Tuesday, September 11, 2012

The Tree Swallows (Tachycineta bicolor) of Middleton Island, Gulf of Alaska

The Tree Swallow (TRES) is a migratory passerine bird that breeds in North America and winters in southern U.S., Mexico, Central America and the Caribbean. They're the species of choice for many North American ornithologists pursuing songbird research. This blog entry presents the results of the first four years of a pilot project involving a TRES population and describes and discusses how these birds established themselves on a remarkable location. It will also look in the future of what possibly is one of the most isolated breeding populations of this species in North America.

Tree Swallows (photo: Chris Gates)

I first found out about the existence of the TRES in the summer of 2005, when I worked abroad, volunteering in the USGS seabird studies out on the small (2 x 7 km) and remote Middleton Island (59°26’N, 146°20’W) located in the Gulf of Alaska. Middleton lies about 110 km from the nearest mainland and 80 km southeast from the nearest island, Montague, which forms the southern barrier of the Prince William Sound. On May 26 of that year I observed three of these birds flying around at the island’s north end. These birds did not stay and the species did not breed on the island during that year, neither did the few birds that were observed by the end of May during a following stay in 2006. I expected that this species was absent from the island as a breeder due to a lack of suitable nest habitat, as their name implies a strong relationship with ‘perennial woody plants of reasonable height’. Middleton, however, is not completely treeless; there are several solitary or clustered windblown spruces scattered around on the island. I doubted that any of these contained cavities for these swallows to nest in. Apart from a lack of nest habitat, the island would probably be an interesting place for them, as there are many fresh or brackish ponds and plenty of flying insects, already satisfying the needs of about 65 - 112 pairs of Bank Swallows (Riparia riparia) annually between 2005 and 2010 (Van Nus, personal records).

Location of Middleton Island in the Gulf of Alaska (source: Google Earth)

Middleton Island is relatively low, flat, and has only a few trees (photo: Lauren Bessey)

Surprisingly, Rausch (1958), who thus far published the only complete breeding bird inventory for the island, mentions the TRES as a breeder in 1956, as he observed a few of these birds nesting in the remains of a building. I would later found out this building was a U.S. Coast Guard station located in the center of the island, which was only operational for a few years during World War II. This building got burned down in 1960, probably eliminating the species from the island as a breeder during the following years.

The former U.S.C.G. buildings where TRES nested in 1956 (photo: Armand Biron).

With an increasing interest in historical records of the breeding species of Middleton Island (I've been working on an avifauna for the island for a while now), I would later find out that during the following decades the TRES made irregular appearances as a breeder, with at least one pair utilizing cavities in a building within an abandoned FAA settlement in the north end of the island where it got recorded nesting occasionally during 1976 - 2002. Unfortunately for these birds this settlement got removed during the summer of 2005, again leaving them without a place to nest on the island.

Part of the former FAA settlement at the north end, removed in 2005
(photo: Tim van Nus)

It occurred to me during a stay in Maine (U.S. east coast), after observing a so called ‘nest box grid’ made up of several dozens of pole-mounted nest boxes placed in an open field, that the TRES on Middleton could possibly be persuaded to nest on the island again. All I had to do was place a few nest boxes! So, I gathered more information about the species, nest box design and - placement. Especially Chris Gates’ website (http://www.treeswallowprojects.com) was of great help, as it describes in detail how to start up a TRES nest box project.
Shortly after arrival on Middleton in early April 2009, I constructed seven nest boxes. The dimensions used were those recommended by the Golondrinas project (an international community of biologists studying swallows (http://golondrinas.cornell.edu)). However, in order to survive Middleton’s stormy conditions year-round, their overall construction had to be a lot more robust than the boxes used elsewhere in the world. Five boxes were mounted on existing wooden poles that hold a non-functional electricity wire in the centre of the island and another two were placed nearby and were mounted on steel pipes. With the absence of ground predators on Middleton I did not have to worry about predator guards.

Box 1, 2 and 3 at an individual distance of about 30 meters
(photo: Tim van Nus).

Instant success! – In 2009 the first TRES (three birds) were seen on May 25. Two of these made up a pair and were highly interested in box 6. Both of these birds were in full adult plumage. A third adult male showed interest in the other boxes, but this bird got accompanied by a second year female only much later, on June 12. These two birds also paired and occupied box 2. During an inspection of box 6 on June 18 a nest was found containing six warm eggs. These resulted in six chicks, which all fledged around July 8. The couple nesting in box 2 laid a total of four eggs, which were observed on June 28, and which resulted in the fledging of 4 chicks around July 21. All these birds seemed to have left the island by the end of July. So, during the first year of this project two TRES pairs laid a total of 10 eggs, which resulted in 10 fledging chicks!

