cavities, where they can stay warmer than when exposed to the night air.
Bushtits don’t do this. Instead, they perch together, in flocks of up to 20-70 birds, in dense tangles of tree branches.
And they huddle.
Yes, these tiny birds seem to snuggle, getting as close as possible to one other to reduce heat loss.
Chaplin found that a Bushtit who huddles with just one other Bushtit reduces metabolic needs by nearly 20%. So the gains are greater when three or four birds snuggle, as seen in the opening photograph that I took at Denver Botanic Gardens in 2013.
Bushtits’ social behavior has another advantage. During the day, it’s not unusual to encounter busy flocks of these birds. (I’ve written about this previously and included a video of one such flock here.)
Foraging as flocks means more eyes, ears, and nostrils seeking foods. Bushtits flocks are more likely to find the new feeder filled with suet, which they absolutely love, or the tree experiencing an aphid outbreak than is a single bird.
Expanding their range
Though not exactly common around Denver, Bushtits are a staple of our suburban and urban landscapes.
But this hasn’t always been the case.
When Colorado birders conducted their first survey of breeding birds in the late 1980s and early 1990s, Bushtits were absent from the northern cities. The birds didn’t nest in Denver, Aurora, Boulder, or Fort Collins
The second survey, which took place between 2007-2012, found them all of the Front Range.
This map shows this. Look, especially, for the blue boxes. These mark new nesting sites that volunteers detected in 2007-2012.
In 2017, area birders speculated about the change. One thing that came up is how the region’s changed landscape is making new possibilities for insects and birds possible. As forest entomologist Dave Leatherman put it, “Colorado is fast becoming part of the desert Southwest” through changing landscape practices.
Specifically, plantings of oaks and conifers may now allow Bushtits to thrive here. These are trees that we favor for their drought tolerance. They’re also trees that attract and host all sorts of insects that Bushtits ruthlessly hunt.
It’s not only Bushtits that are now finding Colorado’s Front Range amendable to them. Other more southerly birds, such as Black-chinned Hummingbirds, have also spread into Colorado’s northern cities. More montane species, such as Dark-eyed Juncos and Western Flycatchers, also seem to becoming down into Denver and its suburbs to breed.
Changes like these are always complex, and they don’t necessarily signal a net gain in biodiversity. Our interventions into the urban and suburban environments of Colorado produce other losses — of native plants and their associates.
But in a time when ecological loss and grief are so heavy, the adorably adaptive toughness of Bushtits is worth celebrating.
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