VENEZUELAN ORNITHOLOGY — FRIEDMANN AND SMITH 465 



US (Tropic-Zone birds, mainly of the savanna and seasonally deciduous 

 forest), we have combined our breeding records with those of Cherrie 

 (1916, upper Orinoco region) and of Belcher and Smooker (1934-37, 

 Trinidad). Wliile the procedure of combining data from different 

 localities is open to criticism, the places are similar in their ecological 

 features, and the entire area involved is only slightly larger than that 

 of the States of Pennsylvania and New Jersey. 



It will be noted that while some breeding activity continues 

 throughout the year, it increases sharply in March, reaches a peak 

 in April, May, and June, and then drops to a minimum in December. 

 The strongly cyclic nature of avian breeding rhythm in our area of 

 South America agrees generally with Skutch's graph of the species 

 breeding at approximately the same latitude in El General, Costa 

 Rica, which shows a sharp peak of activity in April and May, with 

 a corresponding low in October and November. The slight difference 

 may be due either to the different faunal zones involved (he was 

 reporting on subtropical birds at 2,000 to 3,000 feet above sea level 

 whereas we are reporting on lowland Tropic-Zone birds) or to 

 insufficient observations. 



The peak breeding months in our area correspond with those of 

 the North Temperate Zone perhaps more than climatic or photo- 

 periodic conditions would lead one to expect, although it appears 

 that the causes for the pronounced rhythm may be different than 

 those for that of the Temperate Zone. Temperature can hardly be 

 a factor, for the temperature in northeastern Venezuela fluctuates 

 very slightly from the mean of 80° F. For example, in 1944 the 

 approximate average mean temperature (F.) at Cantaura, Anzodtegui, 

 were: Jan. 75, Feb. 79, Mar. 80, Apr. 82, May 83, June 79, July 

 80, Aug. 83, Sept. 82, Oct. 82, Nov. 81, and Dec. 80. 



While an increase in total hours of daylight might appear to be the 

 stimulus, or one of the stimuli, for increased gonadal activity, the 

 rate of daily change in length of daylight at latitude 10° N. is slight 

 (about 15 seconds) and does not correlate with the rapid increase in 

 breeding activity that takes place (fig. 104). And, although our 

 area of the Tropic Zone has nothing directly comparable to the over- 

 cast days of the more northern latitudes, some of this slight increase 

 in dayhght during May, June, and July may be offset, so far as effect 

 on birds is concerned, by the heavy rains characteristic of the period. 

 It must be remembered, too, that migration, which is intimately 

 connected with the breeding cycle, may cause considerable variation 

 between the total hours of daylight experienced by different species 

 (fig. 105), particularly for birds migrating to nesting grounds at 

 latitudes of 40° N. and beyond. Yet the breeding seasons of these 

 different species apparently remain remarkably uniform, even though 