One of the birds from box 6 observing the observer (photo: Tim van Nus)

Box 6 containing six eggs and many feathers of waterfowl
and Black-legged Kittiwakes (photo: Tim van Nus)

2010 – My expectations for the following summer were high; I expected more birds to occupy the boxes, as some of the known breeders could return, as well as some of their offspring and maybe a few more 'floaters'. In order to provide enough nest sites for the near future of the project I added another 10 boxes to the seven that were already there and had survived some severe winter storms seemingly without problems. This made a total of 17 boxes present on the island in 2010 and these are all the project contains until the present day. All boxes from this new series were painted white on the outside for extra durability and were mounted on steel pipes, except for one (box 8), which I mounted on one of two existing slanted wooden poles. These two partly burned poles are the only visible remnant of a U.S. Coast Guard station that once contained Middleton's first TRES nest in recorded history (Rausch 1958). Five boxes (# 10 - 14) were placed along a small water stream in the north end of the island where the former FAA station was located. Another three boxes (#15 - 17) were placed just south of the southern end of the island’s infrastructure, making sure that the birds had the opportunity to nest well away from each other, if they’d prefer. I ended this series by placing a box in front of an FAA weather camera; there are two of these cameras located near the island's airstrip, and both take a photo every 12 minutes to inform pilots about the local weather conditions. The box (9) is facing the southwest camera. These images can be viewed online, meaning that when not visiting the island, the swallows could be viewed from any office (http://akweathercams.faa.gov/sitelist.php). Coincidentally, the first TRES for 2010 got recorded by the camera on May 10, when it perched at the entrance hole of box 9.

 Box 9 facing the FAA weather camera (photo: Tim van Nus)

The first recorded Tree Swallow observation for 2010; the bird is that tiny black spot against the front of the nest box.
Only later did I find out that I'd better placed the box a bit closer to the camera...
(Source: Federal Aviation administration/http://akweathercams.faa.gov/)

Location of the 17 nest boxes on Middleton

After the observation of the first swallow at box 9 on May 10, a second individual was seen the same day. Eventually in 2010 three pairs were present on the island, of which only two nested successfully. These birds, however, decided not to use the box facing the camera. Instead one pair nested in box 6 (same box as used in 2009) and had two eggs on May 28. The second pair laid their first egg on May 30 in box 8. Both nesting attempts resulted in six-egg clutches and six fledglings, which all left their boxes during the first days of July. The presence of a third pair only got recorded twice around box 4 on May 20 and 22. The female of this pair was observed in several severe fights with the established female from box 6 and was not seen afterwards for unknown reasons, but due to the severity of their battles I suspect that she may have been killed by her neighbor. At inspection, their box (# 4) contained only a few straws and feathers, which for me is enough to count them in as the third pair for this year. To summarize: in 2010 three TRES pairs occupied the boxes, of which two produced a clutch of six eggs, resulting in 12 fledged chicks.

After its partner disappeared, this lonely male occupied box 4
for several weeks (photo: Tim van Nus)

Eventual contents of box 4 (photo: Tim van Nus)
Banding - In 2010 all of the 12 chicks were given a small uniquely numbered light-weight aluminum band before their fledging. These bands allow for individual recognition of the birds. If these birds would ever be re-caught or found dead, a lot of knowledge would be gained about the dispersal and survival of TRES from Middleton. Christophe de Franceschi and Kyle Elliot, both well experienced bird banders, helped out with the banding of all of the 12 young on June 26.

Christophe (left) and Kyle (right) busy banding the chicks from box 8,
under the careful eyes of the two breeding birds (photo: Tim van Nus).

Banding in progress... (photo: Tim van Nus)

Chicks of box 8, at about 12 days old (photo: Tim van Nus)

In 2011 I did not visit Middleton due to other obligations, but prior to the summer I asked the USGS seabird volunteers to keep their eyes out on the TRES development. Thankfully they did and by the end of the summer I was pleased to receive an overview of the results of their nest box inspections. It appeared that seven boxes contained at least some nest material, of which four boxes contained completed nests on June 10. Three of these nests contained eggs on this day (box 2: five eggs; box 4: five eggs; box 6: two eggs). During a second inspection on June 16, box 2 and 4 still contained five eggs, box 6 contained six eggs and box 7 contained a completed nest but no eggs. On these dates some nest material was also found in box 1 ('few straws'), 9 ('few straws') and 12 ('cup with no bottom'), but these would not result in clutches, neither did box 7. No inspections were made during chick stage, but the boxes were all checked after chick departure and their contents, as observed by them on August 3, gives good information about the TRES productivity for 2011: box 2 contained an empty nest, indicating that five chicks had probably fledged from this box. Box 4 contained an unhatched egg and the comment that three chicks fledged from this box (which leaves the fate of one egg unclear). Box 6 contained two dead chicks and fledging for the other four was assumed.
Based on these data I suspect that in 2011 at least five TRES pairs occupied the boxes, of which three produced eggs, resulting in a total of 16 eggs, and these birds fledged at least 12 out of 14 chicks. Unfortunately, no chicks could be banded in 2011. This year would be the first year that an unhatched egg and dead chicks were found.

