Our 18th fall banding season took place on 57 days from 17 August to 8 November. An average of 16.5 nets (12 meter equivalent1) were open an average of 4.8 hours per day. This fall followed one of the coolest summers on record, in which all of southeastern Michigan experienced average temperatures at least 2 degrees below normal over the summer months, with July temperatures five degrees below normal, with no 90 degree days. This cool weather was attributed to the prevailing northwesterly flow of air aloft due to the negative North Atlantic Oscillation. This atmospheric climatic phenomenon impacts weather in Europe and eastern North America. Rainfall was above average in June, but most of the rain fell on only two days. July and August were somewhat dryer than normal.
September was extremely pleasant. Too pleasant, really, with little in the way of weather systems that typically assist migrants through the Great Lakes region. This was due to the unusually high-latitude position of the jet stream, which I discussed on the Net Results blog. Migration was generally dismal in much of the western Great Lakes during what is typically the peak period of movement. October was much cooler than normal; it was the third coolest October on record in the U.S. The entire fall season had temperatures in our area that were below average.
We ended up banding 1169 new birds and had 158 recaptures of 66 species (includes three species released unbanded: Ruby-throated Hummingbird, European Starling, and House Sparrow). A total of 1484 birds were netted (this includes birds released unbanded). Our capture rate was 36.7 birds per 100 net-hours.
The number of birds banded wasn’t terribly far off average, but our capture rate was our fourth lowest. Part of this had to do with increased effort, as we had the third highest number of net hours in our history. More hours open, especially in a slower season, translates into fewer birds per hour. There was also the additional handicap of a major construction project beginning at dawn seven days a week adjacent to the banding area.
Declines in the numbers of birds banded probably reflects weather and site conditions this year, versus population declines.
The top ten bird species banded (new captures only) were:
* American Robin — 391 (record high)
* American Goldfinch — 84
* Gray Catbird — 76 (record low)
* Song Sparrow — 53
* Dark-eyed Junco — 39
* Nashville Warbler — 35
* Northern Cardinal — 31
* Hermit Thrush — 27
* White-throated Sparrow — 27 (record low)
* Swainson’s Thrush — 24
The very low number of catbirds and high number of robins resulted in the first shift in the top of the list of most commonly banded species, with robins unseating catbirds for the first time in RRBO history. I have surpassed 300 robins in only three previous falls, and our final total of 391 was 37 birds beyond our previous fall record. Normally, this would not be a cause for celebration, if for no other reason than handling robins can be “messy.” They poop a lot. This year, we wanted to see what they were eating as part of our exploration of the diet of birds at our site. For the first time, we appreciated each “donation”!
The last couple of years I noted a decline in the number of young American Robins banded, which might indicate low productivity. This trend began to reverse this year:
|Percent young birds,
|Percent young birds,
|Percent young birds,
|Percent young birds,
|American Robin (Aug-Oct)||80||72||60||76|
Speaking of robins, we had several unusual individuals. One robin had orange, rather than white, facial markings. Clearly, however, the most interesting was the white-breasted robin shown at the top of the page. You can read all about this bird and see more photos at the Net Results blog or here on our site.
As mentioned, the number of Gray Catbirds was very low. Our average is 93, and we banded only 76 this fall. Corrected for effort, this represents a 63% decline over the previous fall average. I think two factors may be at work here. First, the number of catbirds breeding here on campus seems to have declined a bit as vegetation has gone from favored thickets in old fields to more open early successional forest. Second, like many other species this year, migrants from other areas probably went around us this fall.
Unfortunately for our ongoing study on Catharus thrushes, their numbers were down this year, around 65% below the previous fall averages.
Something vaguely interesting seems to have happened with Northern Cardinals. The 31 banded represents only a slight increase over the previous average. However, over two-thirds of the individuals captured were female. Typically, the sex ratio is close to 50-50, as you would expect. Whether this was just happenstance or if there was actually some reason for this skew (such as increased male mortality this year) remains to be seen in coming seasons.
Baltimore Orioles put in a strong showing, with 17 banded (average 3, previous record high 14). This probably does not reflect any true population change; they are early migrants and some years if they move along early, I miss them.
Twenty individuals of 13 species of passage migrants (those which do not normally nest or winter in this area) were recaptured. Eighty percent of them gained mass. Some mass gains were especially notable. Both Gray-cheeked Thrushes that were recaptured this season gained very large amounts of weight. One gained 40% of its original mass over 11 days, the other gained 20% over 6 days. These birds were quite fat on first capture. The average mass of a Gray-cheeked Thrush with no visible fat deposits is around 31 grams. The second bird mentioned above weighed 42.4 gr when I first captured it, and 51.1 gr on last capture!
A recaptured Blackpoll Warbler gained 19% of its original mass in 10 days, and represented a late fall departure date for Dearborn when we last caught it on 22 October
We also recaptured 15 birds banded in previous years. Of note:
- An adult Red-eyed Vireo recaptured on 25 August was originally banded, also as an adult, in August 2003. This means this bird is at least 7 years old. We’ve had returns of other Red-eyed Vireos before, but prior to this bird the longest period between recaptures was less than 5 years. (You can take a look at RRBO’s age records on this page. It lists a variety of species with at least two years between captures.) Red-eyed Vireos winter in South America, making the minimum distance traveled between Dearborn and the northern coast of Colombia about 2200 miles one-way.This bird, then, has flown at least 36,000 miles on migratory flights!
- On 29 October we recaptured a Slate-colored Junco banded here as a young (hatching year) bird on 24 Oct 2007. Winter site fidelity is well-known in this species, but due to the sheer number of juncos that winter here and the wide area they occupy, we don’t often recapture birds between years.
- A Gray Catbird first banded in May 2005 was recaptured this fall; it has returned and been recaptured here each year.
Weekly summaries are posted during banding seasons at our blog Net Results. Don’t forget to follow along there.
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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. ↩