Figure 14.1 Number of species with median migration dates in each five-day period through the year, based on: (a) 18 species that pass each spring and autumn through Alaska at 64° 50'N; (b) 18 species that pass each spring and autumn through Minnesota at 44° 55'N; and (c) 33 species that pass each spring and autumn through Pennsylvania at 40° 40'N, together with their bisectrix dates (midway between median dates of spring and autumn passage). Migration dates shown in black, bisectrix dates in grey. Note the vastly greater spread in migration dates than in bisectrix dates, and the greater spread in autumn than in spring dates. (a) from Preston (1966), (b) from Winker et al. (1992a), (c) from Benson & Winker (2001).
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