Thursday, March 7, 2013

Winter Bird Population Survey 2012-2013

The 21st year of RRBO's Winter Bird Population Survey has been completed. Over the late-December to late-February survey period, 39 species were tallied. The previous annual average number of species is 38. We added yet another new species this year, a Merlin recorded on 19 December*. This brings the total cumulative species list for 21 years to 73.

Comparing this season's results to the previous 20 years, a few things stand out. First, the number of American Robins was the lowest since 1995. Only previous two years have had lower numbers than this year. While many people still think of robins as harbingers of spring, a lot of robins will winter in southern Michigan so long as they have a food supply -- temperature itself is not the limiting factor. I looked at historical climate data (average temperatures and precipitation for December and January) for the past 20 years and found no correlation between any combination of these weather data and the number of robins counted each year.

For robins, food supply primarily means fruit. Due to the long drought over the summer of 2012, the fruit crop this season in our area was quite poor. Additionally, fruit set was diminished because of the unusual warm period in March, when some fruiting plants set fruit very early, or bloomed but failed to set fruit because pollinators were not yet on the wing.

This robin has staked out some wild grapes.
Photo by alsteele under a Creative Commons license.

For the past 20 years, the mean number of robins seen over the winter survey season was 348, with a mean of 28 robins per visit (each season the area is surveyed/visited an average of 14 times). This season a total of only 47 robins and 3 birds per visit were recorded. The highest counts (although still fewer than 20 individuals) came the last two visits of the season, when robins began to coalesce into larger flocks and move back into the area.

Pine Siskins provided another highlight, this one more positive. This species has only been recorded in three previous years, and this year they were present on all but one visit and in numbers greater than 20 birds each visit prior to mid-January. This winter was excellent for many "winter finches" although strong numbers of siskins were not predicted for this region.

The lack of recovery of American Crows in Dearborn from West Nile Virus (WNV), which first showed up here in 2002, continues. A group of three American Crows flying over campus on January 1, 2013 were the only ones recorded all season. Nine out of the last ten years have recorded fewer than ten crows all survey season. My 2011 post provides more background on the crow decline.

*Technically, this survey is supposed to take place from 20 December to 20 February.  Due to weather and scheduling difficulties, we ran it from 19 December to 21 February this season.

Friday, January 11, 2013

Dearborn portion of Detroit River CBC, 2013

Frozen Fairlane Lake on the University of
Michigan-Dearborn campus.
The Detroit River Michigan-Ontario Christmas Bird Count was held, as it is each year, on January 1. This was the 36th year for the count, which is centered at I-94 and Warren Ave, and the 19th year that RRBO has coordinated the field work in the city of Dearborn.

The day began on a high note. As usual, my husband Darrin O'Brien and I drove friend and participant Cathy Carroll to her starting point, so she can walk along the west side Rouge River back to her car on campus. Due to the fact I have just turned old enough to join the AARP (or, perhaps, a lack of coffee), we drove her to the wrong spot! My dunder-headedness turned out to be serendipitous, as I spotted an adult Bald Eagle perched in a tree along the Lower Rouge River just west of Evergreen Road. As we pulled over to admire it, I got a call from my co-worker Rick Simek, who was covering the south end of campus. He excitedly told me he had just flushed an adult Bald Eagle! Later, Greg Norwood, covering the north end of campus, also saw the eagle; Cathy saw it again later, too. While eagles are getting more and more common in the county and the city, this is only the third time we have recorded this species on the count.

Darrin and I traditionally cover the Ford "sunflower" fields which can vary in productivity from year to year depending on which ones are planted, and with what. Usually, the most productive field is the original field at Hubbard and Southfield. However, the last few years it has been planted in hay. (Curiously, given the very high prices and demand for hay this year due to the drought, the hay was still being stored on-site in the field.)  Sometimes a cover crop is used for winter, but this year it (and the one across the street) was left fallow, save for a narrow fringe of sunflowers along the edges, long since picked over.  There wasn't much at this field, nor any of the others except the one adjacent to Ford World Headquarters.

Hay bales stacked up in one of Ford's fields; The Henry hotel at Fairlane Town
Center in the background.
It seemed that every seed-eating songbird in the city was stuffed into that field along Michigan Avenue. Over 200 Red-winged Blackbirds were there, along with over 3,500 House Sparrows; yes they are tedious to count and require at least two hours of multiple estimates. Prior to these fields being planted in 2002, we would tally a few hundred House Sparrows on the entire count. Now it's usually over a thousand, and this year's total set a record. Here's how the last 17 years* look:

House Sparrow numbers on the Dearborn portion of the Detroit River CBC,
adjusted for effort. The dip in 2010 was due to a rainy day with poor
counting conditions.
All these small birds in one place are very attractive to raptors. At this field were several Red-tailed Hawks, an immature Cooper's Hawk, and a lovely adult Peregrine Falcon, seen earlier in the day winging to this spot from the Oakwood Hospital area. The most unusual species seen at this location was Mute Swan: five were seen flying over and heading east. This is only the second count on which they have been recorded.

