A pair of adult barn swallows., Maria Leung
A pair of adult barn swallows. © Maria Leung

First evidence of double-brooding by a Yukon bird – the barn swallow


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Donald Reid
Emeritus Scientist

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Boreal birds Boreal

Published

2020-09-21

Nesting pairs observed fledging two consecutive broods in one summer.

By Donald Reid and Maria Leung

Mid-August now, and signs of autumn are with us. Leaves are turning colour. Adult warblers, kinglets and sparrows, along with the young they fledged at least a month ago, have flocked together and are passing through on their way south. But on a number of farms near Whitehorse, adult barn swallows are still tending to nestlings, catching insects on the wing throughout the daylight hours. Why so late in the season?

This is the second year that we have observed barn swallows making these late summer efforts to raise nestlings. Part of a study of species at risk on Yukon’s farms, we have followed the nesting success of barn swallows on about 16 farms each year. The pairs that are still raising young in mid- to late August are either dealing with their second brood of the summer, or are re-nesting after having lost an earlier clutch to a predator.

Barn swallows are considered “threatened” under the federal Species At Risk Act, having declined significantly in the southern portions of their nesting range. Loss of nesting sites in older farm structures, declines in insect populations, and cold snaps during nesting are thought to be causing the declines, with mortality during migration and winter another concern.

We are particularly excited to have observed nesting pairs being able to fledge two consecutive broods in one summer. This is the first well-documented evidence for double brooding by a migratory bird in Yukon, although such double brooding is well known in barn swallows further south. In 2019, at least three of the 20 pairs we were following double brooded. In each case, there was only one pair of adult swallows on the farm in question. Laying their first clutches from June 3rd to 9th, they fledged these young by mid July (6-13 July). Very soon afterwards (15-21 July), the females laid a second clutch. One used a different, but pre-existing, nest within metres of the first. The other two used the original nest, though in one case the adults had built a second nest, about 50 cm from the first, before the first brood had fledged. And the adults fledged their second broods between 19 and 27 August. The first broods were 6, 5, and 4 individuals, dropping down to 3, 4, and > 2 individuals in the second broods. Nevertheless, the addition of a second successful brood was a major addition to the pairs’ reproductive outputs and therefore their chances of having young who would return successfully to nest in the north the next year.

In 2020, 4 of the 16 pairs we were following attempted a second brood. Three of these were the only pairs on the farm in question, and we are convinced that the fourth was a double brood because they nested within a metre of the first nest. One of the four attempts failed early on, as a deer mouse predated some eggs before the full clutch was laid. The other three were successful, with 5, 4, and 3 fledglings compared to 5, 5, and 3 fledglings from the first broods.

Double brooding is increasingly being documented in birds that migrate from the tropics to temperate and boreal regions to nest. However, it is tricky to document. Most species hide their nests, and adult birds are rarely marked for individual identification. Barn swallows, however, make relatively easy targets for study because they are habituated to people, and they nest conspicuously in and on farm buildings. Although we did not have marks, such as leg bands, on the adults, we were able to keep track of the adults’ behaviours, such as nest building and egg laying, by visiting frequently and using remote cameras. This, plus the absence of any other pairs of swallows on the farms, in most cases, gave us confidence that the same adults were raising both broods.

For all of this we are very grateful to the numerous Yukon farmers who enthusiastically allowed us on to their properties. Of course, these farmers are themselves keen on having the swallows around, not least because the swallows eat a lot of mosquitoes. Many farmers just like the fact that their barns and sheds are home to these colourful and determined bundles of energy. Some farmers are keen naturalists, and one had regularly observed a pair producing a second brood for several years.

Our cameras helped us figure out why some other pairs of barn swallows were still feeding nestlings in the second half of August. Predators, most often black-billed magpies and occasionally deer mice, took the first clutches. This happened at the egg stage. Deer mice seemed to take advantage of times when adults were away from the nest, specifically before the full clutch was laid and incubation started. We think that magpies aggressively forced the incubating adult off the nest and then ate the eggs. Soon after losing a clutch, we found what were likely the same adults building a second nest in the same building, and two of these at least were successful.

Biologists have long known that migrating birds try to get to the breeding grounds early in the productive spring period. There seems to be high value in starting to nest early, as long as one isn’t so early that the food one needs to make eggs is not yet available. There may be various advantages to nesting early including: making sure that eggs hatch before or during the highest abundance of invertebrate prey required by nestlings, fledging young with more time to mature and learn how to hunt before they have to start the risky process of autumn migration, and giving time for a re-nesting attempt if the first brood is lost. The option of double brooding is yet another advantage to add to this list. The fact that barn swallows can pull it off, even north of 60° latitude, makes us wonder whether other species can too. It also makes us wonder about the particular features of farm buildings and environs that help the swallows to do it, and that is a further part of our study.

After a successful winter and migration to the north, adult swallows can reproduce well on Yukon farms given the right circumstances. This is encouraging news, and will allow us to provide some ideas for how farmers and rural property owners can foster this species. We think that less use of pesticides and greater retention of natural habitats close to farms are helping the Yukon swallows. But farms with successful nesting one year are not always a nesting site the next year, so survival on migration and through winter in central and South America is still a conservation concern.