. Bulletin. Natural history; Natuurlijke historie. MIGRATION OF THRUSHES 57 account when estimating the numbers of migrants. The diflBculty of assess- ing their numbers may be appreciated from the assurance that grouping does occur—probably as a general rule. Confirmation of the belief that, judged by the uneven distribution of call notes, migrants travel in groups spaced at various intervals is furnished by my experience at Routhierville in the Matapedia Valley September 22, 1949. Single birds were heard at 5:09 and 5:26 , 2 at 5:28, 5 at 5:29, 10 at 5:30-5:31, 15 at 5:33-5:35, and 20 dur
. Bulletin. Natural history; Natuurlijke historie. MIGRATION OF THRUSHES 57 account when estimating the numbers of migrants. The diflBculty of assess- ing their numbers may be appreciated from the assurance that grouping does occur—probably as a general rule. Confirmation of the belief that, judged by the uneven distribution of call notes, migrants travel in groups spaced at various intervals is furnished by my experience at Routhierville in the Matapedia Valley September 22, 1949. Single birds were heard at 5:09 and 5:26 , 2 at 5:28, 5 at 5:29, 10 at 5:30-5:31, 15 at 5:33-5:35, and 20 during the next minute—the last birds heard in flight (Fig. 12). That these calls represented isolated groups —not merely vocal thrushes among a steady flight containing many silent birds—becomes evident from the distribution on the ground after dav^m. This showed the same unevenness. I had been walking south from the village along the highway beside the river. Group after group of calling thrushes overtook me and passed out of audible range down the valley. The ecstatic ground notes at dawn were wanting in the vicinity where the descent deadline of overtook me at about 5:40 Nor were any heard as I returned toward Routhierville until 5:47, when 2 hermits. Soufh Figure 12. Migration and morning descent of thrushes in Matapedia Valley. and an olive-back were encountered close to the hotel. Figure 12 explains this distribution. Considerable interest attaches to occasional isolated call notes. For ex- ample, at Sandy Beach on the morning of September 16, 1949, single thrushes were heard in flight at 12:40, 1:12, and 2:08. Each of them piped but once, demonstrating that these birds at times migrate almost silently. Probably others pass without uttering a sound. This is quite a different matter from hearing at long intervals individual birds, or small groups of them, that call frequently in flight. One feels some confidence that the latter are the only ones in the
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