The horse paddock at Greenfield Village once again yielded Horned Lakes (five).  Darrin spotted one first, and it prompted this exchange:

Darrin: I have one. It's on the pile of poop to the left of the side door.
Julie: Got it. And here's three...four...five.
Darrin: What?! Are you on the same poop that I am?

The best time to look for interesting birds at the Greenfield Village horse
paddock is right after they are fed, and hay piles are strewn about.

Lest you think Dearborn is entirely pastoral, here is a photo approaching the massive Ford Rouge complex, often considered the birthplace of mass industry in North America. The city is really more concrete than natural habitat, and in fact we need fewer people to cover it than we used to a decade ago.

The summer drought played a role in our bird sightings. The soft mast crop (berries and other fruit) was the poorest I can remember. Our 66 American Robins was the lowest number we've counted since 1999; the average over the last decade is over 250.

The drought may also be partially to blame for the widespread tree crop failure (seeds, cones, and other hard mast) this year. Earlier in the season we experienced movement of just about every "winter finch" through the area in search of food. Many kept going, but at least a few hung around. Our total of 23 Pine Siskins on campus was part of a flock that has been either visiting the feeders or feeding on European Alder cones. This was only the fourth count they have been recorded, and this is a record number.

Surely the highlight of the day was also one of the wandering winter finches: ten White-winged Crossbills on a spruce tree in Detroit River CBC compiler Jim Fowler's back yard. This is a new species for the count, bringing the Dearborn cumulative species total to 86.

We ended the day with 47 species, which is one above average. We added a count week species the next day (Brown-headed Cowbird) at the campus bird feeders.

*Although RRBO has been coordinating the Dearborn portion of the count since 1995, we've only had uniform coverage of the city since 1997.

Thursday, November 15, 2012

Fall 2012 banding season review

Our 21st fall banding season took place on 38 days from 17 August to 25 October. An average of 17 nets (12 meter equivalent*) were open an average of 4.3 hours per day. This is the fewest number of days we have been open in a fall season, as we lost 14 whole days and curtailed many others due to rain, heat, or wind, including the last week or so of the season due to Superstorm Sandy and her winds (see below).

What we band in fall has a lot to do with weather in the preceding seasons, even those in years past. I'll leave aside the amazingly wet spring and very hot summer we had in 2011 -- not that they can't have lingering effects, but the weather in 2012 was unusual enough. 

We started the growing season well ahead of schedule, with an unprecedented March heat wave. I discussed possible effects in this blog post. Then came the extended hot, dry summer. It began with 11 days over 90 degrees by the end of June resulting in an average temperature for the first six months of the year being the warmest on record for Detroit. Record heat in July (13 more days over 90F) and precipitation well below normal created severe drought conditions (D2 on the U.S. Drought Monitor scale). 

Below is a map at roughly the end of the breeding season for many songbirds showing drought conditions throughout North America.

Click here for the original map in detail from the North American Drought Monitor.
Banding begins in mid-August, and while the month was still warmer and drier than average, most of the month's precipitation fell as we began the season. September was cooler (although still slightly above average) and remained drier than normal. October brought two episodes of high winds, one mid-month and the other at the end of the month as a result of Hurricane/superstorm Sandy. This ended our banding season.

We ended up banding just 616 new birds and handling 72 recaptures of 63 species (includes two species released unbanded, Ruby-throated Hummingbird and House Sparrow, and one species, Downy Woodpecker, in which we only had a recapture and no new individuals banded). A total of 725 birds were netted (which includes birds released unbanded). Our capture rate was 27.5 birds per 100 net-hours. Here is how this fall compared with the 20 previous autumn seasons:

Fall 2012 Previous
fall mean
Days open 38 51
New birds 616 1205
Total birds 725 1541
Capture rate 27.5 49.0
Species 63 70

Bold indicates the lowest numbers in our history for fall banding -- we'll give this some analysis in the trends section, below.

