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BRAW Asks the Professionals
This is another in a series of articles in which BRAW submits questions to Drs. Whittingham and Dunn, professional Ornithologists teaching at the University of Wisconsin, Milwaukee. They are also conducting their continuing research work involving bluebirds, tree swallows, wrens, etc. in Wisconsin. We are very pleased that they have agreed to participate in this series of articles, and that they are members of BRAW's Board of Directors. Their advice and guidance, voluntarily given over the years, has been extremely useful and helpful to our membership. We thank them for all their participation in BRAW.
BRAW: Drs. Whittingham and Dunn, is there any evidence that Tree Swallows prefer, or thrive, or like to congregate around, or forage into areas with surface water?
Drs. Whittingham and Dunn: Yes, tree swallows absolutely DO thrive around surface water and actually prefer an area near water to one that is not. As you know, they eat flying insects almost exclusively and there are a lot of emergent aquatic insects in most water areas; midges, caddis flies and so on. So lakes, swamps, bogs, ponds, rivers, you name it, they love it ! And since tree swallows easily travel
several miles from their nest boxes to feeding areas, the boxes don't have to be right next to the water for the location to be very attractive to the swallows.
BRAW: Drs. Whittingham and Dunn, would you please comment on the difference, if any, between the concept of "territory" when the term "territory" is applied to Eastern bluebirds or to tree swallows?
Drs. Whittingham and Dunn: Tree swallows defend nest boxes or natural cavities as nest sites. Both sexes defend an area around the nest site within about 33 feet. If nest boxes are about 100 feet or less apart males will defend both boxes, trying to attract a female mate to each box. However, regardless of the number of boxes a male tree swallow is defending, they do not defend an area around the nest box as a feeding territory. Tree swallows feed almost exclusively on flying insects, and as a result, they travel widely, seeking out swarms of flying insects which are especially common around water where aquatic insects emerge at hatching. We attached radio transmitters to tree swallows to follow them when they were feeding during the breeding season. Our study took place at the University of Wisconsin-Milwaukee field station near Saukville, WI. We found that tree swallows routinely traveled about 3.7 miles radius from their nest box during the day to feed. Furthermore, we found that they often made several trips per day of about 1 1/4 to 3 3/4 miles each in different directions from the nest box. This behavior is in sharp contrast to the territorial behavior of bluebirds.
Bluebirds defend a territory around their nest box often referred to as an "all purpose territory" because it is used for mating, nesting and feeding. Territories are large and range in size from about 2.7 acres to 20.7 acres maximum. Size variation is most likely related to the abundance of insects in the area. In terms of food, the territory must provide enough food for the breeding pair and their nestlings. The primary food source for bluebirds during the breeding season is terrestrial insects (especially butterfly and moth larvae, grasshoppers and crickets) which they collect almost exclusively from the area defended around their nest box. Short grassy areas around the next box, where such food items are easily captured are preferred.
Overall tree swallows can breed quite successfully when nest boxes are close together because they usually feed far away from their nesting area. In contrast, bluebirds need a high quality feeding site in the area immediately surrounding their nest box because their territories provide all the food for the breeding period in addition to the nest site. As a result, bluebirds defend a larger area around their nest box (2.7 to 20.7 acres) than tree swallows, which defend only a small area around the nest box (within about a 33-foot radius).
References:
Gowaty, P.A. and J. H. Plissner. 1998. Eastern Bluebird. In: Birds of North America, No. 381, (Poole A, Gill F, eds). Philadelphia and Washington, D. C.: Acad. Nat. Sci and Amer. Ornithol. Union.
Robertson, R.J., B.J.M. Stutchbury and R.R. Cohen. 1992. Tree Swallow. In: Birds of North America, No. 381, (Poole A, Gill F, eds). Philadelphia and Washington, D. C.: Acad. Nat. Sci and Amer. Ornithol.
Union.