of an anesthetized bird and searching for neurons that were activated
when sounds were played.As in the behavioral tests,these fledglings
were housed in acoustic isolation so that they had no opportunity to
learn songs.Test stimuli included those used in behavioral tests,as well
as broad band noise bursts and pure tones of varying durations.
Similar to the behavioral results,neurons in and around HVC
responded strongly to normal and isolate white-crowned sparrow song,
and individual phrases such as whistles,buzzes,and trills taken from
white-crown songs. In contrast to the behavioral results,however,
neurons also responded to similar phrases contained in foreign songs.
Such neurons might serve as phrase detectors,and could underlie strong
behavioral responses to artificial songs composed of only one repeated
white-crowned sparrow phrase type.
When results from the behavior test and electrophysiological record-
ings are compared, it appears that fledglings discriminated white-
crowned sparrow song from foreign song more reliably than did the
population of neurons that were sampled.There are a couple of possible
reasons for the difference between these results.The behavioral response
to taped stimuli is the final product of many neuronal inputs and pro-
cessing steps.Thus,one possibility is that we recorded from an area close
to the beginning of the auditory pathway that detects simple acoustic
features of song.These auditory neurons may then project to areas of
the brain responding to more complex song features,eventually giving
rise to species-selective neurons.Alternatively,it is possible that no single
neuron will exhibit species selectivity.Instead,the response of many
neurons in unison may provide a recognition signal to the bird that the
song is to be learned,for example,the synchronous firing of ensembles
of whistle-,buzz-,or trill-responsive neurons.Although some foreign
songs contain these phrase types,they are not composed entirely of
them,as are white-crowned sparrow songs,and thus would not produce
as strong a signal.
To summarize,young birds recognize and choose to learn the songs of
their own species,even when raised in the laboratory in acoustic isola-
tion.In our studies,such white-crowned sparrow fledglings were able to
recognize songs composed of single white-crown phrases,indicating that
recognition of song is not dependent on normal phrase order or song
complexity.This ability may allow youngsters to identify and memorize
the songs of any white-crowned sparrow that they encounter,whether or
not all phrase types are present.Electrophysiological studies with fledg-
lings uncovered auditory neurons that were responsive to phrase type,
although these neurons were not yet selective for white-crowned
sparrow song.However,in adult birds the response properties of audi-
tory neurons in these brain regions are more selective,responding72 Carol WhalingMUS5 9/14/99 11:59 AM Page 72