The top ten bird species banded this fall (new captures only) were:
  1. American Robin -- 107 (low; previous mean 188)
  2. Song Sparrow -- 37
  3. Blackpoll Warbler -- 36
  4. Gray Catbird -- 32 (new record low; previous mean 138)
  5. American Goldfinch -- 32
  6. Swainson's Thrush -- 30 (low)
  7. Magnolia Warbler -- 27
  8. Hermit Thrush and American Redstart -- 26
  9. White-crowned Sparrow -- 21
Our biggest day was on 16 October, when over 100 birds were banded -- 20% of the total that had been banded up to that point. The next day...just 8 birds! In fact, there were only 4 other days when more than 30 birds were banded.

Let's start out with good news. First, we had our first new species for the banding program in a long while. This handsome male American Kestrel was the first banded on campus by RRBO, and represented the 123rd species banded since 1992.

Connecticut Warblers are always a treat -- especially good-looking adult males like the one below, banded on 19 September.

What started out as a gruesome discovery on 28 September had a happy ending. We captured a Swainson's Thrush that had a piece of straw (hay) impaled through its eyelid and the skin past the ear hole. It was embedded at that point, the skin beginning to grow around it. This must have happened at least several days prior to our catching it, when the bird was foraging in some area with straw strewn about. Amazingly, the eye itself was unharmed, there was no infection, and the bird was in good condition.

Under magnification and in just a few minutes, I snipped off most of the exposed straw, severed it where is crossed the ear, extracted the portion between the ear and eye, and finally snipped it close to the point where it was embedded in the skin. I removed over 30 mm of straw! The bird flew off, hopefully to continue a successful migratory journey without the threat of the straw getting caught on something and ripping the skin or puncturing the eye.

Among the 21 White-crowned Sparrows banded this fall was this candidate that had the clear-lored look of the western "Gambel's" form. However, it has the bill shape and color that lean toward more typical eastern forms. This is the first adult of this type we've banded; for a full discussion and more photos see this page on the RRBO web site.

"Gambel's" White-crowned Sparrow?

The RRBO banding program has focused on bird community composition and various metrics relating to the condition of migrant birds at our site. Thus, overall numbers have not been a main theme for us. Limitations and biases inherent in migration banding cannot be overcome, and much has been published in the literature cautioning against the sole use of banding data to monitor bird population trends. However, it's always interesting to take a look at trends of some species, especially in a year like this with unusual climate events and very low numbers. We just need to take it with a grain of salt.

Of the 25 species of which RRBO has banded over 200 birds, five were banded in greater than average numbers and the rest were below average, when effort was considered*. A half-dozen of these species are among our most commonly banded, and all those were banded in numbers far below normal. Low numbers of a few of them (Hermit Thrush, Song Sparrow, American Goldfinch) could be attributed to the premature ending of the banding season, when I tend to band quite a few of these species. Let's look at the others.

Both Swainson's Thrushes and Gray Catbirds continued a long-term decline in our nets that I wrote about last year. Catbirds in particular I think may be responding to landscape-scale habitat changes being engineered by a burgeoning deer herd.

I'm not sure what to think of the -74% departure from normal of White-throated Sparrows. While they do persist longer in the season here than White-crowned Sparrows (many overwinter), they have been scarce at local feeders as well. This is the first fall season when White-crowns outnumbered White-throats, which are typically banded in three times the numbers. If I had to pick a reason, I would guess that White-throated Sparrows, since they most often feed on the ground, may have had trouble finding leaf-litter invertebrates due to dry conditions. This would be especially true during migration, but it could have impacted them on the nesting grounds as well.

Early in the banding season, I wrote about the lack of young birds being banded. This usually indicates low reproductive productivity. Typically, 81% of the birds we band in fall are young-of-the-year (or hatching-year, HY, birds). This year, just 72% were HY. A look at American Robins, our most commonly-banded species, is instructive.

We've banded nearly 4,000 robins in fall, and roughly three-quarters are HY. This year, the figure was just 46%.

I think this is likely attributable mostly to the drought, as robins rely heavily on leaf litter insects and soil invertebrates (such as worms) to feed their young.

Another interesting metric appeared late in the season. By mid-October, most robins have completed molting -- young birds will have lost their distinctive breast spots and adults will have completed molting their wing feathers. This year that was not the case.

Young robins with breast spots after 10 October:
1992-2011: 2.9%
 Fall 2012: 25%

Adult robins still molting primary feathers after 10 October:
1992-2011: 16.1%
Fall 2012: 25%

This indicates that many robins nested later this season. I expect this was due to re-nesting after lost broods (because of the drought), perhaps additionally influenced or initiated by the mid-August rains.

