Chimney swifts eat flying insects, almost exclusively. Beetles, flies, and true bugs make up the core of their diet, with mayflies, moths, wasps, caddisflies, and spiders rounding out the list.1Ornithology. DNA metabarcoding reveals the diet of Chaetura pelagica (Chimney Swift), a declining aerial insectivore These birds catch virtually everything they eat on the wing, scooping prey out of the air during long, darting flights that can keep them aloft for most of the day. What makes their diet especially interesting is that it is not just a random sampling of whatever bugs happen to be flying around. Chimney swifts are pickier than they look, and studying what they eat has revealed surprising things about insect populations, pesticide history, and why these birds are declining.
The Full Menu
For a long time, what we knew about chimney swift diets came from dissecting the stomachs of collected birds or picking through the insect fragments in their droppings. A DNA metabarcoding study published in the journal Ornithology gave a much clearer picture. By analyzing the genetic material in fecal samples, researchers identified prey items down to the order and sometimes the family level. The results confirmed that beetles (Coleoptera), flies (Diptera), and true bugs (Hemiptera) are the most frequently detected prey groups.1Ornithology. DNA metabarcoding reveals the diet of Chaetura pelagica (Chimney Swift), a declining aerial insectivore Beyond those three staples, mayflies, moths, wasps, caddisflies, and even spiders showed up consistently in the samples.
Spiders might seem like an odd item for a bird that hunts on the wing, but many small spiders disperse through the air using a behavior called “ballooning,” where they release silk threads and ride wind currents. A chimney swift cruising through the sky will encounter these airborne spiders along with everything else floating at altitude. The diversity of the diet is notable: these are not specialists locked onto one prey type. They are generalist insectivores that take advantage of a wide range of small arthropods, which makes them sensitive barometers of overall insect abundance in their environment.
Surprisingly Selective Feeders
You might assume that a bird catching insects mid-flight simply eats whatever it runs into. That turns out to be wrong. When researchers compared the DNA metabarcoding results with data from sticky traps placed near nesting sites to measure which insects were actually available, they found that chimney swifts were not feeding in proportion to the surrounding insect community. The birds appeared to actively avoid some of the most common prey groups near their nests, including flies, thrips, wasps, and true bugs, while seeking out prey that was comparatively uncommon in the trap samples, such as mayflies and moths.1Ornithology. DNA metabarcoding reveals the diet of Chaetura pelagica (Chimney Swift), a declining aerial insectivore
This selectivity raises interesting questions. Mayflies and moths tend to be larger-bodied than thrips or small flies, so one possibility is that chimney swifts are targeting prey that delivers more calories per catch. A bird that spends its entire foraging day on the wing, burning enormous amounts of energy in continuous flight, has good reason to prioritize the biggest payoff per insect captured. Another possibility is that the birds forage at altitudes or in microhabitats where the insect composition is different from what the ground-level sticky traps captured. Swifts often hunt high above rooftops, and the insect community up there may look nothing like the one a few meters off the ground.
The practical consequence of this selectivity is important for conservation. If chimney swifts depend disproportionately on certain insect groups rather than simply eating whatever is abundant, then a decline in those specific prey populations could hurt the birds even if overall insect numbers stayed steady. Conserving chimney swifts means understanding which insects they actually rely on, not just which ones fly near their nests.
How They Feed Their Young
Adult chimney swifts do not carry individual insects back to the nest the way a robin brings a worm. Instead, they compress collected prey into a small, ball-like structure called a food bolus, which they then regurgitate directly into the mouths of their nestlings.2Wildlife Biology. Swift sampling of farmland aerial invertebrates offers insights into foraging behaviour in an aerial insectivore Each bolus is essentially a packed pellet of dozens or hundreds of tiny insects glued together with the bird’s saliva. Chimney swifts have well-developed salivary glands, and their sticky saliva serves double duty: it holds the bolus together during transport and also cements the nest materials (small twigs) to the wall of the chimney or hollow tree where they breed.
This bolus-feeding system means that researchers studying nestling nutrition can sometimes collect intact food boluses from nests, dissect them, and catalog the insect fragments inside. These samples have been a valuable window into diet composition for decades. More recently, DNA-based approaches have supplemented or replaced the painstaking work of identifying tiny exoskeleton fragments under a microscope, but the bolus itself remains one of the best ways to sample exactly what a swift family is eating during the breeding season.
