by Carl Strang
Two days ago I reviewed flowering phenology at Mayslake Forest Preserve, which demonstrated that spring is progressing about two weeks ahead of last year. Today I want to consider whether migrant birds are showing a similar pattern. There are two questions that interest me. First, is there a difference between species that winter in the tropics versus those with at least a significant presence in the southern U.S.? If weather is a factor, we might expect the closer birds to be more responsive. Second, did the U.S.-wintering species arrive earlier in March and April this year than last? Here’s one of those species, an eastern phoebe, already incubating a nest at Mayslake.
As of the end of April, not many tropical migrants had arrived. All four of those that I observed were within 6 days of their last year’s arrival date (two were earlier, two later; median 1.5 days earlier). Since they are responsive to physiological clock and day-length signals that are the same between years, this is the kind of tight pattern I would have expected.
The 27 species that wintered in the southern U.S. showed a lot more scatter, with arrival dates ranging from 40 days earlier to 21 days later. The median difference was only 2 days later, however, which leaves me thinking that these birds, as a group, likewise did not respond to the early spring. This, like the flowering phenology, I will want to follow in future years, with the elaboration of looking at the data on a species by species basis.