Wednesday, February 17, 2010

Winter Bird Population Survey, Year 18

Last year I introduced readers to our annual Winter Bird Population Survey. I've just completed the 18th year of this survey, recording two new species.

The two new species were: Mute Swan (two birds flying over on 4 February), and a Long-eared Owl on 21 January. This brings the total cumulative species list to 69. The number of species that have been recorded in only one year is now 15, and the number of species that have been recorded every year is 21.

Every year since 2002, the year West Nile Virus was first detected in Michigan, I have lamented the loss and subsequent lack of recovery of American Crows. This winter, only a single crow was counted during the survey period. This is the seventh year in a row with fewer than 10 total counted on the survey. The average number of crows found over the survey period prior to WNV was 139; the mean since (2003-2010) is 7. I know of only two pairs of crows nesting in Dearborn. I don't know if these four birds have some sort of immunity to the virus, but I find them in the same place every year, and they don't seem to produce any surviving young.

Tufted Titmouse is another species that suffers high mortality from West Nile virus. They seem to have made a come-back in recent years, and this year the survey recorded the highest number since 2002.

Sparrows in general were seen in lower numbers this winter, but this could be due to the fact that the many of the Ford properties, which typically attract thousands of birds and which we tend to see a little overflow, were harvested bare this year.

American Robins were plentiful, as they have been most recent years. This is the eighth year in a row where they have been recorded on 75% or more of the survey dates (the survey period runs from 20 December to 20 February). The average number of robins seen on each visit has also been increasing, as the graph below indicates.

So long as robins are able to access some water (although they will eat snow) and have a supply of food (mostly fruit, but also invertebrates in leaf litter, so snow cover is a handicap), they'll stick around.

Other than those "highlights," it was a rather below average winter. I tallied 37 species, just one species below the average. Other parameters, such as individuals counted per hour or per visit, were also slightly below average, although not dramatically. However dull, long-term data like this is very important for detecting population trends and responses to climate change, habitat alteration, or pathogen introductions, as the robin and crow examples illustrate. And since this survey uses multiple visits over the winter season, it is more robust than one-day counts such as the Christmas Bird Count. In fact, it is probably the unsung hero among the many datasets RRBO has put together over the years.

You can view an introduction to how the Winter Bird Population Survey works, as well as all the results, on the RRBO web site.

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