Notes on reduced bird numbers, winter 2002-2003

[Originally posted to “Birders@umich” listserv, 25 Apr 2003]

Many people have commented on the reduced number of birds at their feeders this past winter.  I have mentioned before several factors that help to explain this.  First, numbers may not really be reduced, but are perceived to be, especially due to heightened awareness after media reports that bird populations are in danger because of West Nile virus.  Second, birds may be reduced in your yards, but the birds are elsewhere.  Finally, some species have experienced reductions in numbers because of environmental factors, in particular two years of summer droughts and last year’s abnormally cold and wet spring.  If you can bear with me for awhile, I’d like to introduce some data that illustrates these factors.

Although I rarely have more than 10 American Goldfinch (AMGO) or a half dozen Dark-eyed Juncos (SCJU) in my yard at any one time, since September Darrin and I have, during periodic weekend banding, banded 137 AMGO and 128 SCJU in the yard and rarely see previously banded birds. This doesn’t surprise me, as I have reported here before that despite seeing only two or three chickadees or one or two Red-breasted Nuthatches, I have over a winter season banded 17 and 12 of them, respectively.  So, there are probably many more of each species in your area than you think there are, and even if you see fewer than before, their numbers have most likely not “crashed.”

Birds are elsewhere
In Dearborn this winter, there were hundreds, sometimes thousands, of seed-eating birds in the sunflower fields planted by Ford Motor Co., and fewer of these species in my yard, for example.  Also, as I’ve mentioned previously, one of the effects of prolonged drought is that plant species produce bumper crops of seeds and fruit.  Natural food supplies were abundant this winter, and many birds chose that over feeders.

Another example is American Crows. In Wayne Co., for instance, we have many fewer crows being counted on our surveys (likely West Nile victims), while the Ann Arbor area has had their usual large numbers.

Environmental conditions
Droughts are hard on birds, who have less food (stressed out plants and fewer wet spots reduced insect prey, for example) available to feed their young.  This alone can result in fewer young birds.  Last spring was cold and wet, and many early broods were lost or nesting was delayed (see the fall 2002 banding summary for more details).  There is another way this type of weather, by delaying nesting, can impact bird populations.

I’ve just read a very interesting paper in the Ibis1, the journal of the British Ornithological Union. It describes that late nesting results in birds molting later (as breeding and molting rarely coincide, since both take substantial resources).  As days become shorter, birds replace feathers faster, but these feathers are weaker and have reduced insulative properties.  These inferior feathers can result in increased winter mortality in resident birds (Blue Tits in one of the cited studies, a close relative of our chickadees).  Presumably, weaker feathers may also have negative consequences for migrant birds as well.

Our banding data for fall 2002 offers proof of a late nesting season, as indicated by late molting. Here at UM-D, 11.9% of birds banded were still undergoing molt of their primary feathers past 1 Oct; the previous 5-year mean is 4.8%% (difference is significant at the p<0.05 level).  While I didn’t do a species-level analysis, this data is is for locally breeding species, as long-distance migrants would have completed their molt before coming south.  It would be interesting to see similar analyses from northerly banding stations to see if late molt indicated a delayed nesting season for these other species as well.

Further, other studies showed that late breeding has been associated with reduced winter survival and lowered reproductive success in the following breeding season, even when nesting was delayed only a week.  Therefore, we may be seeing the impacts of last summer’s late nesting season in reduced numbers of some species over the winter, which may continue to have a ripple effect this nesting season with lower productivity.

This helps illustrate why there are no simple answers in complex ecological systems.

  1. Hinsley, S. A., P. Rothery, P. N. Ferns, P. E. Bellamy, and A. Dawson.  2003.  Wood size and timing of moult in birds: potential consequences for plumage quality and bird survival.  Ibis 145:337-340.  See also studies cited within.

Comments are closed.