At the end of last week I made it over to the 100-acre woods to finish cleaning out the rest of the nestboxes. There were fewer over there, just seven total in two fields compared to the twelve on this parcel. Most of the hundred acres is woods, though, with only a dozen acres or so given over to meadow or scrub. At least that I’ve noticed, all of the boxes are in the meadow areas. It would be interesting to put some up in the forest, though.
It looked like the boxes there hadn’t been cleaned out in some years. One of the main clues that suggested this to me was the above, found in the first box I opened. It’s a very old, very clean Tree Swallow skeleton (you know it’s a Tree Swallow because of, among other things, the many white feathers). This has clearly been there for a while – not only are the bones cleaned off, but the skull is even starting to break in spots. This was a nestling bird, not an adult; you can tell this from two things. The first is that it’s surrounded by many short feathers that are still in pin – half-grown, like a near-fledging-aged bird. The second is that the skull is paper-thin. Young birds grow their skulls in in two phases. While they’re in the nest and growing they lay down a first layer of bone as protection, and after they fledge they start working on a second layer. It can take three or four months or more for the skull to completely ossify. As a bird bander, it’s possible to part the feathers on the head and look through the translucent skin to see how much of the second layer has developed. Early in the fall this can be useful in definitively aging a bird as either an adult or a first-year individual.
In the second box I opened there was a dead butterfly. I thought at first that it was the wings of a butterfly, as in a meal that something had, perhaps brought back for the young just before they fledged. Then I noticed it still had a body. In inclement weather, and on cool nights, butterflies will look for sheltered places such as cracks or cavities. (You can actually buy butterfly houses to put in your garden for his purpose.) A nestbox might do in a pinch. This is a Common Wood-nymph, which are summer fliers; if it was an autumn species I might have suggested that it was killed by an early hard freeze one night. Since it shouldn’t’ve gotten that cold, I don’t know why it may have died in the box.
Box number three seemed to have been home to a paper wasp colony at some point. Paper wasps in the genus Polistes will build these small nests in sheltered places such as overhangs or cavities, and it’s not unusual to find one or two in a couple dozen nestboxes such as we have. Once the winter hits and the colony dies that’s it for this nest; it won’t be re-used, and can be taken down without concern. The queens produced by this nest went out and mated in the fall, then found a secure place to hole up for the winter. Their mother, sisters and suitors are all dead, killed with the onset of winter. Only these young queens survive. Come spring, they’ll search out a new spot and build the first couple of round cells themself. The workers raised from these will help their mother build more cells, and onward through the summer, until the fall when a new brood of queens is raised. As you can see, colonies rarely get very large. I counted 135 cells in this nest, which represents the total output for an entire summer.
See the pupal cases in this one, the dark ovoid things tucked into the grasses? I’m pretty sure these are the “cocoons” of blowflies, flies in the genus Protocalliphora. Nest parasites, the female fly lays her eggs in an active nest. When the eggs hatch, the larvae wiggle up through the nest material and feed on the blood of the baby birds. Now interestingly, the cases you see here didn’t produce blowflies. If an adult fly emerges from one of these cases, the case looks like the end was sawn off. These ones have tiny holes. They’re the exit holes of tiny parasitic wasps of the genus Nasonia that parasitized the larval blowfly.
I was quite surprised when I opened this box to discover a huge stash of milkweed seeds. The meadow surrounding this box abounds with milkweed, so it was an easy resource for the hoarder to collect. I’m not sure if this was the work of a bird or a mammal; species of both groups will store food. However, I lean toward mammal, as birds don’t typically put all their seeds in the same place when caching. Flying squirrels, on the other hand, often build up winter food caches in empty cavities, and I suspect this may be the work of a very busy individual. In any case, whoever this belonged to, they had spent a lot of time at it – that layer was easily half an inch (>1cm) thick.
I noticed the floor of that nestbox had a wooden insert, presumably to make lifting out the old nest easier. When I lifted it up in order to dump out the seed collection (the squirrel won’t be needing it now that the snow’s melted), I discovered this underneath. It’s a colony of carpenter ants, with their intricately carved tunnel system. There were dozens of dead individuals in the tunnels. I have to assume the excavation was all from one summer, and when winter came the thin walls of the box didn’t offer enough protection against freezing – but I’m making a guess there. Carpenter ants will hibernate over the winter, so it’s possible these were all simply dormant ants; I didn’t poke them or bring any home to warm up to check. I returned the wooden insert to the floor of the box and closed it up.