Thursday, May 29, 2014

The Remarkable Oriole Nest‏

Male Baltimore oriole attending nest
Female Baltimore oriole building nest
Baltimore oriole nest in red maple at farm in VA

One of the most beautiful birds in North America, which migrates to the neotropics in winter, is the Baltimore oriole.  The bright orange and black colors of the male, its habit of living around houses and farms, plus a very loud and melodious song make it very conspicuous.  We enjoy seeing orioles in Florida as they migrate north and we arrive at our VA Blue Ridge farm about the same time as the orioles in late April.  The hanging pouched nest of the Baltimore oriole is quite unusual.  We are fortunate to have a pair living in a large red maple in our front yard and can observe all of the interesting breeding activities.  After the male and female have settled in after migration, they are seen searching together for an appropriate nest site.  They favor the ends of long pendulous branches and are likely trying to avoid the depredations of predators such as the fox squirrel and black rat snake which are common in the area.  Have a look at the general nest area and see if you can spot the nest hanging on the left side of the maple over our drive way. For such a distinctive structure it can be hard to find.

The female begins the nest by looping grass fibers around twigs and gradually building up a cup.  Then she goes inside the cup and appears to tie the ends of fibers into one another to knot them together.  In a remarkably rapid fashion she completes the complex nest, in this case between May 5 and 12.   

The degree to which these orioles tolerate and even seek out human-modified landscapes is interesting.  They appear to have benefited from the clearing of the original forests.  They are also attracted to riparian or riverine habitats.  They vigorously defend their breeding territory.  When one of their blackbird or Icterid cousins, such as a redwing, ventures near their nest they immediately chase away the intruder.    

The origin of such peculiar and distinctive nests is a subject of some interest.  It would appear to be a direct result of predation pressures from tree-climbing snakes and mammals.  The mere ability to build such an amazing structure is awesome and presumably instinctive.  The ultimate expression of the habit of building hanging nests is found among other icterids, the oropendolas of South and Central America, which build very long, hanging nests in colonies.  It is easy to see how the orioles could represent an earlier stage in the evolution of highly visible, hanging nests.  It is especially remarkable how the change from the usual condition in birds, highly secretive nest building, progresses to completely visible nests in colonies, which are so difficult to access that most predators are stymied.   

Bill Dunson 
Female Baltimore oriole building nest

Baltimore oriole nest in red maple on VA farm

Fox squirrel

Black rat snake

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