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Point Blue Conservation Science: Monthly Banding Summary, April 2019

This summary was compiled by Point Blue’s Palomarin banding interns Sophie Noda and Sam Snowden with help from Mark Dettling, Banding Supervisor.

About Point Blue: Our mission is to conserve birds, other wildlife, and ecosystems through science, partnerships, and outreach.

Our Vision: Because of the collaborative climate-smart conservation work we do today, healthy ecosystems will continue to sustain thriving wildlife and human communities well into the future.

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Exciting Captures and Observations:

The not-so-new-anymore spring banding interns, Sophie, Sam, Sarah, and Olivia, have been continuing to learn all the tricks of the trade as they finish up the second month of their internship. In April, Swainson’s Thrushes arrived and the stream of Wilson’s Warblers quickened, giving the banders lots of practice in ageing individuals of these species. The arrival of Pacific-slope Flycatchers brought new challenges to bird identification, while Warbling Vireos added new voices to the bird song symphony at Palomarin.

Throughout the month, birds we captured increasingly showed breeding condition (a brood patch or a cloacal protuberance). We only see these characteristics during the breeding season, so seeing them is a sign that the birds are starting to nest. A brood patch is an area on a bird’s belly that loses feathers and fills with fluid to facilitate the transfer of body heat to eggs and nestlings during incubation. In many species only females incubate, and therefore only females develop a brood patch. However, in some taxa, such as Wrentits, woodpeckers, vireos, and a few others, both males and females incubate, and therefore birds of both sexes may develop a brood patch. Cloacal protuberances are the other type of breeding condition that we look for during the breeding season. A cloaca is an opening that birds use to excrete waste, to breed, and to lay eggs. It becomes enlarged in male birds during the breeding season, so the presence of a cloacal protuberance on a bird can help us determine its sex to be male in most species.

The presence of breeding condition in birds is a sign that the birds likely have a nest and that young birds may soon be on the way. In mid-April, the banders indeed caught their first juvenile birds of the year, both of which were Allen’s Hummingbirds. The banders learned to identify juvenile hummingbirds using subtle clues, such as grooves in the birds’ bills that disappear as the bird ages. Differences between adult and juvenile hummingbirds tail feather coloration and width also help in determining the birds’ age and sex. The banders expect to encounter more juvenile birds of different species as the breeding season progresses.

The start of spring migration brought some unusual birds into our nets, including a Yellow-breasted Chat at Palomarin on April 21st, three Western Palm Warblers at Pine Gulch, and a MacGillivray’s Warbler at Muddy Hollow on April 18th. We also banded a Violet-green Swallow, a Steller’s Jay, three American Goldfinches, a Sharp-shinned Hawk, a Barn Swallow, and a Black Phoebe, all of which are common in Marin County, but not often caught at our banding stations.

Female (left) and male (right) American Goldfinches. Photo by Olivia Wang.


Steller’s Jay. Photo by Olivia Wang.


Male MacGillivray’s Warbler. Photo by Sophie Noda.


Western Palm Warbler. Photo by Sam Snowden.


Yellow-breasted Chat. Photo by Sarah Mueller.


At Palomarin, we also caught two different subspecies of Orange-crowned Warblers in the same net. The subspecies that breeds in Marin County, Oreothlypis celata lutescens, has a yellow and olive plumage, whereas the other subspecies that we caught is much grayer in comparison.

Orange-crowned Warblers. The one on the right is of the subspecies that breeds in our area and the one on the left (with the grayer head) is a subspecies that likely breeds further north. Photo by Olivia Wang.


Let’s Do the Numbers:

In 17 days (1727.93 net hours) of mist-netting at Palomarin in April, we captured 55 new birds and recaptured 53 previously banded birds. A total of 108 birds of 22 species were caught. Approximately 6 birds were caught per banding day.

At our other West Marin banding sites, we captured 127 new birds and recaptured 117 previously banded birds. A total of 244 birds of 39 species were caught over 12 banding days in April (638.85 net hours), an average of approximately 20 birds per day.

The highest capture rates at Palomarin and our other West Marin banding sites were on April 23rd at Palomarin with 13 birds, and April 8th and the 25th at Pine Gulch (Bolinas Lagoon Open Space Preserve), tied with 33 birds each.

At Palomarin, the following species were caught in the highest numbers: Wilson’s Warbler (37), Orange-crowned Warbler (13), Pacific Wren (6), Swainson’s Thrush (6), Wrentit (5), and Song Sparrow (5).

Across all off-sites, the highest numbers of captures by species were: Wilson’s Warbler (71), Song Sparrow (33), Wrentit (14), Pine Siskin (11), and Swainson’s Thrush (10).

About these Summaries:

Point Blue interns and staff at our Palomarin Field Station share these blog posts in an effort to further engage the public in our science. We are grateful to our partners at the Point Reyes National Seashore and to our surrounding Bolinas and West Marin County community for their support of our work.

Our Palomarin Field Station is open to the public.  Consider visiting us!  Learn how on our contact & visit us web page.