Bald birds

Joe's comment on my last post reminded me that it's that time of year to address the perennial bald bird issue. It is one of the most frequently asked questions about birds. Northern Cardinals (Cardinalis cardinalis), Blue Jays (Cyanocitta cristata), and Common Grackles (Quiscalus quiscula) are the most common victims of this disturbing-looking feather loss. There's nothing wrong with most of these birds, despite how bad they look. All birds replace all of their feathers at least once a year, and fall is often the time that North American birds undergo this complete molt. Typically, this molt takes place over a few weeks. But some individuals, unfortunately, lose all the feathers on their head at the same time. I've often read that ectoparasites such as feather mites are responsible for this condition. I have never found mites on bald birds that I've banded. All birds have some of these mites -- they feed on oils, fungi, and bits of skin, and don't cause feather loss. It would also be unlikely, if not impossible, that they would cause all the feathers in one area to fall out simultaneously. In fact, they typically move to tail and wing feathers -- cooler locations on the bird's body -- in warm weather such as is typical in fall. There are a couple of other types of mites found on birds, but they have not been found on bald birds. My search for information on quill mites, for instance, didn't even turn up any evidence that they've been found on cardinals or jays. Nor does it make sense that these other mites would cause feather loss on the head but not elsewhere, since birds are not able to remove them easily from anywhere on their bodies via normal preening. Although the role of mites in bird baldness has not been thoroughly studied, the seasonal and localized nature of the baldness and the fact that it is most common in a relatively limited number of bird species leads me to believe that their presence is coincidental and not causative. That this catastrophic molt is "normal" in at least some individual birds comes from Cornell staffer and former wildlife rehabber Laura Erickson. She was in charge of a Blue Jay in captivity that always lost all the feathers on its head simultaneously once a year -- for the 8 years she had it! Sometimes bald birds are seen at other times of the year when they would not be expected to be molting. The reason for their feather loss may be different (running into a window, for instance, could cause at least a temporary loss).

Being bald is not harmful to birds, except perhaps in cold or wet weather. Within a few weeks, the feathers will grow back.

For more photos and information, see the Cornell web site. Top photo of a bald cardinal by Jimmy Smith.

Janet Hug from Commerce Twp., MI sends us this photo of an adult male cardinal at her feeder, on the road to refeathering, but still looking pretty shabby.

Week #4 in review, and longevity records

Another week shortened by rain at the beginning (Sunday) and end (today). In between, numbers were disappointing. Over last weekend one wave of migrants representing good diversity arrived, but it continued to dissipate over the course of the week. Not helping matters was an adult Broad-winged Hawk (Buteo platypterus) that was hanging around the area the entire time. This is the first time in 16 years we have ever had an individual of this species even perch in the banding area, much less stick around for a few days! It hasn't approached the nets (Broad-winged Hawks eat primarily small mammals and amphibians, and rarely adult birds), but has a chilling presence. It may be attracted to by what seems an unusually high number of chipmunks and Red Squirrels in this immediate area. Happily, this issue should be self-limiting, as Broad-winged Hawks migrate almost en masse, with peak passage in this area tightly focused around 15 September. I think these are beautiful birds, but won't be sad to see the last of this one!

I've updated the fall totals in the right sidebar. They are quite dismal for this time of year, and our capture rate actually declined this week. We have only had 5 years out of the last 16 in which our rate was less than 50 birds per 100 net-hours, and only two where it was below 40 (our high was in 1996, when it was 63).

Only a few new species were banded this week, including Yellow-bellied Flycatcher (Empidonax flaviventris), pictured at the top of the post. This is probably the easiest eastern Empid to identify, although I have never been entirely successful at capturing their beautiful yellow-olive color with a point-and-shoot camera.

We did have a few interesting recaptures from previous years. Two were catbirds -- one was first banded as an adult in September 2007, the other as an adult in September 2006. It isn't too unusual for us to get a lot of between-year recaptures of catbirds. The third bird was a female American Goldfinch. She was first banded as an adult in August 2004. Almost four years between captures is not a record for us (that stands at 6 years, 7 months for goldfinches here), but still impressive for a small songbird. I have a compilation of some of the birds that have had at least two years between recaptures at the RRBO web site. You can also examine longevity records based on banded birds at the USGS's Bird Banding Lab web site.

Bill deformities and tumors

The other day I banded this HY (hatching-year) cardinal with a bill tumor. It looks sort of like a scab or blob of gunk stuck on the bill, but it's actually growing out of the tissue near the base of the bill. The dark coloration of the rest of the bill is normal: young cardinals have dusky-colored bills well into fall. It's one of the easiest ways to tell a juvenile from an adult.

Bill tumors and other deformities are not terribly uncommon. They often are seen on young birds, probably because many of the deformities ultimately impact survival. Most tumors I have seen have been due to a virus called avian pox. As I wrote on the RRBO bill tumor page, avian pox most frequently causes lesions on the feet and legs of birds, but also affects other soft parts. Pox lesions are scabby, crusty, or warty growths. If near the nostril they can obstruct breathing, or can obstruct vision if near the eyes. The lesions fall off after about a month when the virus runs its course, but it appears they can sometimes compromise the bill structure -- some post-pox birds have chunks of bill missing. This may not be due to the pox itself, but might be due to secondary infections of the pox lesions. Pox can be spread by mosquitoes, or contact between birds (thus it can be spread at feeding stations).

The tumor on the cardinal does not appear to be due to pox, as it actually seems to be growing out of the horny structure of the bill versus "on top," which is how pox often presents. I don't think this tumor is life-threatening in any way, if it doesn't continue to grow. The inside of the bill and mouth are okay, it doesn't look like it will interfere with feeding or grooming, and the nostril is open.