Nestling chimney swifts grow rapidly. A clutch of three to five chicks demands a constant shuttle of boluses, and both parents share the work. The nutritional quality of those boluses matters enormously. Chicks that receive boluses rich in large, calorie-dense insects like beetles grow faster and fledge in better condition than those fed a steady diet of smaller, less nutritious prey. This is one of the reasons diet shifts over time are a concern for population health.
What Guano Archives Tell Us About Diet Over Decades
One of the most unusual tools for studying chimney swift diets is their own accumulated droppings. Because chimney swifts return to the same roost sites year after year, sometimes for decades, guano builds up in layers at the base of chimneys. Each layer represents a snapshot of what the birds were eating during that period. Researchers in Ontario, Canada, took advantage of a well-preserved guano deposit to reconstruct a 48-year dietary record stretching from 1944 to 1992. By analyzing the insect exoskeletons embedded in each layer, they could track changes in the birds’ prey over nearly half a century.3PubMed Central. Historical pesticide applications coincided with an altered diet of aerially foraging insectivorous chimney swifts
What they found was striking. As DDT use ramped up in the mid-twentieth century, the proportion of beetles in the guano dropped steeply, while the proportion of true bugs (Hemiptera) rose. The timing was not coincidental: DDT concentrations measured in the guano layers tracked closely with the shift in prey composition. Beetles were apparently more vulnerable to DDT than hemipterans, so as beetle populations declined under pesticide pressure, the swifts’ diet shifted to reflect whatever was still available. This is a powerful example of how a bird’s diet can serve as an indirect record of environmental contamination and insect community change.
A parallel study on Vaux’s swifts, a closely related species on the Pacific coast, found a similar pattern using a roughly 26-year guano core from a roost chimney on Vancouver Island. The ratio of Hemiptera to Coleoptera in the guano increased over time, and nitrogen isotope signatures in the layers suggested a possible decline in diet quality. Hemipterans tend to occupy higher positions in the food chain than many beetles, but they are generally smaller and less calorie-dense per individual, so a diet tilted toward more bugs and fewer beetles may deliver less energy overall.4Northwest Science. Linking Population Declines with Diet Quality in Vaux’s Swifts
Diet Quality and Population Decline
Chimney swift populations have dropped sharply across North America since the mid-twentieth century. Breeding Bird Survey data shows steep, sustained declines that have put the species on conservation concern lists. Multiple factors are probably at work: loss of nesting habitat as old chimneys are capped or demolished, reduced insect abundance tied to pesticide use and land-use change, and climate-driven shifts in insect emergence timing. But the dietary evidence adds a layer that is easy to overlook. Even when swifts can still find insects, the composition of what they find may be nutritionally inferior to what their ancestors ate.
The guano studies described above suggest that the shift from a beetle-heavy diet to a bug-heavy one is not just a curiosity. Beetles, particularly the flying adults of ground-dwelling species, tend to be large, energy-rich prey items. True bugs, while abundant, are often smaller and carry less usable energy. For a bird whose entire life strategy depends on catching enough food on the wing to fuel continuous flight and raise a brood of hungry chicks, the caloric value per insect matters. A diet that looks adequate in terms of prey numbers might still fall short in terms of total energy, leading to lower breeding success, slower chick growth, and ultimately fewer birds in the next generation.
This connection between diet quality and population health is difficult to prove directly, because you would need to track individual birds, measure their prey intake, and link it to reproductive outcomes over many years. But the circumstantial case is strong, and it echoes what researchers have found in other aerial insectivores like swallows and nightjars, which are declining across similar ranges for broadly similar reasons.
How Chimney Swifts Compare to Other Aerial Insectivores
Chimney swifts belong to a guild of birds that make their living catching insects in flight. Swallows, martins, nighthawks, and other swift species all share this lifestyle, and their diets overlap considerably. A study of two swift species and a swallow in Central America found that the taxonomic composition of their diets was broadly similar, with no significant seasonal differences in what the swifts were eating.5Ibis. Selective predation by two swifts and a swallow in Central America This makes sense: birds hunting insects at similar altitudes and times of day will encounter roughly the same prey community.
