Nocturnal flight call of bicknell's thrush

Audio recordings of nocturnal flight calls of migrating birds along the east- central Florida coast in May have documented calls that sound similar to those from Gray- cheeked Thrushes (Catharus minimus). Spectrographic comparison of these "Florida gray- cheeked" calls with Gray-cheeked Thrush calls recorded from Minnesota, southern Alabama, and west-central New York State shows that the Florida calls have distinctive acoustic features. Speculation that the "Florida gray-cheeked" calls are from the Gray-cheeked sub- species (C. m. bicknelli), now proposed as a separate species, Bicknell's Thrush (C. bicknelli), is supported by spectrographic comparison with a diurnal Bicknell's Thrush call and the coincidence of time and location of the "Florida gray-cheeked" recordings with the known timing and migration route of Bicknell's Thrush. Received 21 Dec. 1992, accepted 6 May 1993. Since Ball's (1952) description of the nocturnal flight call (nf-call) of the Gray-cheeked Thrush (Catharus minimus), no progress in identifying the nf-calls of migrating passerines has been reported. However, nearly every fall migration summary in the "Audubon Field Notes" since the late 1950s contains one or more accounts of flight calls heard at night. This paper presents analyses of such calls. METHODS Audio recordings were made on evenings when steady calling occurred throughout the following passerine migration seasons in the regions indicated; Minnesota (spring and fall 1987), southern Alabama (Oct. 1989 and spring 1990), west-central New York State (spring 1988 and fall 1988-1991), and east-central Florida (spring 1989 and 1991). The majority of these recordings. and those pertinent to this paper, were made with a Sennheiser 816T "shotgun" microphone with zeppelin windshield, a Shure FP-11 microphone pre-amp, and a Sony TCD-D 10 digital audio recorder. A call-type's presence or absence in a region was associated with known migrants for that area. Similarly, associations between call-types and species were deduced by comparing the dates when call-types were recorded with migration timing derived from historic diurnal observations in each region. In many cases, identifi- cation of an nf-call was supported by comparing it with a recording of an analogous diurnal call made by a visually-identified bird. In the spring of 1989, audio recordings of nocturnal flight calls of migrating birds were made at Merritt Island National Wildlife Refuge (MINWR), 5 km east of Titusville, Brevard County, Florida. The recording effort began on 31 March and continued every evening through 6-7 May. In the early morning of 6 May, two nf-calls were recorded that sounded like those given by the Gray-cheeked Thrush, yet seemed to be different from Gray-cheeked nf-calls that had been recorded from other regions of North America. Because only two of these unusual calls were obtained, and it was known that Catharus thrushes have a fair amount of variation in their calls (Ball 1952), this impression lay dormant. In the spring of 1991, audio recordings of nf-calls were again made in peninsular Florida.