Bill deformities such as crossed bills are a whole other matter. Many years ago, upon capturing a catbird with a crossed bill, I did some research, and ended up publishing a paper on the incidence of bill deformities in passerine birds (songbirds) in North American Bird Bander. Since then, I have kept a compilation of these kinds of deformities in songbirds on the RRBO web site. Many have been contributions from banders and people who have seen the information on the site (I moderate a group on the photo sharing site Flickr on bill deformities which includes all types of birds, not just songbirds).

It is hard to say whether these types of deformities are increasing, or if awareness is just elevated. However, there is one geographic region where something is definitely going on: the Pacific Northwest and in particular Alaska. The USGS's Alaska Science Center has devoted a section of their web site to these abnormalities, many showing up in chickadees.

The USGS Alaskan researcher, Colleen Handel, cited my paper and the RRBO web site several times in her presentation on "The mystery of the long-beak syndrome" at a recent ornithological conference. Colleen and I have discussed these deformities several times over the years, and are planning on collaborating on a paper next year.

Week #3 in review

Although spring warblers are bright and colorful, I have always enjoyed fall banding the most. The pace is steadier and a little less weather-dependent, and I like seeing all the young birds and gauging the success of the breeding season. The nice variety of bird species which inevitably begins in September makes up for the inescapable reality that winter is on the way.

The Nashville Warbler (Vermivora ruficapilla) above was, as are over 80% of the fall birds we band, a hatching-year (HY) bird. Nashville's have a very protracted fall migration, with the earliest records from late August, and some lingering into early November.

Blackpoll Warblers (Dendroica striata) are one of my favorite species. In the field, they can be hard to distinguish from Bay-breasted and Pine Warblers in the fall, and these three species are the classic "confusing fall warbler" complex. In the photo above, you can't see the yellow feet, which help distinguish this species. McGill Bird Observatory has a nice guide to the fall warblers.

Magnolia Warbler (Dendroica magnolia) is one of our top-ten most frequently banded species. In all seasons, they show a pretty wide variation in plumage, from dull young females to bright old males. Because of this variation, banders are not "allowed" to sex HY Magnolia Warblers (if we submit records to the federal Bird Banding Lab that say a HY Magonlia is a male or female, we are told to change the sex to "unknown"). However, very dull birds with short wings are likely female, so for this bird, I noted "female" -- but only in the comments of our data form.

At the other end of the spectrum are most small Empidonax flycatchers, in which not only do the ages and sexes look alike, but so do many species. Wing and bill measurements, and examination of the shape of some of the feathers helps tell apart many of the species in the hand. For the most part, however, Willow and Alder Flycatchers (E. trailli and alnorum) cannot be told apart in the hand. We submit them as "Traill's" Flycatchers.

Stats for the season so far have been updated in the right sidebar.

The first migrant thrushes

Many new species of migrants are coming through this week. Today I banded the first Swainson's Thrush of the fall season. I am really looking forward to thrush migration this year. The three most common species of migrant thrushes here -- Hermit, Gray-cheeked, and Swainson's -- are the subjects of a major paper I recently had accepted to the peer-reviewed Wilson Journal of Ornithology:

Craves, J. A. 2009. A fifteen-year study of fall stopover patterns of Catharus thrushes at an inland, urban site. Wilson Journal of Ornithology 121:112-118.

I analyzed weight and fat data for over 2,000 individual thrushes banded here at RRBO during the fall seasons 1992 to 2006. I used both a regression analysis for all initial captures, and recapture data to examine weight and fat changes in these birds as they used our site as a migratory stopover.

In the paper, I present and discuss a number of different findings. Among them, the majority of individuals of all three species gained significant fat and weight during their stopover on campus. This is especially important because in an urban site such as this one, migrant birds are faced with unique challenges, including a high level of human disturbance which may decrease foraging opportunities, and a preponderance of non-native fruits.

These thrushes rely heavily on fruit during fall migration. Much of the native fruit, to which they are evolutionarily adapted, has been depleted by other species by the time the bulk of the thrushes move through, in particular Hermit Thrushes, which are late migrants. The remaining fruits are predominantly introduced, invasive species such as buckthorn and honeysuckle.

My findings have conservation and management implications. It suggests that thrushes are in fact able to find sufficient resources in urban natural areas similar to our site. Since thrushes gain mass on a diet high in non-native fruits, it demonstrates that some introduced plants do perform ecological functions.

The next question are which fruiting plants and shrubs commonly found in urban areas -- both native and introduced -- are most important to migratory birds? What is their relative availability? If highly invaded urban natural areas are to undergo restoration efforts, which introduced plants should be left to provide resources for migrant birds while native plants become established? In a future post, I'll talk about exactly how we intend to shed some light on those questions with ongoing research!

The vast majority of published papers on stopover ecology have focused on coastal or rural sites, and frequently cover a study period of only 2 or 3 years -- the length of time a graduate student takes to complete a degree. RRBO's work is unique with its focus on an urban, inland site, and especially its long-term nature. It's not enough to catch some birds and see that they've gained some weight between captures. There are many pitfalls in designing studies and gathering data that can be used to draw statistically and biologically significant conclusions. Large data sets are needed to account for many extrinsic variables (such as weather) and weed out ambiguous or problematic data points. I have raw data for a suite of other species besides the thrushes. At some point, they will also be run through the analytical mill, but first I'll be concentrating on the thrush and fruit connection. Stay tuned!