Where chimney swifts differ from swallows is in their foraging style and flight endurance. Swallows tend to forage closer to the ground or water surface and return to perches frequently. Chimney swifts rarely perch during the day at all. Their tiny feet are designed for clinging to vertical surfaces, not for landing on branches, so they stay airborne for most of their waking hours. This means their foraging range is enormous compared to a barn swallow working a field. A chimney swift may cover many kilometers in a single foraging bout, sampling insect communities across a wide area rather than concentrating on one patch.
This wide-ranging foraging habit is part of why chimney swifts are considered good indicators of regional insect health. A swallow working one field reflects the insects in that field. A chimney swift roaming across a city reflects the insect community of the entire area. When swift populations decline, it signals something bigger than a local problem.
Do They Eat Mosquitoes?
This is one of the most common questions people ask, and the answer is: yes, but probably not as many as you hope. Mosquitoes are small flies, and flies in general are a major component of the chimney swift diet. But individual mosquitoes are tiny, and chimney swifts seem to prefer larger prey when it is available. The metabarcoding study’s finding that swifts appear to avoid some of the most common small flies near their nests while targeting larger, less common insects like mayflies and moths suggests that mosquitoes are more of an incidental catch than a primary target.1Ornithology. DNA metabarcoding reveals the diet of Chaetura pelagica (Chimney Swift), a declining aerial insectivore
That said, a chimney swift probably eats thousands of individual insects per day over the course of its marathon foraging flights, and some of those will inevitably be mosquitoes. Communities that encourage chimney swifts by building or maintaining swift towers sometimes promote the birds as natural mosquito control, and while the birds do eat some mosquitoes, their real value as pest controllers is broader. They consume enormous volumes of flying insects of all kinds, including agricultural pests like aphids (which are true bugs and show up in the diet data) and pest beetles. Framing swifts purely as mosquito killers undersells their ecological contribution and sets up unrealistic expectations.
What Chimney Swifts Do Not Eat
Unlike many insectivorous birds, chimney swifts do not eat seeds, fruit, or nectar. They do not forage on the ground or glean insects from leaves and bark the way warblers or chickadees do. Their entire food supply comes from what they can catch in open air. This makes them extraordinarily dependent on flying insect populations in a way that a more flexible species is not. A robin can switch from worms to berries when conditions change. A chimney swift has no such fallback.
They also do not eat at feeders. There is no bird feeder design that attracts chimney swifts, because they do not land to eat. People who want to help chimney swifts cannot do so by putting out food. The most effective actions are habitat-related: keeping old chimneys uncapped during the breeding season, building purpose-built swift towers, reducing pesticide use in the surrounding landscape to support healthy insect populations, and planting native vegetation that supports the insects swifts depend on. The relationship is indirect: you feed a chimney swift by feeding the ecosystem that feeds its prey.
The Seasonal Dimension
Chimney swifts are long-distance migrants, spending winters in South America and breeding across eastern North America. Their diet shifts as they move between continents, not because their preferences change, but because the available insect communities differ. During spring migration, swifts moving north through the Gulf states encounter different insect assemblages than they find on their breeding grounds in the Northeast or Midwest. During fall migration, they may encounter late-season insects like swarming ants and termites.
On the breeding grounds, diet composition also changes as the summer progresses. Early-season meals may lean heavily on spring-emerging insects like mayflies and caddisflies, whose aquatic larvae hatch in enormous numbers near rivers and lakes. By midsummer, beetles and moths become more prominent as those populations peak. Late in the season, true bugs may make up a larger share. The study of Vaux’s swift guano found no dramatic seasonal differences in the broad taxonomic composition of the diet within a given year, but the relative proportions of different insect orders do shift with the local phenology of insect emergence.4Northwest Science. Linking Population Declines with Diet Quality in Vaux’s Swifts
On the wintering grounds in the upper Amazon basin, almost nothing is known about what chimney swifts eat. The birds are notoriously difficult to study in South America because they mix into enormous communal roosts with other swift species and forage over vast tracts of tropical forest. Presumably they take whatever flying insects are abundant in that ecosystem, but detailed dietary data from the wintering range remains a major gap in our understanding. Filling that gap matters because winter survival is likely a significant driver of population trends, and winter diet quality may be part of the story.