A Portrait of Illinois’ Swallows
The late July heat slowly dissipated as the sun approached the horizon. Above the wide expanse of the Illinois River, the air was filled with swallows. All six species common to Illinois were present, though most were tree and bank swallows. And all were doing the same thing: foraging on the wing for flying insects. I found it difficult to make sense out of the confusion of birds. The different species seemed all mixed up, each swallow foraging over the river on an independent flight plan. It was an impressive display of life.
Gazing upon this scene, I could not help but reflect on a basic principle of ecology which states that no more than one species can fill exactly the same role or niche in an ecosystem. Yet here were six aerial insectivores, apparently all feeding in the same way, in the same area, on the same food items. This summer post-breeding spectacle, however, portrayed only one aspect of their lives, probably the portion where the most competition occurs. While all of Illinois’ swallow species – tree, northern rough-winged, barn, bank, cliff, and purple martin – may behave in similar ways during some periods of time, I knew that I would have to more closely investigate their behaviors during the breeding season (when the swallows are in Illinois) to see how each species uses the available habitat in a different manner.
Indeed, once the swallows arrive on the breeding grounds from distant overwintering areas, each species begins to diverge rather obviously from the others in habitat choice and behavior. They partition the landscape’s resources so that all species may co-exist, reproduce, and provide a new generation for the future. How each type of swallow does this illustrates the concept of niche separation.
Rough-winged and bank swallows nest in burrows in vertical soil exposures such as those found along streams. Rough-wings also nest in other types of holes and crevices. Bank swallows actually excavate their own burrows several feet into the soil and form small to large colonies. It is not certain whether rough-winged swallows excavate their own burrows or always use abandoned burrows of belted kingfishers or bank swallows. Although the rough-wings are the most solitary of the Illinois swallows, sometimes they will place nests on the edge of a bank swallow colony. Here, given this high degree of resource-use overlap, we might expect the more numerous bank swallows to drive out the rough-wings, but both species seem to tolerate each other. Because nest predation is greater near the edge of the colony, perhaps the bank swallows find an overall benefit by having some of the edge nests occupied by rough-wings. But other than not having to excavate their own burrows, how do rough-wings benefit?
There are many disadvantages of nesting in large colonies. One study in Michigan by John L. Hoogland and Paul W. Sherman showed that for different sizes of bank swallow colonies, as colony size increases, there is more competition for nest sites, nest materials, and mates. At larger colonies, there is also an increased chance that parents may not recognize their own still-dependent young after the young aggregate into larger groups with other fledglings. Parents might then expend time and energy to partially raise another pair’s young, possibly at the expense of their own. Another disadvantage of colonies is that as colony size increases, fleas and swallow bugs (wood tick-sized, bloodsucking bedbugs) are more easily transmitted among individuals. So again, why should rough-winged swallows accept all of the disadvantages and located themselves on the edge of a bank swallow colony?
Even though large colonies have many disadvantages to birds, and they can actually attract the attention of predators, colonies are highly effective in predator detection – and defense in some cases (mobbing the predator). While walking along Salt Creek in Logan County one spring, I noticed a bank swallow colony just below the steep drop off of the creek bank. When I was directly above the burrows, the swallows formed themselves into a great whirling cloud that circled directly overhead, with each bird vociferously calling. I am not sure if this tactic could actually repel a determined predator, but it surely kept my attention and made me desire to get away. A large bank swallow colony is a chaotic affair, much like a school yard full of highly energetic children during recess on the first warm, sunny spring day. If I were a bank swallow, I would probably locate myself as far away as possible from the colony to dig my burrow, and then choose not to breed. But bank swallows seemingly thrive in the chaos.
A dead tree full of bank swallows, near the Illinois River.
Cliff swallows are even more highly colonial and social than bank swallows. They build their gourd-shaped nests made of mud on cliffs, on the undersides of bridges, and on buildings. Because of this, cliff swallows are not in competition with bank and rough-winged swallows for nesting sites. Probably because of the use of bridges, cliff swallows have actually been increasing in Illinois. A very large colony, for example, is under the U.S. Route 136 bridge to Keokuk, Iowa, directly over the Mississippi River. Cliff swallows are so highly social that even what might easily be a solitary activity, such as gathering mud for nest construction, has a social context. Cliff swallows in groups spend less time looking for predators and are more efficient at gathering mud than lone swallows. The cliff swallow colony, in fact, is an integrated unit. Breeding is highly synchronized within the colony, and the colony acts as an information exchange center to the location of food resources. Well fed individuals are followed by others as they leave the colony en route to known food concentrations. But all of the disadvantages associated with nesting in a colony, such as increased pest transmission, competition for resources, etc. still apply. In fact, the highly social cliff swallows themselves compound the disadvantages. A female may not only lay eggs in another cliff swallow’s nest, but may also toss out the host’s eggs before physically transferring her own egg to the host’s nest.
Nest infestations of fleas and bugs at cliff swallow colonies can defy the imagination: In an intensive, long-term study in Nebraska, Charles R. Brown and Mary Bromberger Brown found as many as 2,500 swallow bugs in some nests. Nestlings in highly infected nests fared worse than those in cleaner nests. Bug infestations can actually be so bad that cliff swallows may not use a colony site every year, presumably to give bug populations a chance to die out.
Although aggregations of barn swallow nests are referred to as colonies, the species is much less social than the bank and cliff swallows. Small colonies may form in appropriate locations, but the birds seem to go about their activities solitarily, not exhibiting the social qualities of the tightly-knit cliff or bank swallow colonies. Barn swallows, as the name implies, build their bowl-shaped nests constructed of mud pellets mainly on artificial structures such as buildings and bridges. Historically, nesting sites may have been limited to cave entrances and cliffs. So barn swallows, like cliff swallows have increased with the advance of roads and buildings across the landscape. In nest site selection, barn swallows are now very similar to cliff swallows, and the possibility for competition between the two certainly exists.
Barn swallow nestlings, almost ready to fledge.
Their foraging patterns are slightly different, however. Barn swallows tend to forage low over the ground, following the contours of the landscape or low over the water. Cliff swallows, in contrast, may fly to considerable heights in close-knit flocks following swarms of insects rising on warm air. This small difference in foraging style may be enough to reduce niche overlap so that both species can better co-exist in the same area. But cliff swallows are more aggressive than barn swallows, and may usurp barn swallow nests at sites where both species occur. And cliff swallows are increasing in Illinois while barn swallow populations appear to be somewhat stable.
Of the Illinois swallow species, the barn swallow is the only one that regularly has two broods in succession. I observed second brooding several times over the last few years at a small barn swallow colony located inside an abandoned boat house along the Illinois River. I was surprised each year as some nests contained nestlings in the early stages of development, long after other nests and other species of swallows had fledged young. The energy requirements necessary to prepare for a long fall migration to overwintering areas in South America must be great enough, a second brood notwithstanding. But by locating their nests along the Illinois River, the barn swallows I observed using the boat house were very fortunate. The river and its backwaters are a prodigious source of insects. Many insect species spend the immature stages of their lives on the bottom of the river and the adjacent floodplain lakes, later emerging in great numbers on hot summer days.
[For a continuation of this story, see Wings Over the River A Portrait of Illinois’ Swallows - Part 2]
[Note: This story originally appeared in slightly different form in the summer 1999 issue of Illinois Audubon magazine, number 269.]
