VENEZUELAN ORNITHOLOGY — ^FRIEDMANN AND SMITH 467 



lating the physiological reactions governing the migration and breed- 

 ing cycle, it hardly appears to be an important factor for migrants 

 which "winter" in the tropics close to the equator, where the length 

 of daylight is so constant. One is indeed hard put to postulate a 

 periodic, external stimulus capable of initiating the migration of birds 

 "wintering" near the equator. For example, at approximately 

 latitude 9°30' N. in Venezuela the regular northward withdrawal in 

 the spring of such migrants as the dickcissel (Spiza americana) and the 



DJFMAMJ JA SONDJ FMAMJJ ASOND 



Figure 105.— Approximate hours of daylight for certain migrant birds compared to hours 

 of daylight (sunrise to sunset) at lat. 40° N. and lat. 10° N. (vertical scale, 10 mm. 

 equals 1 hr.) : a, Muscivora t. tyrannus; b, Pluvialis d. dominica; c, Dendroica striata; 

 d, d. petechia aestiva. 



yellow warbler {Dendroica petechia aestiva) appears to be without 

 observable external "cause" and in spite of increasingly favorable 

 conditions at the wintering grounds. By the first of May, the hours 

 of light per day have increased but little (around 53 minutes) over 

 the December minimum; there is nothing obvious about the climate 

 that would induce the birds to leave; and their departure comes at a 

 time close to the peak breeding season of the resident population. 

 Moreover, the closely allied yellow warbler {Dendroica petechia 

 rujopileata) remains in the same general area throughout the year. 



