Thank you Dr. or Mr. Stoker for pointing out Bob Dooling's extensive research
(much of which is on budgerigars, to be sure) and other important points. The
literature on this topic is not thin or hard to find.
I would like to add that there are species of nearctic/neotropical Dendroica
warblers whose songs extend above 6 and even above 9 or 10 kHz, as can be
readily seen in the classic Robbins et al. guide published by Golden Books.
These are different species from those in question, of course, but they
certainly are songbirds. The target listeners are probably conspecifics.
Ron Larkin
