Climate Change Reduces Nesting Success of Local Songbirds
Several times this column has mentioned the chain of nest boxes along Putah Creek that the UC Davis Museum of Wildlife and Fish Biology oversees. It is one of the best things anyone has done for songbirds in the region. More than two hundred boxes now line thirty miles of the creek, from Fishing Access #4 in Putah Canyon downstream to Mace Boulevard. They have been so successful that Yolo Audubon members have taken the practice to the North Davis Channel, Woodland Regional Park Preserve, Slide Hill Park, Davis West Pond and who knows where else.
The boxes along Putah Creek are closely watched for what they can tell us about the nesting habits of the songbirds that use them. In eleven recent years biologists followed the fate of 2305 nests, including 1141 of tree swallow, 579 of western bluebird, 368 of house wren and 217 of ash-throated flycatcher. This is a solid, reliable number of nests from which to draw conclusions. And the conclusions do not portend well for nesting birds as our climate evolves.
Climate scientists expect two important shifts in Sacramento Valley weather as the earth warms overall. One is hotter springs and summers. The other, more surprising to me, is a shift of some of the rain from winter to spring. So prepare for both hotter and wetter springtime, which is when local birds nest. The peak of egg-laying for house wren is late April and early May; for western bluebird, about a week later; for tree swallow, mid-May; and for ash-throated flycatcher, late May and early June. Swallows and bluebirds often have a second brood, with a peak of egg-laying in late May and early June for swallows and mid and late June for bluebirds.
Of course, the weather differs from spring to spring. And the differences turn out to matter for nesting success in the four species. The researchers compared the number of young fledged in normal versus hot and normal versus wet springs over the eleven–year period. They found significantly skinnier nestlings and fewer fledglings in the years now called abnormal, years which will likely become the new normal if our climate stays on its current track.
So why does the weather affect nesting success? To begin with, nestling birds can’t self-regulate their temperature the way you and I can. Their bodies run hot and cold with the weather. Unusually hot days can overheat and weaken or kill them. Rain comes with cold weather, which also can weaken or even kill nestlings. For adult birds, extreme heat is a physiological strain which makes them less active and means fewer trips in search of food for nestlings. Rain grounds otherwise flying insects, and presumably reduces the food available for aerial hunters—including ash-throated flycatcher, tree swallow and western bluebird. Notably, wet springs did not reduce the nesting success of the flycatcher during the study, doubtless because it nests after any spring rain might occur.
That is the story in a nutshell. The full study was published in the journal Biological Conservation.
For explaining the many details of songbird nesting and climate variability, I am grateful to the lead scientist on the project, Jason Riggio.
The nesting season is getting under way this month. Go out and watch those nest boxes.
This male Western Bluebird has used the same box for three consecutive years
photo by Kelli O’Neill
— Michael Perrone, Conservation Chair