Barn swallows at Point Pelee, Ontario (photo from Wikimedia Commons)

12 August 2025

At this time of year barn swallows form flocks as they migrate south. During the day you may see them swooping through bug-laden air or perched on dead branches to pause and preen. Are they traveling with friends or in a flock of unrelated birds?

Barn swallow flock in South Africa (photo from Wikimedia Commons)

We assume that, unlike crows and geese, barn swallows don’t travel in family groups. However, an intriguing study published in 2015 discovered that at least one pair stayed together all year. This discovery was made possible when barn swallows breeding in Spain were fitted with small geolocators to find out where they went. Among them two breeding pairs were tagged and tracked.

The results of a study using light-level geolocators suggest that the male and female of a breeding pair could have migrated and wintered together, as they left their breeding area in northern Spain on 9 September, arrived to a same overwintering area in West Africa around 20–21 September, started the northward trip on 20 March and were back in the breeding site around 10–11 April, the coincidence in the temporal timing seeming highly unlikely to occur by chance for individuals that are not associated.

The screenshot below gives you an idea of the size and shape of light-level geolocators. For example, Lotek’s MK 5720 Geolocator for birds are quite small. The background photo shows a snow bunting wearing one.

MK avian geolocator on a snow bunting, the size of a paperclip (image from Lotek.com website)

The simplest geolocator analysis derives latitude from day length and longitude from solar noon, which is relatively straightforward (Hill, 1994).

When two mated pairs of barn swallows completed their journey the data provided pinpoint locations all the way to West Africa.

One of the tagged pairs stayed together on migration, at the wintering grounds, and returning to the breeding grounds. Another tagged pair did not.

There’s no way we’ll know if any North American pairs stay together year round, but it’s nice to imagine such fidelity.

Meanwhile, watch for barn swallows passing through our area on their way to Central and South America for the winter. They’ll reach peak numbers this week and taper off quickly. By the week of 30 August they’ll be scarce in southwestern PA and be gone completely by mid September, as shown on the 9 Aug and 30 Aug abundance maps below from eBird.

Barn swallow abundance, weeks of 9Aug and 30Aug, eBird Status and Trends