The 2012 season: a good start, but a near total failure... - This year the TRES activity got monitored by Mark Baran and Kyle Elliot, who both visited Middleton as part of their seabird research out there. Kyle informed me the first TRES was seen on May 12. The first eggs were laid probably just after Kyle left the island for his mid summer break, but throughout the summer Mark did an excellent job in recording the development in and around the nest boxes and even managed to band the birds this season. 
In 2012, five pairs laid eggs during the first two weeks of June. I assume the earliest laying date was June 2, as five eggs were found in box 8 on June 7 (the largest clutch on that date; they lay one egg a day, I believe). On this date box 1 contained one egg; box 6 contained three eggs, box 9 contained four eggs and box 17 also contained four eggs. During the following inspection on June 15 most nests were found to contain completed clutches: box 1: six eggs; box 6: six eggs; box 8: six eggs; box 17: seven eggs (the first seven-egg clutch recorded for this project!). Box 9, however, only contained three eggs and one broken egg. Mark noted that this box had been abandoned by the birds at this time.
During the inspection of June 22 not much changed, but box 1 lost an egg (broken) and went back to five, box 8 contained the first (two) chicks for the season and one egg disappeared form the seven-egg clutch from box 17, which would later be found broken. A little bit of nest material showed up in box 4, but this would not result in a finished nest and was probably not the work of another pair. On this date Mark managed to capture three of the breeding birds from their nest for banding (from boxes 1, 6 and 8). With still four pairs, 21 eggs and two chicks on the list it would not appear to be a bad season.
Unfortunately, during the following chick stage the situation would turn very bad. Box 1 contained four chicks and one egg on June 30, but these chicks were all found dead on July 7 and the remaining egg would not hatch and was later found broken. By June 30, box 6 contained five chicks and an egg. Of these chicks four were still alive by July 3 and these were banded, but all were found dead in their nest by the end of the season and the remaining egg apparently never hatched. In box 8 five chicks were present on June 29, and these were all banded on July 3. During the final inspection on August 11, however, two of these chicks were found dead, resulting in three fledged chicks that left their box during the second half of July. On June 27, box 17 was found to contain five chicks and one egg (which would later be found broken in the nest). Of these chicks only three were still alive on July 7 and these were banded, but only two managed to fledge during the second half of July; the three dead chicks were removed from the nest box during the final and cleanup round on August 11. Based on this information, in 2012, five pairs occupied the nest boxes and these laid a record number of 29 eggs, but these resulted in the fledging of only five chicks. Besides these breeding pairs, there may have been one or more lonely males occupying nest boxes for a while, but without success.

Contents of Box 17 on July 7, 2012 (photo: Mark Baran)

Contents of box 17 on July 7, 2012: three live chicks and two dead; only two would fledge (photo: Mark Baran)

Results - an overview (2009 - 2012):

The first four years of this project have shown that, as throughout most of the North American continent, on Middleton Island the TRES can easily be persuaded to nest in nest boxes. Throughout Middleton’s recorded history, the TRES has shown to have a close relationship with human presence on the island, as the species appears to be totally dependent on human constructions for its nest sites. Without people’s activities, there most likely would not be any TRES nesting there. So, why would I put effort in establishing a TRES population on Middleton if the species would not nest there without the presence of man? Well, I could come up with several reasons:
- History has shown that there are TRES that apparently want to nest on Middleton. I'd like to help them doing so.
- As long as there is human activity on the island, I believe it’s good to do something positive for the bird communities by increasing the island’s breeding bird diversity. I expect the presence of a TRES breeding population to (thus far) be of a minimal influence on the other bird species present out there.
- Personally, after my first close encounter with this species in 2009, I believe they’re one of the most enjoyable species around. I was very pleased to see their playful flight and hear their friendly chattering calls above Middleton’s grassy plains, which otherwise can be a bit boring. I hope other people enjoy this too.
- Due to the island’s isolated location, size and the presence of an onsite weather station, Middleton Island would theoretically be a perfect place to study a TRES population. I am hoping that by having a descent number of these birds nesting there annually, monitoring these could make an interesting contribution to scientific research in general, in order to find out more about the island’s bird populations, as well learning more about the TRES itself.
- And finally, I don’t like bugs very much. The TRES eats loads of bugs, so as far as I’m concerned they’re welcome there.

