As for the breeding season, there is good evidence of differences in time of completion of clutches. Since date of completion can be expected to change from place to place, comparisons must be made at one fixed locality. Of 15 nests of black throated green warbler found by Harlow in the Poconos of Pennsylvania, the mean date of clutch
completion was June 3, and of 21 blackburnian the mean date of clutch completion was June 1. Thus, for this region, and there is no reason to think that the relative dates are different in other regions, there is little difference in time of nesting between blackburnian and black-throated green warblers. From the extensive collections of P. B. Philipp near Tabusin tac, N. B., now in the American Museum of Natural History, and from a smaller number colleered in the same region by R. C. Harlow, now
in the Museum of Comparative Zoology, baybreasted and Cape May warbler nest dates can be
compared (Figure 9). It is quite clear that the bay-breasted with the median date of nest discovery of 25 June (95'10 confidence interval for the median 23-27 June) nest substantially later than the Cape May whose median date is 17 June
(95'/0 confidence interval for the median of 16 J une-20 June). As the figure shows, the small number of nests of black-throated green and myrtle from the same region show a fairly wide spread but strongly suggest median dates intermediate between Cape May and bay-breasted. (The dates recorded by Palmer (1949) for Maine give a roughly similar sequence; myrtle, 30 May 6 June; black-throated green, 26 May-20 June;
bay-breasted, after 7 June.)