Wednesday, June 27, 2012

Bluebird Trail, Week 13 - Fledging Tree Swallows

Much of the action in the bluebird boxes lately has been from tree swallows. A couple of broods of house wrens have also fledged.

Sadly, about two weeks ago we lost the first brood of our second-round bluebird nestlings at just a few days of age. We had seen them as new hatchlings, but they were all dead in the nest when we returned the following week. We can only presume that something happened to the mother (possibly in the day of big storms that hit Northfield that week, though based on the nestlings' development we think it was probably a couple of days earlier) and the little ones perished without her. As we understand it, while the males help feed the babies, they do not enter the nest and brood them to keep them warm, which they need until their feathers come in and they can regulate their own body temperatures. So they really need their mothers for the first week or more. It was quite a shock -- our first bluebird casualties.

However, the tree swallows have given us much to observe, as the photos below reveal.

In the first photo are six tree swallow nestlings a few days from fledging. They are certainly crowded in the round Gilbertson nestbox, but all the tree swallow nestlings we've seen seem to figure out that heads pointing out is the best arrangement. We have not lost any tree swallow nestlings to overcrowding, though we understand that it can be an issue since they have larger broods than the bluebirds do.

Six tree swallow nestlings

In the next photo, tree swallow nestlings approaching fledging age were peeping out of their box, awaiting food.

Tree swallow nestlings looking out at the world

This past weekend we checked a box where we were pretty sure fledging would already have occurred, and we found two nestlings were still in the box. We must have caught them on fledge day -- or perhaps it took a day or two for all the birds to fledge. Dave checked again the next day, and they were gone.

Fledge day - two left

And here (below) is a shot showing the remarkable construction of the tree swallow nest, revealed when we cleaned out a nestbox after tree swallows (the ones peeping out in the earlier photo) had fledged. We found an unhatched egg in this nest -- in this one case, we'd never been able to get a final count of the eggs before hatching, so we were not aware until the end that one had not hatched. This was the location where I got the lovely photo of the mama incubating, surrounded by white feathers.

Tree swallow nest after fledging

Unlike bluebirds, which typically raise two or sometimes even three broods in a season, tree swallows are usually finished after one brood, though there are exceptions. (The tree swallow page on has more good information about tree swallow nesting and development.) So the nestboxes that are gradually being vacated by the fledging tree swallows will become available for bluebirds or other cavity nesters that are ready to lay again at this point in the summer, and the tree swallows will not compete for those nesting locations unless their first attempt has failed.

We try to clean the nestboxes out as soon as we confirm that fledging has occurred, to ensure a more sanitary and pest-free setting for the next nest (tree swallows leave pretty dirty nests by the time they're done), but it's been amazing how quickly a new tenant can move in. We recently came back to one nestbox intending to clean out a vacated tree swallow nest, only to find a complete new bluebird nest with an egg in it already, built right on top of the tree swallow nest.

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