Sunday, April 27, 2014

Hybrid Audubon's x Myrtle Yellow-rumped Warbler

Hybrid Audubon's x Myrtle Yellow-rumped Warbler. March 30, 2014. San Elijo Lagoon, California. Greg Gillson.
Click for larger view.
Hybrid versus intergrade--what's the difference? Well, a hybrid is a cross breeding between birds of two different species, while an intergrade is breeding between two well-marked subspecies of the same species. So, technically, this post should be discussing intergrades between Myrtle and Audubon's Warblers. But why be pedantic? These were considered two different species until 1973 and may be again some day. Where the ranges meet in NE British Columbia they breed together producing offspring with mixed characters.

You may know that the western Audubon's form of Yellow-rumped Warbler has a yellow throat and the eastern Myrtle form has a white throat. But, of course, that is not the only field mark.

Myrtle Warblers have thin white eyebrows while Audubon's do not. Myrtle has two white wingbars while Audubon's has a big white wing patch. The under tail on Audubon's has black feathers all the way across the tip while on Myrtle the white reaches the end of the tail. The Myrtle has a hard "check" call note while Audubon's is a higher "tchip."

Intergrade Yellow-rumped Warblers are more often noticed in photos than in the field. In the photographed bird above, there is a bit of white around the edges of the yellow throat. And there is a short white eyebrow behind the eye (it was slightly longer and more obvious on the other side of the head). The wingbars form a messy patch. The under tail feathers are white to the tip in the center. Really, in most aspects, this bird is midway in field marks between Audubon's and Myrtle, except for the more Audubon's-like throat.

Here are two photos of more typical representatives of each form for comparison.

Audubon's Warbler
Audubon's Warbler. Forest Grove, Oregon. March 23, 2012. Greg Gillson.
See? Throat, eyebrows, wingbars, undertail.

Myrtle Warbler
Myrtle Warbler. Forest Grove, Oregon. April 29, 2011. Greg Gillson.