>While in Texas in April I heard a male Black-throated Green Warbler giving
>its chip note, which, to my ear, was identical to that of a Townsend's
>Warbler. By my side was Michael O'Brien, who has a superb ear for
>separating calling warblers (especially migrants overhead in the dark). We
>then briefly discussed chip notes of Townsend's/Hermit/Black-throated Green
>Warblers. He seemed to think the call note of these three warblers were all
>very similar. I got the feeling he would be reluctant to bet on an
>identification without visual confirmation. Dunn and Garrett's Peterson
>Field Guide to the Warblers states they are all very similar also, but does
>note Townsend's is slightly higher.
>
>With the above in mind, I would be hesitant to place much faith in data
>for this species pair based solely on call notes, especially from calling
>birds 200' up in a fir forest. Rohwer and Wood discuss characteristics of
>hybrids in the zone in question at length in their recent paper in the Auk
>(115:284-310. 1998). Eight characters (color and patterns of mid flank,
>lower flank, bib corner, face, extent of yellow, intensity of yellow,
>crown, and back color) are used in their study to delineate the hybrid
>zone. Theirs is a marvelous study employing methods that are very difficult
>to assess accurately in the field.


If field studies are not adequate for sight ...... well,

I have been waiting for someone to explain to me why I felt that ID by ear
is not acceptable - just in case somebody *really* thought it was .....
and this seems to be a fairly good explanation on why NOT .....

Toni