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BIRDWISE: Battle of the boxes – a matter of timing

The unlikely circumstance of nestings by multiple tree swallows and purple martins in a four-unit apartment complex have only been possible because of the timing of arrival of both conspecifics and potential competitors.

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Two first-year male purple martins and one tree swallow, at right, using my apartment complex.

Purple martins are large, dark-colored swallows that are most familiar to us as garrulous occupants of multiple-compartment nest boxes.

Although they are natural hole-nesters, nearly all martins in the eastern U.S. now nest in artificial boxes. Articles about local martin aficionados who have had success establishing substantial “colonies” of these birds on Long and Fish Hook Lakes have appeared in the Park Rapids Enterprise now and then.

Like most everyone, we love the chortling sounds (listen at https://www.allaboutbirds.org/guide/Purple_Martin) and aerial acrobatics of martins.

For four years, I’ve tried to establish our own colony by placing a modest, four-box apartment unit, purchased from the Purple Martin Conservation Association (https://www.purplemartin.org), over the water on our lake, which, to my knowledge, has no recent history of nesting martins.

To date, our results have been meager but not without interest.

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The first year a pair of tree swallows, much smaller cousins to the martins, appropriated one of the four boxes. No martins appeared and the swallows successfully fledged a brood of young. Like martins, tree swallows nest in either natural or man-made cavities. But unlike martins, tree swallows are much less tolerant of conspecifics (i.e., members of the same species) nesting close by. Their territorial behavior normally prevents conspecifics from nesting closer than 10 to 15 meters.

In year two, a pair of tree swallows attempted nesting, but abandoned it when disturbed by our first pair of purple martins, which arrived in early June.

The single pair of martins did indeed establish themselves, to our delight, and eventually fledged a brood of young on July 23.

Year three was pretty much a repeat of the first, with a successful brood of tree swallows and just occasional occurrence of martins but no effort to occupy a box.

2021 has been the most interesting of all, and has provided some insight into how the biology of these birds changes over time.

Our first takers, as usual, were a pair of tree swallows, about the middle of May.

Surprisingly, a second pair of tree swallows took over another box, while the first pair was incubating in theirs. I believe that the first male tolerated the new arrival only because his mate was far enough along in her reproductive cycle that the second male posed no threat. His territorial tendency, which is related to mate protection, had waned.

Best of all, purple martins arrived en masse (well, six) on the Fourth of July. By this time, the first pair of tree swallows had fledged, but the second pair of tree swallows was still nesting and gave chase to the martins often, preventing them from taking over their own box. If the martins had arrived earlier, the swallows might have left, but by this time they were too invested in raising their brood. The martins investigated all the other boxes, but didn’t evict the tree swallows.

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As of this writing, I believe a pair of martins is in the egg-laying stage in one box and an unmated male is desperately calling for a mate from another box. A third box has been investigated by another martin. The second pair of tree swallows continue to feed their young and appear to be doing fine.

The unlikely circumstance of nestings by multiple tree swallows and purple martins in a four-unit apartment complex have only been possible because of the timing of arrival of both conspecifics and potential competitors.

I believe hormonal changes in the nesting tree swallows during incubation and brood-rearing result in increasing tolerance of conspecifics and more aggressive behavior toward other species (e.g., martins) that could otherwise appropriate their nesting box and possibly evict the inhabitants.

Marshall Howe is a retired biologist with the U.S. Fish and Wildlife Service. He specialized in bird population studies. Howe has been a Park Rapids resident since 2010.

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