Taking the Long View: An inside look at the goings-on at the longest running avian ecology field station west of the Mississippi.

Monthly Banding Summary, April 2016

This summary was compiled by Palomarin banding interns Thomas Ryan and Kayla Putty with help from Mark Dettling, Banding Supervisor.

Exciting Captures and Observations:

The month of April is a time of transition at the Palomarin Field Station. The spring migrants who first reared their heads in March are trickling in at a faster rate while the wintering birds head north for greener pastures. Where once there were Ruby-crowned Kinglets dancing in branches, male Wilson’s Warblers now use vociferous melodies to establish themselves on the space in which they hope to attract a lady-friend and raise a nest of youngsters. Hermit Thrushes, who lay silent for months in the understory, suddenly began to announce their presence and then, poof, were in a few short weeks replaced by their more ethereal doppelgangers, the Palo-breeding, Swainson’s Thrushes.

Not knowing what mix of dispersers and incomers each day will bring adds a special pinch of surprise to each check of the nets. By months end, our Fox Sparrows and Varied Thrushes are long gone, but soon the young fledglings, and the occasional lingering or lost migrant bird, will happily gobble up the burgeoning fruit from the productive native plants left behind. May awaits.

Our first indications of breeding females back in March were the featherless, fluid-filled brood patches on the breast and belly of some of our caught birds. But some spring arrivals manage to produce many eggs and young each year without ever showing this crucial brood patch indicator. How can this be? Well, the trick is in never actually incubating your own eggs! Female Brown-headed cowbirds, an occasional capture at Palomarin and West Marin offsites, are renowned for laying their eggs in the nests of any songbird who will be tricked into sitting on an egg and feeding insect morsels to a chick that isn’t their own. Why lose those insulating belly feathers if you haven’t a need to warm your own eggs? Bird lovers occasionally scorn the brood-parasitic cowbirds for their trickery, but such an intriguing adaptation adds flavor to avian ecology and a wrinkle to what constitutes “breeding condition” as observed by us banders at Palomarin.

Female, Brown-headed Cowbird (Molothrus ater). Photo by Garrett Duncan
Female Brown-headed Cowbird (Molothrus ater). Photo by Garrett Duncan

The aforementioned Swainson’s Thrush returned from their wintering grounds in Mexico toward the end of April.Typically among migrants, males arrive earlier than females to establish territories on breeding grounds. The Swainson’s Thrushes are recognizable by their haunting, ascending song and their ‘drip’ and ‘goat-whinny’ calls. They are not a sexually dimorphic species, meaning that males and females look identical, but through careful observation of developing cloacal protuberances and recapture reports demonstrating the observed sex in previous years, we’ve seen a heavy influx of male Swainson’s Thrush arriving before the females yet to come.

Swainson’s Thrush (Catharus ustulatus). Photo by Garrett Duncan
Swainson’s Thrush (Catharus ustulatus). Photo by Garrett Duncan

Other exciting captures for this month include the Audubon’s Warbler. A male was captured on April 5th at Pine Gulch (in the Bolinas Lagoon Marin Open Space Preserve). Audubon’s Warbler is a subspecies of the Yellow-rumped Warbler, and can be differentiated from the other subspecies, the Myrtle Warbler, using its yellow throat and bright white wing coverts as well as a several more subtle features. Both subspecies are found here in the winter, but they leave for the breeding season (Audubon’s move higher in elevation and Myrtle move north), so this was a somewhat late capture date.

For the second time this year, some beautiful Red Crossbills were banded. Three males were caught at Muddy Hollow (in Point Reyes National Seashore) on April 29th. Red crossbills travel in gregarious flocks among the tops of trees making their capture particularly rare for a year-round resident bird. As such, there are gaps in our knowledge of their natural history, but among that which is known is their ability to breed any time of year. Unlike most breeding birds in temperate climates, Red Crossbills can, and do, breed whenever and wherever there are enough seed cones to raise nestlings, making them a true challenge for age determination for us banders! On April 30th, our first Olive-sided Flycatcher was caught at Palomarin. These flycatchers are heard singing around the station but are rarely caught since they tend to move higher in the canopy.

Olive-sided Flycatcher (Contopus cooperi). Photo by Kayla Putty
Olive-sided Flycatcher (Contopus cooperi). Photo by Kayla Putty

A Rufous hummingbird was caught at Pine Gulch on April 22nd. This female was caught alongside an Allen’s Hummingbird presumably chasing or being chased by the other. Unlike the Allen’s Hummingbird, this species does not breed in Marin County and will be presumably moving further North to its breeding territories.

Let’s Do the Numbers:

In 16 days (1638.07 net hours) of mist-netting at Palomarin in April, we captured 29 new birds and recaptured 44 previously banded birds. A total of 73 birds of 19 species were caught this month. Approximately 5 birds were caught per banding day.

At our other West Marin banding sites, we captured 57 new birds and recaptured 71 previously banded birds. A total of 128 birds of 28 species were caught over 12 banding days this month (497.69 net hours), an average of approximately 12 birds per day.

The highest capture rates at Palomarin and our other West Marin banding sites were on April 4th and April 30th at Palomarin with 7 birds and April 29th at Muddy Hollow (in Point Reyes National Seashore) with 22 birds.

At Palomarin the highest numbers were captured for the following species: Wilson’s Warbler (16), Oregon Junco (9), Orange-crowned Warbler (7), and Chestnut-backed Chickadee (6).

Across all offsites, the highest numbers of captures by species were: Wilson’s Warbler (29), Song Sparrow (16), Wrentit (11), Bewick’s Wren (10), Fox Sparrow (6), and Tree Swallow (6).

About these Summaries:

In an effort to share our science with the public, Point Blue interns and staff at our Palomarin Field Station (Palomarin or “Palo”) in Point Reyes National Seashore near Bolinas, CA produce these monthly bird-banding summaries. Our science interns create these summaries as part of their science outreach training. Our Palomarin Field Station is open to the public.  Consider visiting us!  Learn how by visiting our mist-netting demonstrations web page.