Bushtits showing a female with light eyes on the left and a male with dark eyes on the right. Photo by Bob Boekelheide

Flocking Madness: The Story of Bushtits

by Bob Boekelheide

March 2024

Bushtits are spritely little birds, usually seen flying in loose, bouncy flocks from bush to bush. They are the antithesis of long-distance migratory birds, staying in the same general area year-round and typically flying only far enough to reach the next bush or tree. Their flocks act more like superorganisms, moving through the brush as if with one mind, kept together by their chattery call notes.

Bushtits are the smallest passerine on the north Olympic Peninsula, weighing just over five grams, or about the same as two pennies. This makes them only a couple grams heavier than the smallest bird on the Olympic Peninsula, the Rufous Hummingbird, and about half a gram less than the relatively chubby Golden-crowned Kinglet. If you’re birding with someone who says, “There goes a ton of Bushtits,” remind them that it would take about 170,000 Bushtits to weigh one ton.

Bushtits range along the Pacific coast from British Columbia to Central America, as well as inland through the Great Basin and southern Rocky Mountains to west Texas. The local subspecies, Psaltriparis minimus saturatus, is the northernmost subspecies of Bushtit, found from southern coastal British Columbia through the Olympic Peninsula and Puget Sound. At the other end of their range, the southernmost subspecies of Bushtit is found as far south as the mountains of Guatemala. In between, Bushtits probably reach their peak densities in the oak woodlands of California and the mountains of Mexico.

The genus of Bushtit, Psaltriparus, comes from a combination of two other genera: Psaltria and Parus. Or, as described in Words for Birds by Edward Gruson, “Psaltry” is an ancient stringed instrument, and “Parus” is the genus of titmice. Technically it translates to something like “lute-playing titmouse,” but, to be honest, their chittery vocalizations don’t sound like a lute to me.

Bushtits belong to the family Aegithalidae, known as the “long-tailed tits.” The family Aegithalidae contains 11 species, of which the Bushtit is the only one found in North America. All the other members of the family live in Europe and Asia, particularly around the Himalayan Mountains, including some species ranging up to 15,000 feet elevation. It is hypothesized that ancestral Bushtits moved into North America by crossing the Bering land bridge sometime during the Ice Ages.

There are a multitude of interesting stories about Bushtits, many of which can be found in Birds of the World, the wonderful on-line resource from Cornell Lab of Ornithology. For one, Bushtits are eating machines, having to consume approximately 80 percent of their body mass each day to maintain body temperature and avoid losing weight. For comparison, if a 150-lb human ate like a Bushtit, they would have to eat 120-lbs of food every day. That’s a lot of insects!

Bushtits are classic “foliage-gleaners.” Their diet is mostly insects and spiders, with a smattering of small seeds and other plant material, particularly during winter when insects are scarcer. Their preferred diet seems to be scale insects and moth caterpillars, especially when feeding chicks. Their penetrating little eyes are able to spot the smallest insects and insect eggs, mostly impossible for human’s eyes to locate. If you have a feeder, you know that Bushtits also love suet, furiously gobbling mini-mouthfuls of high-calorie fat.

Speaking of eyes, do you know that you can determine the sex of Bushtits by their eye colors? Females have light eyes, whereas males have dark eyes. Females hatch with dark eyes, but their eyes turn light within one month of fledging.

*Bushtits showing a female with light eyes on the left and a male with dark eyes on the right. Photo by Bob Boekelheide*

How does a tiny bird like a Bushtit survive freezing nights, particularly when temperatures drop into single digits like we had in January this year. There is no evidence that Bushtits use hypothermia or torpor, the way that hummingbirds reduce their body temperature and slow their metabolism on cold nights. Instead, Bushtits are huddlers, gathering their flock together into a tightly-packed clump lined up in thick foliage within a bush or tree. This increases their collective volume while decreasing each individual’s exposed surface area. Adults also huddle within their nest during the nesting season, warming with their chicks and sometimes other adults.

Bushtits have one of the most unique bird nests in North America. Their nest is a “hanging sock” made out of spider webs, lichen, and bits of plant material, fashioned into a foot-long pendulous tube that looks like it is part of the plant. The entrance hole is under a hood at the top of the sock, whereas the nesting chamber is at the bottom in the “toe” of the sock. The spider webs create flexibility, so that the nest stretches as the chicks grow. One nest in Arizona even had four adults and ten chicks huddled in it at night, a testament to how flexible and strong they can be.

*Bushtit nest under construction in a red alder. Photo by Bob Boekelheide*

On the north Olympic Peninsula, one of Bushtits’ favorite places to build their nests is within Oceanspray, Holodiscus discolor. They even build nests within thick Douglas-fir foliage. They often build their first nests in early spring before the deciduous trees and shrubs have started to leaf-out, making their nests look very vulnerable to predation. Studies in Arizona, however, have shown that exposed nests experienced about the same level of predation as hidden nests, so maybe their nests already provide enough camouflage to confuse some predators.

Inside their sock nests, Bushtits lay an average six to seven eggs, ranging from four to ten eggs. Despite the bird’s tiny size, the female lays only one egg per day, the same as many other birds, including chickens. Bushtits don’t begin incubation until the last egg is laid, so even though the incubation period is listed as 12 to 13 days, some of the eggs may be in the nest for up to three weeks before they actually hatch. Curiously, even though both the male and female incubate, only the female develops a vascularized brood-patch.

