Banding summaries: 2004-2008

Fall 2007

Overview

Our 16th fall banding season  took place on 43 days from 21 August to 3 November.  An average of 16.5 nets (12 meter equivalent1)  were open an average of 4.75 hours per day.  The head bander was ill for several days early in the season, but this period coincided with very hot, dry weather with little bird movement. In fact, it was the unseasonably hot weather 2 which extended well into October, that was the hallmark of fall 2007. August, September, and October all had above-normal temperatures, with October averaging over 7 degrees above normal, the 6th warmest on record for Detroit. Rainfall was below normal for every month from June through October, with the exception of August. However, a large percentage of August’s rainfall fell in one day (19th-20th), and subtracting that, rainfall was normal. For more on weather effects, see numbers and trends, below.

We ended up banding  1230 new birds and had 163 recaptures of 69 species (includes three species released unbanded: Ruby-throated Hummingbird, House Sparrow, and European Starling).  A total of 1499 birds were netted  (this includes birds released unbanded).  Our capture rate was 46.7 birds per 100 net-hours our fourth lowest.

Fall 2007 Previous
fall mean
Days open 43 50
New birds 1230 1234
Total birds 1499 1584
Capture rate 46.7 52.0
Species 69 70

The top ten bird species banded (new captures only) were:

American Goldfinch — 283, with 249 banded the last 14 days of banding. Our previous fall mean was 89.5, with a record of 247 in 2005.
American Robin — 215
Gray Catbird– 104. Second lowest fall total, with a previous mean of 152.
Nashville Warbler — 44
Song Sparrow — 43
White-crowned Sparrow — 40. This is the first year we banded more White-crowned Sparrows than White-throated Sparrows.
White-throated Sparrow  — 38. Our lowest total ever; previous mean 80. Not only were they scarce, only 60% were young birds, versus 82% in previous years, indicating low productivity (see more in numbers and trends, below).
Dark-eyed Junco — 29
Northern Cardinal — 27
Swainson’s Thrush — 23. Lowest fall total, with the previous being 32, and the mean being 80.7

Notable species
A very special new species for RRBO was banded this fall: a Northern Shrike on 22 October.  This was also a new early date for Dearborn, the previous being 10 November. Our second-ever Northern Mockingbird, and first for fall, was banded on 2 October. Two Rusty Blackbirds, only our second and third ever, were among the final birds banded on our last day of the season 3 November.  Other less-frequently banded species we had this fall included a Red-breasted Nuthatch on 27 August, heralding an unprecedented invasion year; and Connecticut Warbler on 4 October, a late date for this species.

Numbers and trends
This was a truly dismal fall season in terms of numbers of birds, with nearly every species in near-record-low numbers. Attempting to explain the reasons for trends over a single season is highly speculative, but we can point to the weather having some influence.

The long, hot, dry stretch of weather this fall resulted in few weather systems (low pressure systems with north or northwest winds) that tend to push migrants into an area in obvious pulses. Many birds may have by-passed our area, continuing over us in fair weather. We observed migrants that did end up on campus tending to use wetter areas near the lake and in the swamp rather than the parched areas that characterize our banding site.

Numbers don’t tell the whole story. On 27 September, we had our first day of banding over 40 birds. Of the 41 birds of 14 species of passage migrants banded, only 53% were young birds. Generally, we would expect over 80% young birds. The table below gives some examples of final tallies of young versus adult birds banded for species in which we had >15 captures.

Percent young birds,
fall 1992-2006
Percent young birds,
fall 2007
Orange-crowned Warbler 65 43
Magnolia Warbler 86 73
White-crowned Sparrow 63 57
White-throated Sparrow 82 60
American Robin (<15 Sep) 86 50
American Robin (Aug-Oct) 80 72

While I have not worked through this to determine for which species these differences are statistically significant, this is suggestive that productivity was low for the species with low numbers of young birds. I know of two other banders in the region who also saw lower-than-usual numbers of young White-crowned or White-throated Sparrows this fall, so this was not restricted to our site. Two factors may have played into this. First, I would suspect that this could be due to the prolonged spring freeze. Perhaps fewer birds arrived on the nesting grounds, or those that did arrive got there in poor condition, or too late to nest. Second was again the very dry summer — and the drought in some parts of the far north was even worse. Very dry conditions can lead to a decrease in the amount of insect prey available to birds to feed their young. We will have to see some final results of other banding stations to see how widespread this trend may have been.

Seed and cone crops across much of Ontario were extremely poor this summer. Whether this was connected to the drought, or played a role in productivity, is not known. But the forecast is certainly playing out around the region — it should be an interesting winter!

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  1. 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.
  2. RRBO weather policy. Three weather variables factor in to whether or not RRBO bands on any given day: precipitation, wind, and temperature, and our guidelines are explained below.

    Precipitation: We do not band in any sustained precipitation, including light drizzle or mist that last more than one hour, or if any sustained precipitation is forecast or seen approaching on radar.  When birds become damp or wet, including from wet nets, they can easily become chilled and stressed, even in fairly warm temperatures, especially if there is a breeze.

    Wind: Generally, RRBO does not keep nets open in wind speeds much above 10 MPH, depending on the wind direction and temperature. Any wind above 15 miles an hour certainly presents danger to birds in most situations.  As nets move about in the wind, they can easily pull and strain, dislocate, or break wings or legs, or in some cases strangle birds.  This is especially true of small birds such as warblers and kinglets. While these cases might be uncommon in wind speeds of 10-17 MPH or so, we don’t feel it’s worth the risk.

    Temperature:  Unless nets can be monitored nearly constantly — and in our case the nets are not where they can be under continual surveilliance — banding in temperatures below 40F requires careful consideration of wind speed and cloud cover (how quickly temperatures will rise).  RRBO rarely bands in temperatures below 40F.  High heat, especially when combined with direct sun, will also be grounds for closing.

    Perhaps RRBO errs on the side of caution.  This seems the only prudent way to go.  While banding large numbers of birds is interesting and can be fun, the point is not our own amusement or satisfaction, and 10 or 100 birds either way is not going to have any more overall meaning to a long-term banding project.  In our case, where we focus on mass gains and condition of birds, we also feel stressed birds are more likely to be recaptured (or die and never be recaptured), thus introducing bias into our results.  Therefore, we avoid banding in poor conditions and our experienced banders strive to process birds quickly and safely.