32 BIRDS OF THE NEW YORK CITY REGION 



The fourth "wave" of the season is what is technically 

 known as the " height of the migration," and takes place 

 chiefly between May 16th and May 19th. If all goes well, 

 and the season is normal, the maximum number of birds is 

 present at this time, and those who are so fortunate as to be 

 afield the day when this "wave" arrives, have recorded 100 

 or more species. It will bring the birds listed below, plus any 

 in the earlier groups which have not yet appeared : 



(May 10-May 19) 



Yellow-billed Cuckoo Wood Pewee 



Black-billed Cuckoo Indigo Bunting 



Cedar Waxwing 



The above summary may be described as the normal or 

 ideal May migration. As there are obviously an infinite 

 number of combinations of weather and temperature which 

 can occur in 19 days, it follows that this ideal migration rarely 

 takes place without more or less extensive modifications. It 

 must be remembered that a "wave" is usually due to un- 

 favorable weather damming the flood of birds moving north- 

 ward for several days. A change to ideal conditions means an 

 opportunity which the delayed host immediately seizes, and it 

 rushes forward en masse. However, should the unfavorable 

 weather continue over a protracted period, two or more 

 "waves" are telescoped, the species arriving irregularly and 

 in small numbers at a time. Long-continued ideal weather 

 may have exactly the same result. Birds move forward every 

 night throughout the period, and on no one day is there a 

 sufficiently marked concentration to be called a "wave." 

 Such seasons of both classes are particularly unsatisfactory 

 and disappointing to the observer, who usually misses most 

 if not all of the rarer transients, which as a rule are only re- 

 corded on days when other birds are particularly abundant. 

 Perhaps the table below, giving days of the month on which 

 "waves" have arrived during 12 years in Central Park, based 

 on my own daily observation, will give a better idea of the 



