So far the fall of 2012 has been the best in years in the northeast US for irruptive finch species from the north. This weekend I recorded the flight calls of three of these species at Pennypack Trust in Huntingdon Valley, Pa, just north of Philadelphia. The first two, Common Redpoll and White-winged Crossbill, have "tchet" calls that can easily be confused when, as with these flyover birds, they aren't giving other flight calls, (the redpoll also gives a trill, the crossbill a House Finch-like vheet). Fortunately, the sonagrams are very different.
This distant Red Crossbill was a type 3, the most likely type in this part of the country. The call is quite distinctive, it sounds a little like a Tree Swallow.
Red Crossbill calls are at 3-4kHz
The chance of detecting these birds is far greater by listening for their calls as they fly overhead than by finding them feeding in their favorite trees. Recordings help confirm their identification, and are especially helpful in the case of the Red Crossbills, which have at least ten call types.