I've noticed quite a bit of conversation in OBOL re the heat dome and resulting bushtit numbers. Rather than comment in OBOL, I thought I'd offer my thoughts here so that a larger audience can see them if they want. These are just thoughts. I have no answers!
First, every single banded adult at the eight nests survived the event. So, at least from that small sample, there wasn't any adult mortality. In addition, many other banded birds have been seen since. I'll have a better idea of adult mortality when the breeding season begins next month. But even then, I won't know the cause if the % survival is lower than usual.
We have found about 100 nests/year over the last 4 years in Portland. I have never had a nest with dead nestlings before. The only other time was just once in Arizona after a freak cold spell: 3 out of 4 banding-age nests contained dead nestlings. The only surviving brood had 3 adults feeding at (and sleeping in) the nest.
On the other hand, predation rates were exceptionally high this year in Portland. Personally, if there is a dip in bushtit numbers, I would expect that had a greater effect on the population than the heat dome, given that the heat came very late in the nesting season.
Bushtit numbers in the winter are hard to come by. As they are in large and mobile flocks, you either see them.....or you don't. Many CBCs miss bushtits. But that doesn't mean they aren't there! I think the best way to get an indication of the robustness of a population is to count flock size. Not estimate. Count. That means catching them as they cross a street or from tree to tree and counting them as they cross. Big flocks in winter=healthy population.
The problem is the occasional small flock doesn't mean much. This November and December, I found the same bird in both a monster flock of >50 and messing around for over an hour in a smaller flock of 10. So....she's was doing both. I think if you go out often and see only small flocks of <15 birds, that may indicate a problem.
The heavily-feathered bushtit nest is great at insulating against cold. But it also serves to protect against heat. I did a short study in AZ several years ago and found that the interior of a bushtit nest stays at a reasonable temperature even in full sun on a very hot day when the exterior nest surface temp rose above 120 degrees. In Portland this year, WT 12---the 3-bird nest that lost all 6 nestlings during the heatwave---was a small nest with very little insulation. It was mostly in the shade, BTW. The one nest with eggs during the heatwave was a pretty bizarre nest (the 3rd nest for the pair that year) with uncharacteristically thick sides and many feathers. Perhaps that saved it? Who knows. It was also partly shaded as were most of the nests at that time.
One more point: Nests that survived were significantly closer to water. Water is essential to prevent dehydration which is the primary cause of death in the heat for birds. We (Amit Gordon, Ian Connelly, and I) wrote all these observations up for publication and it is currently in press in The Wilson Journal of Ornithology.
Hope all that is of some interest! Comments and additional ideas welcome.
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