Wednesday, January 14, 2015

The Tree Swallows (Tachycineta bicolor) on Middleton Island in 2014

2014 was the sixth year of the Tree swallow (TRES) nest box project on the remote Middleton Island. The first nest boxes were put up there in 2009, allowing the projects' first two breeding pairs to nest there successfully that same year. Until 2013, the presence of 17 nest boxes resulted in an annually increasing number of breeding pairs. For the past four years the boxes were monitored by volunteer - and professional ornithologists from the I.S.R.C. (formerly USGS) who visited the island as part of the ongoing seabird research that takes place on the island. Initially I was concerned not receiving any TRES data for this year, as Middleton did not get visited by researchers during the summer months. Luckily, several different people visited the island in April and August and these were nice enough to pass by the boxes and inspect these thoroughly for their contents.

Prior to the swallow's arrival
The presence of people in April allowed for a box cleanup before the first swallows would show up (last year these had not been cleaned by the end of the breeding season). This not only gave interesting additional information about the 2013 breeding season, but would also provide the 2014-birds with clean boxes. On April 19 and 21, all boxes were emptied out and cleaned. Old nests were removed and photographed.

Old swallow nests from the 2013 season.

Together with the data collected in 2013, the data revealed that in that year a total of 12 boxes contained at least some nest material. Of these, 10 contained (beginnings of) nests, of which 7 contained eggs and 6-7 contained chicks (of which 22 got ringed last year, from 4 nests). No unhatched eggs or dead chicks were reported, making it seem likely that in 2013 a record number of up to 40 chicks fledged!

One of the old nests which resulted in fledged chicks.

Box 17 during the April cleanup. The thin metal pipe is to provide a perch to the swallows and prevent Bald Eagles using the boxes as a perch at the same time.

The nest boxes appeared to be still in good condition. I was informed box #1 was missing a nail, but no further problems were reported. It seems they survive Middleton’s harsh conditions quite well. After the cleanup in April the boxes were ready for the upcoming breeding season.

The 2014 breeding season...
Nobody would inspect the nest boxes during the breeding season this year, so all I could do was occasionally check the status of box # 7, the nest box facing the FAA weather camera. Based on the regular nest box visits made by a few swallows, it got occupied.

Thankfully, in August Middleton got visited again by seabird researchers. On August 12, Martha Hatch inspected and cleaned out all 17 boxes. Box #7 indeed contained a nest from which young fledged, but further results were rather surprising: only 6 boxes contained at least some nest material. Of these five contained (beginnings of) nests (not just a bit of nest material). Of these three had contained eggs and chicks. Although two unhatched eggs were found, no dead chicks were reported, so most of these will probably have fledged.

Based on this data, it can be concluded that in 2014 the TRES on Middleton Island had a poor breeding season. Their breeding population decreased for a first time, with at least 50% compared to the previous breeding season. In numbers, the 2014 population was similar to 2011. Exactly how many chicks fledged remains unknown.

A possible explanation for this year's poor breeding results was given by Scott Hatch, who commented: "Lots of cool, wet weather which probably kept the insect supply down". I took a brief look at the historical data collected by the island's weather station (see the link for this elsewhere on this page). Strangely, during the period of the TRES' arrival and establishment (10 - 25 may) and the period in which the first eggs are laid (26 may - 2 June, based on data gathered during previous years) the average daily temperatures were higher than during the record TRES year 2013. At the same time the amount of daily precipitation in this period was lower or equal to that 2013. Did in spring the climate really prevented more TRES to start up their breeding cycle on Middleton Island?

A bit of an odd figure: daily average temperatures and precipitation totals on Middleton Island in 2013 and 2014. The yellow line indicates the period of arrival. The green line indicates the period during which the first eggs are laid, based on data from 2009 -2013 (read from right to left).

Alexandra Rose operates a TRES-project at Long Lake, almost exactly 100 miles northeast of Middleton Island. In 2012 chick mortality was very high at Middleton due to poor weather conditions and this was also the case at Long Lake that year. Alexandra informed me that in 2014 her swallows had a pretty good year, as far as the weather goes. They had a pretty mild summer with no real severe cold snaps. The past several summers had been terrible in terms of overall box occupancy there, but 2014 turned out to be a good year for the swallows.

Unfortunately, climate conditions consists of many more parameters that could have influenced the swallows on Middleton, like wind or fog, which I did not look at (although all of these should have been recorded by the island's weather station). Conditions prior to the bird's arrival may also be important, as these will be crucial for the startup of insect activity on the island.

The direction of the wind may also be of influence, as this could determine how many swallows eventually end up on the island. Before this project started the number of TRES observed on the island annually during spring migration ranged from 0 - 8. Maybe the birds nesting on Middleton in 2014 were primarily the 'hard core' Middleton birds and no or hardly any 'floaters' found their way to the island? Were these swallows pushed more towards Long Lake this year?

The number of unhatched eggs that Martha found in the boxes in August (2 among 3 nests, more than usual) indicates that poor weather conditions could have affected the early stages of egg-laying and breeding, but would this have resulted in the establishment of fewer birds?

Besides climate, other factors such as winter mortality and predation could have been involved. High winter mortality, however, probably wouldn't allow for the high box occupancy recorded at Long Lake this summer. With the absence of ground predators and only an irregular occurrence of potential aerial predators (primarily Peregrine Falcons) in past years Middleton's swallow population has always been relatively free from predation. The fact that none of the nests found in August show clear signs of being aborted by the birds (the nests that were finished all resulted in fledged young) and the distribution of occupied boxes is similar to the first years of the project (they all used the central cluster of nest boxes and the one closest to this, the northern and southern nest box clusters were not used this year), however, indicates that predation probably was not an issue this year. I am very much getting the impression that in 2014 fewer birds found their way to Middleton Island, resulting in a smaller breeding population that year.

Unfortunately, without people present on the island, more constant ringing and ring-reading efforts, any questions will remain difficult to answer. I can only hope that during the upcoming breeding season the swallows get the attention they deserve.

For the last 4 years, field work for this project has all been conducted on voluntary basis, not by me, but by seabird researchers that visited the island. Many of them I don’t even know personally. I must thank them all for their efforts!!

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