Posts Tagged ‘Field Marks’

My brother and I visited Hawk Hill (just about our favorite place in the world) in the Marin Headlands, just north of San Francisco, CA.  One migrant that passed us by was an Osprey, and this bird got me thinking.  The bird had very solid, dark plumage on its back which meant that it was an adult bird as hatch year birds show scalloping of white on the tips of the feathers on the dorsal surface.  On the ventral side, it had the iconic, pure white breast and belly, and this is what got me thinking.  Some Ospreys have a necklace of dark streaks that run in an arch across the breast.  I remember that there has been some confusion and debate over this necklace as to what it indicates.  Does it mean that the bird is a male or a female?  Is there overlap between the two with some males having a necklace and some females not having one?  So, I did some reading, and here is what I found.  Most females have a darker necklace than males, but there is some overlap with some males having a pretty strong necklace.  This is most dramatic in the North American Subspecies, Pandion haliaetus carolinensis.  However, I could not find any reference that said that females could have a necklace so light that it disappeared.  Only the males can show the pure white breast with no necklace at all (from what I have read so far).  So this means that this field mark has a sort of asymmetrical usefulness.  If you see an Osprey that has a necklace, this could be a female or a strongly marked male, so the field mark is not particularly useful.  However, if you see an Osprey that has no necklace, like the one my brother and I saw on Hawk Hill, this is likely to be a male bird, so the field mark may be much more useful!

References Used:

Crossley, Liguori, and Sullivan. 2013. The Crossley Guide: Raptors.

Dunne, Sibley, and Sutton. 2012. Hawks in Flight, 2nd ed.

Ferguson-Lees and Christie.  2001. Raptors of the World.

Liguori. 2011. Hawks at a Distance.

Poole, Bierregaard, and Martell. 2002. Birds on North America: Osprey.

Wheeler. 2003. Raptors of Western North America.

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