Adult male Tree Swallow on Middleton Island in 2025 (photo by Karim Bouzidi)
2025 was the 17th year for the Tree Swallow (TRES) project out on the remote Middleton Island. Thankfully, this year again the Institute for Seabird Research and Conservation (ISRC) had its field crew present on the island in spring and summer. Shannon Whelan and her colleagues, who are always very occupied with seabird research while out there, found time to monitor this year's TRES population that resides on the largely treeless Middleton Island in summer, solely in nestboxes since 2009.
As during previous years, the TRES season started off in spring, with a checkup of the nest boxes still present on this island, to see what condition the previous winter left them in and whether replacement is required or not. Last year, Shannon had taken GPS coordinates of each nest box and provided the following map:
Out of the project's maximum of 20 available nest boxes, one was still gone (#10) and two (#'s 12, 15) were not in good shape but still usable to the swallows (as we'll see later?). So, similar to last year, 19 nest boxes were available to the first arriving TRES.
Box 15: No decent nest cup means no nest? As pointed out in some earlier posts, it is often difficult to estimate the exact number of occupied nest boxes. To attract a female, males can sometimes bring in good amounts of nest material, if I recall correctly. But if she then decides to abandon the hovel because the newborn family would be better off in the adjacent Cedar villa (#16, photo below), had nest box #15 truly been occupied or not? Or officially given up on by the TRES after a change of wind direction? (Note the holes in the back wall) Anything could have happened here; I'd like to stay conservative and stick to 5 occupied nest boxes/"pairs" in 2025. I know I might be wrong.
Overview of nest box history and occupancy (note: total seasons with young could not be accurately determined for some boxes in some years, as the condition of the nest found was not recorded):
On Middleton, the nest boxes remain outside year-round. In a quest for the most sustainable nest box design, during the past 17 years the TRES have successfully been using various models of nest boxes. By now there are several different series from several different makers, different in their design and materials used, all of which have proven to be successful to some extent. Except for one model.
Acknowledgements: I thank Scott Hatch and the 2025 ISRC crew for keeping their eyes out for the TRES this summer, from recording observations to taking photos, with Shannon Whelan in particular, for still putting her time in the project and for producing sharper nest box reports by the year.
Results of the spring nest box inspection on Middleton Island (April 2025):
Out of the project's maximum of 20 available nest boxes, one was still gone (#10) and two (#'s 12, 15) were not in good shape but still usable to the swallows (as we'll see later?). So, similar to last year, 19 nest boxes were available to the first arriving TRES.
The first TRES sighting of the year was made on 6 May (2 individuals, by Karim Bouzidi, and again 2 TRES were seen by him the following day (Ebird.com)). This is the earliest TRES record for Middleton Island in recorded history. DeCicco et al. on spring migrants: earliest dates 12 May (one, 2012; KHE) and 14 May (eight, 1981; DDG). Thus far our nest box study's database contains an earliest arrival date of 10 May (2010) and several more before May 14. Perhaps from now on, on Middleton one should also be on the lookout for the first TRES in the first week of May. This year the first TRES visiting a nest box was seen on 10 May.
The crew could give little attention to the TRES during the breeding season, but (as in previous years) an inventory of the 2025 breeding population was made by means of a nest box inspection in early August, shortly after the swallows and their offspring had left the island for their fall migration:
Box 15: No decent nest cup means no nest? As pointed out in some earlier posts, it is often difficult to estimate the exact number of occupied nest boxes. To attract a female, males can sometimes bring in good amounts of nest material, if I recall correctly. But if she then decides to abandon the hovel because the newborn family would be better off in the adjacent Cedar villa (#16, photo below), had nest box #15 truly been occupied or not? Or officially given up on by the TRES after a change of wind direction? (Note the holes in the back wall) Anything could have happened here; I'd like to stay conservative and stick to 5 occupied nest boxes/"pairs" in 2025. I know I might be wrong.
Single unhatched eggs that remained in nests from which siblings successfully fledged were found in box 5 and 16.
However, box 14 contained a complete clutch of 6 unhatched eggs. Shannon reported they contained no obvious embryos, perhaps indicating the nest was abandoned only shortly after laying. A Peregrine Falcon was reported irregularly on the island in 2025 (eBird), but the cause of failure here will remain unknown. There are presumably still no ground predators on Middleton, but the TRES there has shown to sometimes be very hostile towards its own species.
But thankfully an unknown number of chicks appeared to have fledged on Middleton Island in 2025, from four nests. Hopefully enough to help sustain their population and have them find their way to The Rock in future years (photo by Éliane Miranda).
Distribution of Tree Swallow nest boxes on Middleton Island and their status in 2025 (note that the satellite map above, provided by Shannon, shows a much better picture of exact box locations):
But thankfully an unknown number of chicks appeared to have fledged on Middleton Island in 2025, from four nests. Hopefully enough to help sustain their population and have them find their way to The Rock in future years (photo by Éliane Miranda).
Distribution of Tree Swallow nest boxes on Middleton Island and their status in 2025 (note that the satellite map above, provided by Shannon, shows a much better picture of exact box locations):
Overview of nest box history and occupancy (note: total seasons with young could not be accurately determined for some boxes in some years, as the condition of the nest found was not recorded):
On Middleton, the nest boxes remain outside year-round. In a quest for the most sustainable nest box design, during the past 17 years the TRES have successfully been using various models of nest boxes. By now there are several different series from several different makers, different in their design and materials used, all of which have proven to be successful to some extent. Except for one model.
And that's this one, the 'Plastic roof' model. Three of these were produced, custom-made in The Netherlands, shipped all the way to Middleton and installed there in spring 2021, to replace lost boxes #'s 6, 9 and 11. In the past, those were VERY popular and productive nest boxes, but after replacement, they remain strangely unused.
This new series was mounted at the exact location as the previous boxes. This makes it likely the design that deters the TRES from occupying these. Finding and fixing these flaws by making some adjustments to these three particular boxes would be a valuable activity for the future year of the project.
Acknowledgements: I thank Scott Hatch and the 2025 ISRC crew for keeping their eyes out for the TRES this summer, from recording observations to taking photos, with Shannon Whelan in particular, for still putting her time in the project and for producing sharper nest box reports by the year.
