AVIAN MIGRATION AND REPRODUCTION 711 



demonstrated a tendency toward larger testes in the regressed condi- 

 tion. 



During the last few years additional studies have been made of 

 breeding cycles in the Tropics, a few experiments have been per- 

 formed, and the effect of external factors, particularly drought and 

 rainfall, on reproduction has been studied (Keast and Marshall, 1954; 

 Marshall and Disney, 1956, 1957; Miller, 1954a,b; Serventy and 

 Marshall, 1957). 



Miller has shown that in the uniform environment of the Mag- 

 dalena Basin in Colombia (3°12' N. Lat.) 8 out of 10 species studied 

 showed acyclic and uncoordinated breeding; two species showed co- 

 ordinated and cyclic breeding. All the species showed "evidence of 

 possessing the same innate mechanism basic to cyclic breeding as 

 north temperate species. This consists of need for rest, or assumption 

 of a refractory state, and a basic tendency to progressive recrudes- 

 cence." Miller regards these physiological attributes, and the fact that 

 young passerines are able to breed at ages of 4 to 9 months, as the 

 only known elements to be thought of as an inherent rhythm. He re- 

 gards day length, or light stimuH, as sufficient to surpass threshold 

 needs in the innate mechanism, and as a regulator of the duration of 

 refractoriness, rate of recrudescence, and duration of breeding condi- 

 tion. 



In the cyclic species, he suggests that other stimuli also come into 

 play to retard or advance the phases of the innate rhythmic tenden- 

 cies. (It is also possible that these species differ from the acyclic 

 species in their response to photoperiods and dark periods.) Miller's 

 data and interpretations are in general agreement with the hypothesis 

 and interpretations put forth earlier (Wolfson, 1952a) and elaborated 

 in this paper. 



Marshall and Disney (1956) have tested the response of an equa- 

 torial bird. Quelea quelea, to increased photoperiods. One experi- 

 mental group received 5 min of light daily at dusk; the other received 

 an additional 5 hr of light. Treatment was given for eight weeks. The 

 birds which received only 5 min additional per day failed to respond, 

 whereas the birds receiving an additional 5 hr per day responded. On 

 the basis of the data presented in this preliminary paper, the response 



