Fall banding 2002 took place on 48 days from 15 August to 4 November. After a bone dry summer, rain came at inopportune times, and we lost 7 full and 1 partial mornings to rain, and several to wind. Early cold in October has us struggling with frost for the last 2 weeks of the season (frosty nets, when they can be opened, look like big white sheets). An average of 18 nets (12 meter equivalent1) were open an average of 4.25 hours per day.
We banded only 901 new birds and had 215 recaptures of 67 species A total of 1199 birds were netted (this includes birds released unbanded). Our capture rate was 32.4 birds per 100 net-hours. Previous fall means are 1182 new birds and 1511 total, and 54.9 per 100 net-hours. This was our slowest fall season yet; previously our capture rate has never been below 41 birds per 100 net-hours, and it has only dipped below 53 four times. While we typically have many mornings of over 50 or 60 birds, our best day was 8 October, when we handled 56 birds. We had only 3 days over 40 birds!
Numbers and trends – what does it mean?
Summer 2002 was the second nesting season in a row of very dry weather. We probably began the season with fewer breeding birds. An even more serious problem was the spring rain and cold, which coincided with the beginning of the nesting season. An examination of weather maps for the eastern breeding range of migrants that pass through our area showed that temperatures for the period from late February to early June were below average; the only exception was a warm spell in April that was most pronounced in our area (lat 42N) which became less pronounced further north and disappeared by about lat 48N. North of that latitude, temperatures were below normal for virtually the entire spring season. Temperatures were furthest below average around mid-March and again in late April through late May. Many birds delayed nesting (which meant they did not raise an extra brood) or lost their first broods.
The ratio of young to adult birds banded suggests this lack of productivity. We looked at 6000 fall birds of our most frequently banded species, and found on average that 85.6% are hatching-year (HY) birds. This year it was 79.8%. The difference is even more pronounced in American Robins, a species very common in our nets. Our catch is typically comprised of 83.8% HY birds; this year only 50%! This would seem to be weather related, as robins nest quite early in this region, and we had a wet, cold spell in late April (it snowed on 21 Apr) into early May. Meanwhile, Gray Catbirds, which nest a little later, actually showed up in our nets with a higher ratio of young than usual.
When we examined the variation in numbers of birds banded this fall from previous years, we found that nearly all species showed modest declines. The most accurate data can be obtained from the 15 species we band most frequently (Gray Catbird, American Robin, American Goldfinch, White-throated Sparrow, Swainson’s Thrush, Yellow-rumped Warbler, Song Sparrow, Hermit Thrush, Magnolia Warbler, Slate-colored Junco, Black-capped Chickadee, Northern Cardinal, White-crowned Sparrow, American Redstart, and Nashville Warbler). Comparing this fall’s capture rate versus the previous fall mean, two (White-crowned Sparrow, American Redstart) showed increases of an average of 25%. The rest showed decreases of an average 51%. Overall for this suite of birds, the deviation from the mean was -41.4%, which represents a decline of 358 birds. This does not appear to be biologically significant for a single-year occurance, and none of the declines is beyond what has appeared in the annual variations over time at our site.
A notable characteristic of this fall’s migration was that most migrants arrived later than usual (probably due to delayed nesting). Two of the species that decreased, Slate-colored Juncos and White-throated Sparrows, were late arrivals. By the time their numbers peaked here, we were stuck with frosty nets and falling leaves that dramatically decreased our overall capture rate.
There has been a great deal of concern over the impact of West Nile Virus on birds this year. If WNV was causing significant decreases in bird numbers, we might expect to see more dramatic decreases in particular species (since some are more vulnerable than others) or in birds nesting in particular geographic areas.
There was no taxonomical pattern to the decreasing species in our sample. Those 13 most frequently banded species which experienced declines are represented by 7 different families (Blue Jays were banded in normal numbers, but sample sizes are small).
There was no geographical pattern to the decreasing species in our sample. Of our 13 decreasing species, 6 nest locally and 7 do not. One might expect species nesting locally to have been harder hit by the virus, as the outbreak was considered moderate in southeast Michigan. Severity of WNV can be expected to decrease in northern latitudes. This is because the virus must reach a threshold in the mosquito population before many birds get infected and the virus can only overwinter in adult mosquitoes, which are not hardy in far northern latitudes. Thus, the virus must be reintroduced through the seasonal immigration of infected migratory birds, and build up again to the threshold level. Highest concentrations of breeding populations of our 7 non-locally nesting species were north of lat 48N, where the mean low temperature in January is -4F.
Our data are suggestive that the cause of the declines we documented in our banded birds this fall is more widespread and that would have a similar impact on a wide range of species, such as weather. The only thing our 13 declining species had in common was below-normal temperatures during the nesting season.
Two exciting birds were banded this fall. First was a Yellow-billed Cuckoo on 13 Sep. This was our first one, and represented the 118th species banded since 1992. The other was a Northern Flicker showing characteristics of both the Yellow-shafted and Red-shafted forms. You can see more pictures and text here.
We also did some Northern Saw-whet Owl banding, with 3 banded. Details here.
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- In order to compare different locations or years that may operate the same number of hours but with more or fewer nets, capture rate is calculated by “net-hours.” One net hour is one 12-meter net open one hour, or two 6-meter nets open one hour, etc. This rate is often expressed per 100 net-hours for more manageable numbers. ↩