Here are just a few of the notes I made while monitoring Tree Swallows at the man-made San Joaquin Wildlife Sanctuary in Southern California this year.
The beginning of the season is always tedious. A plethora of measurements need to be taken, and the time it takes to monitor is dragged up by a hour or two.
This time isn't without its excitements, however. Nests often exhibit unexplained anomalies: this year I have observed two nest boxes with gymnosperm nest material, specifically an exotic introduced type of juniper. This seems to be an odd choice when much more conventional nesting materials are readily available. I wonder if the acidity or some other property of juniper may be responsible for the decision -- yet another topic to be marked for further research!
The Tree Swallows have begun the lengthy process of egg-laying and incubation. While monitoring at this time of the year, I'm always doubly excited to open the box, with high hopes that my fingers will lay upon warm eggs or an incubating bird rather than feathers and grass.
There is one nest box (A47) here that always strikes me as strange. Upon my approach to this particular box, a swarm of four or more swallows immediately springs from the surrounding area and raises a ruckus above my head. The nest also has an unusually large amount of eggs - six --which, in our population of Tree Swallows, is slightly out of the realm of a normal-sized brood.
It is rather far-fetched, but I wonder if brood parasitism such as that which happens with Barn and Cliff Swallows is going on here, or even something along the lines of cooperative breeding. I will have to mark this down as another mystery to investigate the next season.
Most of the nest boxes are now well on their way to raising a successful brood, except for C nest boxes, which have had been a great deal of trouble recently. Two nest boxes of Tree Swallow have took on it upon themselves to be rivals. Such aggression is rare in our population of Tree Swallows, likely since both food resources and suitable nesting locations are ample (the habitat near the C boxes seems just as good as any!).
A few weeks ago, the parents of one nestbox destroyed the eggs of another. The nest of the attacked box remained inactive, so last week someone recommended that I clean it out. Now, I have returned this week to find that the attacked Tree Swallows have built a poor and improvised nest--2 centimeters tall, with loose construction and few feathers--and laid two eggs on it. Given the gentle conditions and climate here, I will not be surprised to see the eggs hatch and chicks fledge successfully.*
*Postnote: They did.
It’s my last day in Alaska. Soon I’ll be returning to Southern California to again monitor the Tree Swallows of San Joaquin Wildlife Sanctuary. Today, I visited a lagoon near Anchorage and was pleased to notice a smattering of active swallow nest boxes throughout the boardwalk. What was not so pleasant was the voracity the swallows attacked me with. While I was simply standing on the walk ten feet away from a fledgling, a Tree Swallow came within centimeters of my face in its vicious dive-bombing attempts.
While the Tree Swallows back home dive bomb me as well, it it much more gentle and usually ends after the first two weeks or so of the season. They certainly do not come as close to my face and with such a venomous attitude as the Alaskan Tree Swallows did. I suspect the aggression of Tree Swallows varies greatly on the mildness of the environment. Alaska, even in summer, is a far harsher place than Southern California for Tree Swallows to nest, full of frequent bad weather and potential predators.
San Joaquin Wildlife Sanctuary is full of provided and constantly monitored nesting sites, in which there are always a dozen boxes left over. Since it is a located in the middle of the city and suburbia, it also lacks predators.
It's nearing the end of the season. With the banding of the last few nests has also come a strange phenomenon. Last week when I checked next box B26 to to see if the large chicks had fledged yet--they should have, but had not-- a juvenile Tree Swallow (with yellow gape and all) assaulted me. I guessed the young birdling was defending relatives, having fledged earlier than its siblings, or been a member of the previous brood of B26.
This week, however, when I banded D01, which is a nest box right next to B26, I was again confronted with a frantic juvenile Tree Swallow, who was far more concerned about me than any other swallow in the vicinity. Long after the parents had left, the juvenile continued to circle around me and utter alarms. D01 has not had a previous brood.
In all my cynical anti-anthropomorphism, I doubt that this was an act of altruism. If the attacker was indeed the same bird as the one that defended the adjacent nest box, an explanation might be that inexperienced fledgling Tree Swallows are anxious of “predators” in general, as they might be threats to multiple nest boxes, not just that of a neighboring nest box. In that way, driving away a predator from another nest box before the predator reaches their own could be advantageous.
Until the origin and number of the protective juvenile(s) are clarified, I'm afraid this conundrum will have to remain a mystery. Either way, the fact that a juvenile Tree Swallow shows concern for anything other than its belly is intriguing.