Finally, while I've already provided the caveat that our banding program is not designed to accurately monitor population trends, I have to look at the overall trend in our capture rate for the last eight fall seasons:

Prior to that, there were ups and downs but our effort and capture rate remained fairly stable. We have been banding in the same location all these years, and strive to keep the vegetation structure at its original stage of succession. This may be an indication that populations of the species we typically band in fall are truly declining, and/or that the landscape surrounding our banding area is changing through urbanization, fragmentation, the interactions of deer and canopy loss due to emerald ash borer, or a combination of these and other factors. The value of long-term data collection is having these trends to look at, even if we are not sure how to interpret them at this time.

Twenty-two individuals of 10 species of passage migrants (those which do not normally nest or winter in this area) were recaptured. Seventy-seven  percent of them maintained or gained mass.

The old man cardinal. More RRBO longevity records here.
We also recaptured 13 birds banded in previous years. The oldest was a male Northern Cardinal first captured as a second-year bird in April 2001, making it 12 years old.

Six Gray Catbirds from the past were recaptured; there were two from 2008 that were both originally captured as adults. A Downy Woodpecker first banded in 2006 as an adult at least three years old was also back again.

Finally, I’d like to thank this year’s banding crew: Shelley Martinez, Mike Sullivan, and Dana Wloch. RRBO couldn’t operate without you!

*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.

*Weather statistics from the National Weather Service.

Monday, October 22, 2012

Our online seed guide

As regular followers know, RRBO's primary research focus is the ecology of fall migrant birds in urban areas, and one of our main project is examining the fall diet of birds at our urban site.

To do this, we collect fecal samples from the birds that we band, and identify and catalog all the seeds we find. We initially compiled a book of photos of seeds from plants known to occur in the area, as well as a large reference series of seeds collected on campus and nearby.  This has enabled us to identify all but 9 out of nearly 6,500 seeds we have collected in fecal samples from 2007 through 2011.

Although our big binder of seeds and photos was handy, we found we also wanted to have other reference material in it. We also found there was a lot of interest in this project from other researchers as well as land managers and homeowners interested in what birds are eating. We conceived an online reference guide that would include all this information in the form of species accounts.

Thanks to support from the Jimmy F. New Foundation and the Michigan Audubon Society, our web site is coming together. You can take a look at the first set of species accounts: Fruit Seeds of Southern Michigan.

The guide provides accounts of plants that produce fleshy, bird-dispersed fruits, with special emphasis on the identification of seeds. It will focus on fall- and summer-fruiting plant species found in southern Michigan, but since many of these species are widespread in North America, it will be useful for a broader region. Both native and non-native species are covered.

The financial support has provided a stipend for UM-Dearborn graduate Dana Wloch to work on the site. As a student, Dana worked for several years collecting and compiling seeds from our banded birds under two independent research projects. As the "Number 2 of Number 2" she is uniquely qualified for this task, and I'm grateful for her help and work on the web site, as well as sorting through, identifying, and compiling all the seeds pooped out by every species except the Catharus thrushes for the fourth year in a row.

Take a look at our site. It's a work in progress, so there will be many more species added, revisions to the existing accounts, and the bibliography and resource pages will be completed.

Thursday, October 18, 2012

"Gambel's" White-crowned Sparrows

A western form White-crowned Sparrow (Zonotrichia leucophrys) was banded by the Rouge River Bird Observatory on 20 October 2006, described below and compared with typical eastern and “intergrade” forms. There is a very nice article on identifying subspecies, with range maps, in a 1995 issue of Birding (27:182-200). Two other web pages, Oiseaux Birds and David Sibley’s blog also have a nice set of comparative photos of some of the variations.

Adult White-crowned Sparrows of all subspecies have black and white head stripes, while juvenile birds have brown and tan head stripes. Here are some examples of juvenile White-crowned Sparrows banded at RRBO, concluding with the western “Gambels” type banded in 2006.

Above is a  typical eastern hatching-year White-crowned Sparrow (Z. l. leucophrys).  Characteristics include a deep-based, pinkish bill with a dark tip, a medium-brown crown stripe, and a dark line through the eye that connects to the crown stripe in front of the eye.

This White-crowned Sparrow, banded on 16 Oct 2006, shows traits of an intergrade between the eastern form and the western “Gambel’s” White-crowned Sparrow form. Bill shape and color and crown color are similar to the eastern form.  The lack of dark feathering between the eye and crown is more like the western forms.

This White-crowned Sparrow banded on 20 Oct 2006 is most like the western “Gambel’s” form (Z. l. gambelii).  It not only lacks dark feathering between the eye and crown, but has a thinner, orange bill which is yellowish at the tip (described as looking like “candy corn”), and a more rufous crown stripe coloring.

This adult White-crowned Sparrow banded on 22 Oct 2012 is also a good candidate for the western “Gambel’s” form. It obviously lacks dark feathering between the eye and crown, the bill shape is farily narrow, but the bill color is a little ambiguous