by Robert McFarlane, Houston Bird Survey Coordinator
Several groups of birds have independently evolved the capability to capture insects while in prolonged continuous flight. Their common adaptation is a short bill but a wide mouth, fringed with stiff rectal bristles, that opens to provide a gaping insect trap. The Houston Bird Survey provides information on eight of these insectivorous species: the Chimney Swift, Common Nighthawk, and six species of swallows. If we use the number of survey sites that report these species as a surrogate indicator of abundance, we can decipher evidence of the adaptation of these birds to urban environments. All of these birds are essentially summer residents although there are rare reports of Tree and Rough-winged Swallows and Chimney Swifts in our winter surveys. These reports are too rare to be very informative.
Does an urban habitat provide an insect resource sufficient to sustain these insectivorous specialists?
The Chimney Swift is the most widespread of these flying bug-traps (see maps), occurring widely across Harris County and penetrating deep into the urban area, including downtown. Chimneys and incinerator smokestacks may provide nesting sites for these swifts. Curiously, there are few reports from eastern Harris County, which may reflect a paucity of surveys rather than birds.
Common Nighthawks have been reported from more sites inside of Beltway 8 than beyond it. Flat-roofed buildings are known to provide nest sites for nighthawks and may influence this distribution.
Purple Martins are our most abundant swallow, widespread across the county and deep into the urban zone. They may be assisted by the presence of ever-popular martin houses for nesting. Barn Swallows appear to be our second-most abundant swallow. They also penetrate deep into the inner city. Cliff Swallows are widely distributed although less frequently encountered. Although they penetrate into Loop 610 and near the downtown area, they may have less of a tendency to do so. Tree swallows are not as abundant in June as they are during migration. There is only one survey report inside of Beltway 8. Bank Swallows are very scarce, all beyond Beltway 8 in eastern Harris County. Northern Rough-winged Swallows are rare, from only one area northeast of Beltway 8.
Thus some, but not all, aerial foragers appear to do well in the urban habitat. Suitable nest sites may play a role in determining which of these far-ranging birds that fly many miles each day are able to succeed as urbanites.
