Saturday, May 13, 2017

Bullock's Orioles

A fiery flash darts through the riparian canopy of oaks, pouring forth a beautiful song.  For a moment you are stunned.  You grab your binoculars and search for this flaming apparition.  After following the song and scanning the trees, your eyes are drawn to a patch of bright sunshine in the canopy.  Your binoculars come to rest on a brilliant, almost tropical-looking bird: a male Bullock's Oriole (Icterus bullockii).  If you're relatively new to birding in California, particularly the Central Valley, you might be very surprised to learn that a bird this bright and beautiful spends the warm months with us!
 
A bright and flashy male Bullock's Oriole

Bullock's Orioles breed across much of the western United States, migrating to Central America for the winter.  In the Great Central Valley of California, they seem to favor Valley Oaks (Quercus lobata) in riparian areas.  Cottonwood trees are also favorite nesting and foraging sites, and especially in the arid West, they usually stick close to water. 

Male Bullock's Oriole; notice the prominent black line through the eye.

Bullock's Orioles feed by gleaning insects and small spiders from the canopy.  They also eat caterpillars, fruit and nectar.

Male Bullock's Oriole, showing off his white wing patch

For quite a while, Bullocks and Baltimore Orioles (Icterus galbula) were lumped together as Northern Orioles.  The two species do hybridize in the Great Plains where their regions overlap, but researchers don't believe they are closely related genetically.


Bullocks Orioles may spend up to two weeks building intricate hanging nests, the male and female of the pair working together to weave grasses, twine and other fibrous materials into the structure.  The nest is then lined with soft material, like feathers or the downy "cotton" of cottonwoods and willows (which is really part of the seed structure of those trees; the fluff encloses the seed and aids in wind dispersal).  The nests are typically suspended high above the ground from flexible branches.  This nest placement discourages predators, though on windy days as I watch oriole nests bounce in the breeze, I wonder that the young orioles don't get motion sickness!

A Bullock's Oriole pair's woven hanging nest in a Valley Oak

Female Bullock's Orioles are more drab in color than males, but can still be distinguished from other species by their long, thin bills and faint eye line.  Like many other species, orioles are affected by habitat loss.  Since they are insect eaters, pesticide use may have a detrimental effect as well.

Female Bullock's Oriole

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