Ok, I have a confession to make. I’ve been getting quite impatient waiting for the Tree Swallows at the Salmon Creek nest box grid to start nest building for real. After all, swallows have been here nearly seven weeks now, and pairs began forming six weeks ago, but aside from a few bits of dead grasses there’s nothing in any of the eight boxes remotely resembling a nest. So, to see if my impatience was justified I decided to review records from the past nine years to determine if nest building truly is abnormally late this year.
First, let me explain how I define the start of nest building. As we’ve seen, female Tree Swallow often bring a few bits of vegetation into their nest cavities early in the season. These have been termed “claim grasses”, but if they are meant to claim a cavity the claim certainly isn’t honored by other swallows. A pair must often spend weeks repelling other Tree Swallows who would like to kick the original owners out, claim grass or no claim grass. Personally, I believe these early bits of vegetation signal a brief post-migration transition into the nesting season, a slight tweaking in the female’s hormonal balance, nothing more. The shift to nest building comes several to many weeks after arrival when, like a motor that sputters and sputters before it catches, the females rather suddenly start bringing large amounts of vegetation into their boxes. It really is like an internal switch has been thrown.
When, in checking boxes, I find a substantial ring of vegetation around the outside of a box’s floor, a ring that wasn’t there the previous day, I label that day the start of nest building in that box. And when the female has covered the box bottom with vegetation and formed the cup that will hold her eggs, I note in my records that her nest building is complete as of that date. I do not include feather-gathering by males or females in this formula, since feathers may continue to be added for many more weeks.
So, here’s some statistics. We had 86 nestings at Salmon Creek between 2003 and 2011. Four of these had takeovers that interrupted nest building, and 16 were late nestings in boxes added in May to reduce competition. But of the remaining 66 nestings where nest building began “normally”, 50 of them, or 76%, began during the eleven day period between 4/29 and 5/9. In other words 2012 could still easily fall in the normal range if most of our females begin to build in the next ten days. And actually there were some encouraging hints today that nest building is about to start in earnest. As I sat with the swallows at Box 2 the female began gathering dried grasses vegetation from the ground, and she went right into high gear, bringing piece after piece into the box. For the half hour I watched she stopped only to help the male ward off floaters or to dash away in response to raptor alarms. Box checks later revealed some additions of vegetation in Boxes 1 and 3 since yesterday also. I think the female Tree Swallows of Salmon Creek are on the verge of an intense bout of nest building and, if so, they’ll be right on time after all.