Friday, May 31, 2013

Tree Swallow

Tree Swallow Tree Disaster

Tree Swallows flit around my back yard a lot. They feed on the insects over the pond below the house. The Tree Swallows routinely investigate the bird houses on my fence every year but a couple has only chosen this site once in the last 8 years.

There are nesting boxes along the highway fence that borders the Elbow Valley Constructed Wetland and the swallows always nest in these boxes.

This year I happened upon a conventional nest along the Elbow River, about a hundred meters from where the Bank Swallows nest. I spotted a Tree Swallow flying into a hole in an old tree about ten meters back from the edge of the river. A flicker and and a Starling nest hole were within twenty meters of the site, so I am a regular visitor in the area.
The photo above shows a Tree Swallow in the nest hole. There were several excavated holes. The holes immediately above and below were not more than a few centimeters deep. There is another nest hole, not visible in the photo, to the right and a meter above the occupied hole.

The first observation of the nest hole was on May 26, 2013. The photos above were taken on May 27, 28, and 29. On May 30 I did not see the swallow in the nest hole.

This morning, to my great surprise, the tree was gone! Further investigation revealed that the tree had fallen down. The lower nest hole was facing downwards and to the side and the upper nest hole was facing right down and there was no reasonable access to this hole. There were obvious twigs (nest material) in the upper hole. In the lower hole (the one in the above photos) I could not see anything.

While I investigated the fallen tree several Tree Swallows landed on a living tree above me and they were clearly making a lot of excited noise. I presume these were the parents. 

I decided to try to rearrange the log so that there was easy access to both nest holes. After this was accomplished I realized that the lower hole would be flooded in rain, so I created a roof to protect this hole. The upper hole (with twigs evident inside, and therefore more likely than not a true nest site, had excellent access and no risk of flooding. The nest holes, however, are both at ground level, and therefore clearly at risk from four-legged predators.  
 Lower nest hole with log roof. 
Upper nest hole, with satisfactory orientation to avoid flooding. This orientation is no different than the side of the upright tree.

I wondered why the tree had fallen over. The old tree was dead and the stump was rotten. The tree was already leaning over. However, there was no real wind last night. Nearby I found a new empty beer can and also evidence where a ground log had been recently removed. Clearly some boys had been around. I suspect, sadly, that the boys pushed over this tree. 
Beer can. Evidence that boys were recently hanging out at the site. I hike this trail almost every day and the beer can was new. 
Site were a log on the ground was recently moved. The Elbow River is visible at the top left. Likely the boys lifted up this 8 to 10 cm diameter log and threw it into the river. 

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