Background Notes: San Joaquin Wildlife Sanctuary, located in Southern California, is a man-made marsh in the middle of sprawling city and suburbia. The sanctuary defies all odds assigned to its unlikely location; it is a paradise of wildlife. Nest sites are provided for multiple species of birds via boxes, and there is monitoring of nesting sites for wasps, ants, mites, and various other offenders on a regular basis here. Certain areas have been planted to cater to birds and insects. As you can probably guess, the birds thrive here.
2016, Early March
It's been a long five months without practice. Today, I reviewed the banding and monitoring protocol of the Tree Swallow program at the marsh. Last year, the curator of the project informed me that the program has revealed unusual breeding behaviors of the birds I’ll soon be working with. Apparently, in this Tree Swallow population, males incubate. I wonder what new discoveries I can contribute here.
The beginning of the season is the most tedious. A plethora of measurements need to be taken, and the time it takes to monitor is dragged up by a hour.
This time isn't without its benefits, however. It is interesting to observe the nest material each bird uses for its nest. 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 the much more conventional nesting material (dry grass) is readily available. I wonder if the acidity or some other property of juniper may be responsible for the decision. In fall when the season has concluded, I will have to go back on the data and see if there is any difference in mortality rates or brood size of the juniper nests.
*Extra notes: Both boxes are located in close proximity of each other, and both of them are close to the busy road.
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. Whenever I open it, a swarm of four or more Tree Swallow immediately springs from the surrounding nest boxes 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 may be 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 something to investigate the next season.
Most of the nest boxes are now well on their way to raising a successful brood, except for the 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, I would guess since both food resources and suitable nesting locations are ample. 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 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.
While half-way through my usual swallow monitoring route today, I was puzzled to find a nest box missing. After five minutes of searching for it, I found it half sunk into the water, with dozens of large Asian Carp circling ominously around it. Strangely enough, although the nest in this box was well-built with many feathers, no eggs had been laid in it for 1.5 months after its completion, and luckily so. The nest was easy to clean out.
Other broods are well on their way to fledgling. However, I have noticed a high rate of chick mortality this season compared to the past season, namely the mortality of chicks around 5-7 days old. Strangely enough, the nests of the dead chicks almost have no signs of mites or ants and the dead chicks almost always comprise 40% to 60% of the brood, with several healthy surviving siblings successfully fledging. A random birder mentioned to me that there was a fatal fungus going around the Tree Swallows of the marsh. I could not find confirmation from any source, but it might explain the mysterious chick deaths.
It’s my last day in Alaska and soon I will be returning to Southern California to again monitor the Tree Swallows of San Joaquin Wildlife Sanctuary. Today, I visited Westchester Lagoon near Anchorage and was pleased to notice a smattering of active Tree Swallow nest boxes throughout the boardwalk. Far more jarring was the voracity the Alaskan Tree Swallows attacked me with. While I was simply standing on the walk ten feet away from a fledgling, a Tree Swallow began non-stop dive-diving me, coming within centimeters of my face.
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. The parents certainly do not continue their aggression after the chicks have fledged. I suspect the aggression of Tree Swallows varies greatly on the mildness of the environment. Alaska, even in summer, might be considered a harsher place than Southern California for Tree Swallows to nest, full of frequent bad weather and more potential predators.
San Joaquin Wildlife Sanctuary is full of provided and constantly monitored nesting sites, in which there are always a dozen or so quality nest boxes left over. Since it is a man-made marsh in the middle of the city and suburbia, there is also a shortage of predators.
It's nearing the end of the season. I banded several nests today, and also noticed a strange phenomenon that has been on my mind for a while. Last week when I checked a nest box a few feet away from D01 to see if the chicks had fledged yet (they had not), a juvenile Tree Swallow assaulted me. It was hard to clear to get a good look, but it appeared to have hints of a yellow gape.
I was confused by the identity of the fledgling, but concluded that it had fledged earlier than its siblings, or been a member of the previous brood of the nest box I was checking, and was simply defending its relatives. It must have stuck around only because its parents were continuing to feed it.
This week, however, when I banded D01, I was again confronted with a frantic juvenile Tree Swallow, who was far more concerned about me than the actual parents of the 8-day-old-chicks I was banding. Long after the parents had left, the juvenile continued to circle around me and utter alarm calls. As far as I know, D01 has not had a previous brood, although since I was gone shortly during the season I could be wrong. I will have to check.
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” (humans) 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.
However, 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.
Another thing to mark down for next season, I suppose.