Wednesday, September 8, 2010

Vultures and Migration

The horaltic pose - sitting with wings widespread - perhaps to gather the sun's warmth.
In August, the turkey vultures (Cathartes auracontinued to land on the barn every few days for short periods of time.  Often I noticed them at midday, and several times it was exactly 11:00 when they circled the farmyard.  One day as I leaned back to watch the vultures chase their loops directly overhead, I heard loud whooshing sounds.  Whenever the vultures approached the side of the spiral closest to the barn, they tucked in their wings and suddenly raced through that part of the turn with loud wing turbulence.  Coming out of the bank, they pointed skywards and slowed down to their typical dihedral and wobbly flight above the orchard.  The red-tailed hawks were also circling, and the vultures would dive bomb any hawk below them on the fast side of the thermal.  Eventually, the red-tails pulled out and flew to Mindego Hill to claim their own breeze.

Four turkey vultures and a red-tailed hawk riding a thermal above the farmyard
Does the late summer sun warm the hillsides above the Dipper Ranch in a way that creates unique morning wind patterns?  Is that why the vultures are suddenly appearing on the barn roof?  If so, why haven't I noticed this pattern any of the other three summers I lived at the Dipper Ranch?

Or, is this barn roosting habit just a temporary phenomenon as the turkey vultures prepare for their winter migration?  Are turkey vultures migratory in the Santa Cruz Mountains?  I should know this with all the time I spend outside, however, since turkey vultures are such common birds, I rarely note their presence in my field notebook.

So I've checked references and gotten mixed answers because as a species turkey vultures are partial migrants, that is, turkey vultures in the northern United States are migratory, but less so in the warmer states, and are generally noted as residents in coastal California. The Dipper Ranch is in coastal San Mateo County near the ridgetop of the Santa Cruz Mountains which also spans into San Francisco and Santa Clara Counties, California.  In its 2006 checklist, the local Sequoia Chapter of the Audubon Society lists turkey vultures as fairly common in San Mateo County year round.   In the 2007 Breeding Bird Atlas of Santa Clara County, Bill Bousman reviews historical accounts by Grinnell and Miller (1944) that a "large share" of the turkey vultures in California move southward in winter, however, the numbers of turkey vultures reported since the 1950s in Christmas Bird Counts in San Jose have increased significantly and the number reported in Palo Alto are similar in summer and winter, thus he concludes that TVs are now resident in the lowlands of Santa Clara County.  In nearby Santa Cruz and Monterey Counties, turkey vultures are noted as uncommon in the winter.  On the Peninsula Birding on Yahoo group, I found several observations of turkey vultures at nearby Russian Ridge Open Space Preserve and other highland locations in the Santa Cruz Mountains in winter months.  One mountain neighbor says he sees turkey vultures year round.

In other parts of the state, migrating vultures are a big deal.  There is even an annual Vulture Festival at Audubon's Kern River Preserve where an average of 25,000 TVs have been observed on their southern migration in the months of September and October.  They explain that clusters of vultures spiraling up a thermal to gain altitude are called "kettles" (like bubbles floating upwards in a kettle of boiling water).  TVs are notorious for using wind patterns to assist their long distance flights with a minimum of wing flapping and expenditure of energy.

Eventually I found the Turkey Vulture Migration Project in which scientists with the Hawk Mountain Sanctuary have tracked 21 turkey vultures with radio tags during migration and over 100 TVs with colored wing tags in North and South America.  Check out the Google maps of Rosalie's migration between Mexico and Washington State since 2006. During its southerly migration, this bird often passes over inland California in mid-October, approximately 50-75 miles east of Bakersfield which puts it in the general vicinity of Kern River Preserve.  Other tagged vultures also appear to prefer a more inland route across central California, thus suggesting that groups of vultures I might see over the Dipper Ranch in the next 2 months are resident birds that do not migrate.

This is one of the anomalies of living in the mild climate of the California coast - some birds decide to stay whereas their inland or northern compatriots of the same species migrate to Central and South America in what is called leap-frog migration.

The dihedral wobble
You can bet I'll be looking for TV kettles in the next few weeks, and this winter, I will write down any dates I see vultures.  It would be exciting to see a vulture with a radio or numbered color tag, and I would rush to see if it matched the birds tracked on the Turkey Vulture Migration Project website, but I suspect our local turkey vultures are homeboys and not part of this cross-country study.

I just found out that September 4th was International Vulture Awareness Day.

Next up:  Vultures and Death


  1. Cindy
    Just a couple of thoughts. Have you removed or has a tree fallen near the barn lately? If a nearby roosting area has changed it might have upset the social activity around your place.
    I have also noticed changes in wildlife behavior (including TV’s) on some of my cameras when the cattle are moved. Were the cattle recently moved?

    Also, hope the Bobcats are okay in Rancho. I will be watching that one. DT

  2. That's deep ecology thinking! Trees have been going down with Sudden Oak Death in the area and the usual end-of-the summer/dry season succumbing of the old and diseased trees. Several times in the last few weeks while hanging outside, I have heard a crash somewhere off in the forest. Nothing has gone down right next to the barn but it could be something in the forest nearby. Cattle left in July; we have a seasonal January-July grazing program. Didn't notice anything around that.

  3. I love the last photo so much... good capture...


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