Thursday, September 28, 2017

The Tree Swallows (Tachycineta bicolor) on Middleton Island in 2017

2017 was the ninth year for the Tree Swallow (TRES) research project out on the small (8x2km) and remote Middleton Island, located in the Gulf of Alaska (59°26′59″N 146°18′26″W). It’s one of the most isolated breeding populations and research sites (that’s thus far assumed to be largely predator free) for a beautiful bird that is considered a ‘model species’ in science. This year the Institute for Seabird Research and Conservation (ISRC) provided information on the TRES breeding season after a single nest box inspection (and cleanout) after the breeding season, of the 20 boxes currently present on the island. No in-depth study on their breeding biology this summer, but basic data on box occupancy and an indication for reproduction in 2017 instead. Some additional info and photos came from ISRC 2017 crew members. On 19 September Scott and Martha Hatch recorded the following nest box contents:

1. nothing
2. nothing
3. nothing
4. complete feather-lined nest with feces
5. nothing
6. complete feather-lined nest (feces?)
7. complete feather-lined nest with feces
8. complete nest with 3 dead, nearly fledged chicks
9. large straw nest base but no feather lining
10. nothing
11. complete nest with 5 dead, nearly fledged chicks
12. nothing
13. nothing
14. complete nest with 1 unhatched egg, also feces
15. nothing
16. nothing
17. nothing
18. nothing
19. nothing
20. nothing

(Complete nests with feces are expected to have resulted in fledged young)

Besides these data, I received four photos and some opportunistically noted observations of the birds around their boxes, in particular box 7 (the one facing the FAA weather camera), from which chicks fledged, which seemed to get visited by a TRES as late as July 17 (also the last recorded sighting of a TRES that I received).

Another year added... (By the way, after analyzing the data again and for accuracy I adjusted the number of occupied boxes for 2013 from 10 to 9 (not counting a few straws as an occupied box anymore)).

Distribition of occupied nest boxes in which eggs were laid in 2017.

The FAA weather camera ( was offline throughout most of the summer, due to the installation of a new camera and further maintenance, but it’s up and running agian now and box # 7 is still visible.

Discussion (& some serious speculation...)
2017 was a relatively good year as far as the number of nesting TRES pairs goes. Total reproduction remains unknown, but was likely relatively low due to chick mortality occurring shortly before fledging. During the previous years of the project chick mortality has been low and dead TRES chicks were a rare sight, at least for me. The thought that came to mind among some was that perhaps a predator had been picking off the adults this summer, cutting off food supply to their young. Has predation become an issue for the largely predator free population? ISRC crew members observed a Peregrine Falcon Falco peregrinus, the first predator that would come to mind, on the island but only in May. Additionally, I suspect that an adult TRES would not be an easy prey for a falcon, while around the same time the island would be full with other, more easy prey types (i.e. young sparrows, snipe?) (But Peregrines will lose many larger prey items to the Bald Eagles Haliaeetus leucocephalus there, perhaps the reason they're not nesting there, and this could increase predation risk for the TRES(??)). But no Peregrine was seen in June and July, when the swallows have young.

Previously, on Middleton chick mortality only had a significant influence on reproduction in 2012. The chicks involved that year were also large and close to fledging. Bad weather (cutting off the adults from their food supply) was the obvious cause that year. When looking at the historic climatic data for 2017, the end of June/first half of July appears to be a good period for TRES ( Unfortunately data from this on site weather station appears to be incomplete again and therefore unreliable). The only significant change to bad weather seemed to occur around July 14 - 18 (complete days missing here), but it's the type of weather that I would expect to kill TRES chicks. During my stays, however, TRES chicks fledged by the end of June/first half of July, so were these boxes still occupied by the time real bad weather appeared? Perhaps they were, since the last sighting of a TRES made by the 2017 crew was on July 17, of a bird exiting box # 7. On Middleton, the TRES do not appear to visit their boxes after fledging (and box 7 resulted in fledged chicks). And when looking at the historical data of this project, first eggs can be laid as late as June (June 2, in 2012), which could indicate that boxes can still contain chicks during the second half of July. So, with only little data to support this wild speculation, 2017 was also a late breeding season. It would be interesting to compare these thoughts on the TRES breeding season again with whatever happened at Long Lake (the nearest TRES study site in Southeast Alaska) this summer, but unfortunately I don't seem to be able to get in touch with the responsible there.

Update 10-25: here's what Chris Gates ( wrote me:

Hi Tim,

Thanks for Middleton update. It's always nice to hear some of these hardy birds made it to the island and nested successfully. I put some of your information about the blog and links to it on the facebook group Tree Swallows, so others here in the US can learn of your efforts.

I do have a comment regarding the possibility of predation on TRES by peregrines and other falcons. In eastern North America TRES form very large flocks after nesting, which usually stage for the summer months in and around large marshes. By late summer and autumn the flocks begin to migrate south with heavy concentrations along the Atlantic shore. Here, the swallow flocks are followed by falcons, peregrines and especially merlins, which apparently take them with relative ease. I've attached a photo of a merlin "packing a lunch." Although the swallow in the photo is an adult I expect hatch-year swallows are heavily targeted. Also, in one of my favorite books, "Curious Naturalists," Niko Tinbergen describes behaviors of hobbies hunting swallows along the North Sea coast. So I guess if peregrines or merlins happened to summer at Middleton some year they could be potential predators of both tree and bank swallows there.

Regards, and thanks again,

Chris gates

(I've seen Merlins there regularly in spring, but never in summer. I recall installing two artificial stick-nests for the species in 2006, hoping to attrack a pair...) 

A Merlin taking a TRES... (photo supplied by Chris Gates)

A big thank you to Scott and Martha Hatch for their participation in the project, as well as to the 2017 crew members supplying some additional information and these photos of the 2017 MDO TRES breeding season!!

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