MIGRATION AND ASPECTIOX 225 



regional variations in the advance of spring. This regional discrep- 

 ancy is exhibited likewise in the ratios between returns at extremes 

 and at other years; these are respectively 2:3 at Ann Arbor and 1:7 

 at Oberlin for the grebe, 1:4 and 0:6 for the loon, and 1:6 and 0:7 

 for the chimney swift. However this may turn out, it seems evident 

 that there is no direct correlation between sunspot numbers and time 

 of return. This is indicated furthermore by the occurrence of returns 

 at both maxima and minima, in spite of differences of 50-100 spots. 

 It is also indicated by the fact that the number of early returns 

 within 1 year of the various extremes, beginning with 1883, bears no 

 consistent relation to the number of spots, as seen in the following: 



Year 1883 



Spots 64 



Returns 3 



Year 1889 



Spots 6 



Returns 14 



Migration and Aspection. As in the case of the plant matrix, birds 

 and insects also tend to appear in more or less definite seasonal 

 aspects. In the deciduous forest area at about 40° north latitude, 

 the ]irevernal group of birds begins to pass through in February or 

 ]\larch and continues until late April; IMay is marked by the second 

 or vernal grouj), and this is followed by a few species that are estival 

 and serotinal. In these northward movements the birds appear in 

 so-called weaves (Wood, 1906; Smith, 1930), approximately a dozen 

 of these being regularly noted in the region of Oberlin (Jones, 1931). 

 The assumption is that species move forward until low temperatures 

 halt them, a considerable accumulation forming, and that the mass 

 resumes the northerly movement as soon as this barrier is lifted. 

 Stone (1891) has pointed out that the greatest migration takes place 

 on warm nights with falling or low barometer and the least on cool 

 nights of rising pressure, and the conclusions of Cooke (1913) and 

 Smith (1930) are in essential accord. Smith, who carried on observa- 

 tions from 1903 to 1922 at Urbana, Illinois, states that "the greatest 

 migratory activity in spring occurred at times when the weather maps 

 showed an area of low barometric pressure approaching from the west, 