But what about the TRES itself? Is life good for a TRES on the remote Middleton Island? At first sight, it seems that food appears to be widely available to them and with the absence of ground predators, only an irregular presence of almost only a single aerial predator species during the breeding season (the Peregrine Falcon, Van Nus, personal records) and the absence of competing species around their nest sites, a TRES can certainly have a good time out there. However, although the first two years of this study have shown that these birds can indeed have great breeding success, hatching a good number of eggs and fledging an equally good number of young, the results of this study’s third and especially its fourth year show that this is not always the case.
By now, the main cause of breeding failure seems obvious: with the exception of a some rainy periods during my stays on the island in 2009 and 2010, I suspect that continuing periods of bad weather that later field crews had to endure determined the relatively poor breeding results of the 2011 and 2012 seasons. After the almost disastrous 2012 season, both Mark Baran and Kyle Elliot informed me that bad weather conditions out there during the chick stage most likely were the cause of the poor breeding results in that year (and I can’t recall seeing a single sunny day when checking the FAA weather camera regularly throughout the 2012 summer). It seems obvious that eventually bad weather periods limited the foraging opportunities for the adult birds, resulting in starvation of many of the chicks. The relative large number of eggs that did not hatch in 2012 may also have been a result of the poor feeding conditions during an earlier stage of the breeding cycle. In 2012 the earliest egg laying date (June 2) was 7 days later than in 2010 (May 26), indicating that conditions were probably not very good at the start of the season. This is all not unique and apears to occur regularly among TRES populations elsewhere, i.e. Kyle informed me he noticed the same thing happening with the TRES nesting at the Long Point Bird Observatory in Canada and Alexandra Rose informed me she had a very bad 2012 season as well at Long Lake (almost exactly 100 miles northeast of Middleton), with massive nestling mortality. Unfortunately, only current weather data from the island appears to be available online and I can’t find a climate summery for any of these four summers anywhere, which would have been nice just to show you what the Middleton birds had to deal with during the first four years of this project.
But has, thus far, life been really so bad for them? Throughout the first four years of this study, the number of pairs laying eggs appeared to have gradually increased. Their eventual success on Middleton Island will probably depend on the overall climate conditions in summer. We can only hope for these birds that during future summers the climatic conditions will be better than they were in 2012, but they certainly will not always be.

Even with some very good breeding results and over the years still a good number of young fledging, compared to mainland colonies, the population increase that occurred during the first four years seems to be somewhat slow (I would love to compare this with data from the establishment years of other colonies, but I haven't managed to find this). Though the reason for the slow population growth has not been studied, I speculate that thus far the following scenario may have taken place:
Regardless of the bird’s breeding results in any previous year, the number of birds that nest on the island largely depended on a number of breeding birds from a previous year that managed to return to the island + an additional small number of ‘floaters’ (young, inexperienced birds) that shows up there in spring, usually late May. I do not know much about the annual survival of the breeding birds, but based on the records of the number of TRES that pass by the island in spring (before the establishment of this breeding population) I think that these could well account for the additional number of pairs that were found with each new year. I do not have many records of TRES observations in spring, simply because they were rarely seen, but because of the number of birds observed on these occasions (0-8, mostly 2-4, Van Nus, personal data), as well as the timing of the majority of these birds (last week of May) in relation to the initiation of nesting of the breeding pairs, I suspect that Middleton does not make part of the normal flyway for this species in spring and TRES would only show up there irregularly. With a number of birds arriving relatively early during the later three years of this project (May 10, May 12), I suspect that at least some of the experienced breeding birds are aware of their destination and these were deliberately heading to a Middleton Island nest box.
Whatever happens to the young that are born on the Middleton Island, thus far remains a mystery. As there appear to be no recorded observations for this species on Middleton in the fall, I suspect that Tree Swallows do not migrate above the Gulf of Alaska easily. I suspect it will not be easy for them to make their first fall migration, as they may have to cross several hundreds of kilometers of ocean. This makes me very uncertain about their survival, as well as their return rate to the island during a following year.