Chicks are totally altricial, naked until they grow their first down at three-days old, and not opening their eyes until eight-days old. Based on the size of Bushtits, it is likely that their chicks fledge in a couple weeks or so, when the fledglings depart their nest and never come back. Their age at fledging is a bit uncertain, because tracking individual chicks in enclosed nests is difficult without destroying the nest. After their first chicks fledge, most Bushtit pairs attempt a second clutch most years, sometimes using the same nest and sometimes building a new one.

Studies have shown that flocks forage over an area of about 250 acres during the year, but flock members typically place their nests within an area of about 80 acres, which may facilitate awareness and interactions with other flock members. Even though individual pairs maintain their own nest sites during the breeding season, they apparently still make contact with fellow flock members throughout the nesting period. If a pair loses their nest, they may join adults at other nests, where they might help feed the other pair’s chicks or sometimes even take over the nest. Although it is hard to imagine what Bushtits are thinking, they appear to be remarkably non-territorial with other Bushtits, letting other Bushtits approach their nest and chicks without concern.

In at least the southern part of their range, from Arizona to Central America, Bushtits are cooperative nesters, with attendant birds helping at the nest. Typically, the helpers are adult males, apparently because the sex ratio in these populations is skewed towards more males than females, producing a surplus of males. More studies are needed to determine if Bushtits use helpers in the Pacific NW.

Social interactions like these suggest that members of Bushtit flocks may be closely related. More data are needed to reveal how closely related they really are, but the flocks we see during the non-breeding season could possibly be mixes of parents, brothers, sisters, aunts, uncles, cousins, and other relationships. How do they keep from interbreeding? Apparently, there are times during the non-breeding period, like in late summer and early fall, and again in early spring, when multiple flocks join together to form “superflocks,” within which individuals switch between flocks. Despite small sample sizes, it appears that female Bushtits are the ones most likely to disperse into new flocks, thereby mixing genomes between flocks.

Sequim-Dungeness Christmas Bird Count (SDCBC) data for the last 30 years reveals that Bushtits are doing quite well in the Sequim area (Figure 1). Despite lots of interannual variation, likely due to weather on different count days, the number of Bushtits counted per CBC party hour has increased by fits and starts over 30 years. Between 1995 and 2005, we tallied an average number of about one Bushtit per party hour. Between 2015 and present, the average count increased to about 1.5 Bushtits per party hour.

*Graph showing yearly changes in the number of Bushtits counted per party hour on the SDCBC.*

Similarly, the Wednesday morning bird walk at Railroad Bridge Park, which has occurred along the same stretch of the Olympic Discovery Trail every week since 2001, also shows a slight increase in Bushtits over the last 22 years (Figure 2). During that time, we observed Bushtits on 60.4 percent of Wed am bird walks. Even though Bushtits are somewhere in the area of Railroad Bridge Park throughout the year, we don’t encounter flocks every week.

*Graph of yearly changes in the number of Bushtits observed on Wednesdy morning bird walks at RR Bridge Park, 2002 – 2021.*

Data from 22-years of Wednesday morning bird walks at RR Bridge Park also show the annual cycle of Bushtit abundance in the Sequim area (Figure 3). Peak numbers of Bushtits occur in late summer, after new fledglings join the population. Numbers steadily decline through fall and winter, likely due to winter mortality, reaching lowest numbers during the next breeding season in April and May. Another reason we likely count fewer birds during the breeding season is because that is when pairs occupy individual nest sites, so instead of counting birds in flocks we’re instead counting adults present at the few nests visible along our sample route.

*Graph of the Annual Cycle of Bushtit abundance observed on Wednesday morning bird walks at RR Bridge Park, 2002 – 2021.*

The longest-living Bushtit on record, from USGS Bird Banding Lab data, was 8 years 5 months old. In actual breeding studies, one marked male nested for four consecutive nesting seasons, but most Bushtits do not reach these “old” ages. Another study showed that only 27% of marked Bushtits showed up again one year later. This appears similar to the annual cycle data at RR Bridge Park (Figure 3), where we recorded a high-count average of over 30 Bushtits just after the nesting season in August, to less that 10 Bushtits just before the next nesting season in March. This suggests that only one-fourth to one-third of Bushtits survive each winter. It’s not easy being a little Bushtit.

Here is your springtime homework assignment. Find a Bushtit nest, then every week spend an hour or two watching the birds coming and going from the nest. Are there more than two adults attending the nest? Can you tell whether they are incubating eggs or feeding chicks? How often do they feed the chicks? Can you follow males and females (remember their eye colors)? How do the birds interact?

Or, if you’re really fast, try following a Bushtit flock as it moves through the landscape. I’ve tried to do this, but I must admit that I can’t possibly move as fast as a Bushtit flock. I can follow them for a few minutes, but then “Poof,” they disappear. After you’ve gathered your data, please write an article for the Harlequin Happenings telling us all about your observations. Thanks!

Many of the interesting facts about Bushtits contained in this story came from Birds of the World, an on-line resource available through Cornell Lab of Ornithology. I highly recommend that all bird aficionados subscribe to Birds of the World, both for the information and to support the Lab of Ornithology.