Thursday, October 15, 2015

The Tree Swallows (Tachycineta bicolor) on Middleton Island in 2015

Tree Swallows near the military pond. Middleton Island, June 23 2015 (photo by Angelika Aleksieva).

2015 was the seventh 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. Before these boxes were placed the TRES nested on the island irregularly between the 1950’s and 2002 in abandoned buildings, probably no more than a single pair. All of these buildings got removed before 2006, leaving the swallows with no place to nest. From the first two occupied boxes in 2009, the TRES population gradually increased to 10 occupied boxes in 2013, but it subsequently dropped to five occupied boxes in 2014.

Tree Swallow peeking out of nest box 7. Middleton Island, May 30 2015 (photo by Angelika Aleksieva).

During the breeding season of 2015 the boxes were not monitored (though I did finally receive some fantastic photos of the birds occupying them!), but thankfully by the end of the summer Scott and Martha Hatch inspected and cleaned out all of the 17 nest boxes and reported their findings to me (as they did last year). Their data show that in 2015 at least 7 boxes got occupied, 6 of which contained eggs and resulted in fledged young.

Nest box contents as found by Scott and Martha after the breeding season (nests need to contain a feather-lined nest cup in order to count as being occupied by a pair).

Scott and Martha provided photos of each of the nest boxes’ contents, making it easier to distinguish the presence of true nesting pairs from boxes that were only temporarily occupied (often by a solitary male).

An ‘unoccupied’ nest box: little nest material indicates this box was not in use by an established pair. Middleton Island, August 24 2015 (photo by Scott Hatch).

Nests from which young fledged are characterized by the presence of feces. Middleton Island, August 24 2015 (photo by Scott Hatch).

Distribution of used and unused TRES nest boxes on Middleton Island in 2015.

Unfortunately, due to the absence of nest box checks during the breeding season, no information is available on clutch - and brood sizes for 2015. However, the information gathered by Scott and Martha does show that the 2015 TRES population managed to recover from previous year’s low (as far as the number of breeding pairs goes). Although the numbers of eggs and fledged young remain unknown, the number of egg laying pairs and those fledging young in 2015 got close to the known record year (2013).

To what extend weather conditions were of influence on this breeding season remains unknown. Unfortunately, early in the season Middleton’s on site weather station stopped recording precipitation. This seems to be a reoccurring problem. For future analysis it would be useful to have access to correct climatic data for each breeding season.

Only time and continuing inspection efforts will tell how this remarkably remote and isolated TRES population will develop in the future. For this, the current 17 nest boxes will also need to stay available to the swallows. The photos that Scott and Martha took show that 6 – 7 years of Middleton’s harsh weather conditions have started to affect the boxes. In order to have them last for many more years, a paint job will become necessary in the near future.

Additionally, it could be questioned whether the number of nest boxes determines the maximum number of breeding pairs on the island. Although the boxes have been spaced by the recommended minimum of 100 feet (28 meters) from each other (with the exception of one or two boxes), a look at the maps of occupied boxes of the most successful years (2013 and 2015) indicates that these birds have a preference for nesting as far away from other pairs as possible. During my stays on the island I also noticed that some birds defended areas around their nest box much larger than the recommended 100 feet between two boxes. This could negatively influence the number of available nest sites and the total number of breeding pairs. Because of this, I doubt whether the 17 nest boxes will ever result in 17 nesting pairs. In order not to have nest box locations limit the number of breeding pairs, 2 – 3 additional boxes should be placed, away from the existing grids and solitary boxes (this would be preferred to replacing existing nest boxes). This might allow a possible further increase of the population to take place and be noticed more easily.

Tree Swallow carrying insects for its young. Middleton Island, June 23 2015 (photo by Angelika Aleksieva).

I thank Scott and Martha Hatch for their help, as well as Angelika Aleksieva for the photos she sent me.

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