With the re-establishment of the TRES on Middleton, however, I am suspecting that with a number of these birds flying around there in spring, this could be a great attraction for floaters to the nest sites that are offered on the island, which otherwise may have been overlooked by these birds. Singing males will attract females, but possibly also other males. But again, these are all speculations, and not the result of a scientific study. The only way to get more insight in the way this remote population works is by continuing the TRES banding activities out there.
At this point the only thing I’m pretty sure about is that slowly more and more Tree Swallows are finding their way to Middleton Island and there are currently more pairs nesting out there than ever before.

The spring and fall migration routes that are used by Middleton's Tree Swallows remain unknown.
Do they take the shortest way? Do they fly hundreds of kilometers over the North Pacific Ocean?
Do the chicks all migrate south and find a watery grave?

The future of this project and priorities for further research 
How many TRES pairs Middleton Island eventually can host remains unknown. The number of Bank Swallows nesting there (65 - 112 pairs (2005 - 2010)) could indicate that swallows may be able to hold large populations out there, but as this is a different species, it will be hard to make a comparison. In fact, in 2009 the first two TRES pairs seemed to be doing very well, whereas during the same season the 65 pairs of Bank Swallows remained largely unproductive... (Van Nus, personal records)
With 17 nest boxes still present and only a maximum of about seven boxes occupied (including the non-breeding and lonely males) it seems that no new boxes need to be added yet. The people who kept their eyes on the TRES during the previous year informed me that the boxes are still in good shape and do not need to be replaced yet. As a side note: as mentioned before, the current boxes have been built by using the sizes recommended by the Golondrinas project. However, except for one. Throughout the first four years of the Middleton Island project, the swallows have occupied and laid eggs in seven of the 17 boxes (1 x in box 1; 2 x in box 2; 1 x in box 4; 4 x in box 6; 2 x in box 8; 1 x in box 9 and 1 x in box 17). According to this information, nest box 6 (shown on a photo above) has been the most popular box thus far, being used in every summer and producing more fledglings than any other box (16 fledglings in total). Box 6 was made about an inch less deep and the internal sizes are about half an inch bigger than the others. I suspect the birds prefer the dimensions of this box more than the others, which seems obvious for a secondary cavity nester, but it could all just have been a coincidence. In any way, these alternative sizes may be recommendable when adding an additional number of nest boxes in the future. 
The presence of the onsite weather station is a bonus for this project. However, it would be very useful when the gathered climatic data would actually be stored somewhere and when this would become available online (at least I'd never managed to find this).

Above all, it has become important that, in order to continue to generate good quality data, in future years a standardized method for monitoring the TRES population development on Middleton Island, in and outside of their nest boxes, gets applied. In order to get to know more about the structure of this population and the project’s future perspectives, during the upcoming years banding of the chicks and (re)capturing and banding of the adult breeding birds should be considered a priority. Middleton may be the ideal study site for a number of reasons, but this has yet to be proven.

Short video footage of the swallows of Middleton Island in 2010.

I thank all the people that were involved and that helped me out with the TRES-project thus far. Thank you for either helping building and placing some boxes or helping me conducting nest box checks or ringing chicks of the few pairs in 2009 or 2010 (Veronique Frochot, Christophe de Franceschi, Lauren Bessey, Joel White, Paul Solis and Kyle Elliot). During 2011, Lena Agdere, Mike Johns, Sharon van den Eertwegh, Chris, Lucy (sorry, couldn't find your last names) and Thomas Merkling all did a great job monitoring the boxes. In 2012 Mark Baran did a great job too, monitoring and banding the swallows during his rainy stay and he got thoroughly instructed by Kyle Elliot. I'd like to thank Scott Hatch (USGS) for letting me use the necessary power tools to construct 17 nest boxes from scratch and also for supplying the TRES bands. I thank Chris Gates for supplying me with the required information about starting up a project involving this fascinating species.


Rausch R. 1958. The occurrence and distribution of birds on Middleton Island, Alaska. The Condor 60: 227-241.


  1. Hey Tim,

    Really great stuff!!!!
    2 things that come to mind.....Looking at the photo of you banding chicks...have you ever thought about the effects of leaving the nest box open while doing so? We've always shut the box and sealed off the entrance hole (with a towel or something) so that the parents didn't see an empty nest. (precautions to reduce abandonment)
    Secondly....I didn't come across an obvious spot on your blog to find your contact information (email or phone number). Perhaps you want it that way, not sure.

    The work looks great! I'm jealous of your location.

    - Justin Proctor

  2. Hi Justin,

    Thank you for your reply; you have just successfully contacted me!
    Thank you also for the comment about closing the next box door during banding; these six chicks still fledged successfully, but next times I hope the box entrances can be covered.