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PROCEEDINGS OF THE NATIONAL MUSEUM 



the annual fluctuation in hours of dajiight differs according to their 

 various migratory habits. 



Further compUcating any attempt to correlate the migration and 

 breeding cycle with changes in the hours of daylight experienced by 

 any particular species is the fact that even in northeastern Venezuela, 

 where the majority of breeding forms have no known migration, a 

 seasonal fluctuation in numbers indicates at least local and perhaps 

 extended movement. Here it must further bo noted that while we 

 may have a logical explanation for a limited seasonal north-south 



Figure 104.— Approximate avian breeding periods for northeastern Venezuela (solid line) 

 and for northeastern United States (dash line) compared with length of day from twilight 

 through twilight at lat. 10° N. (solid line) and at lat. 40° N. (dash line). 



movement, we have little explanation for the extended and at times 

 trans-equatorial migrations of some races, as well as for the aston- 

 ishing temporal precision with which they are effected. In fact, some 

 of the races which breed farthest north are among those which winter 

 farthest south, as for example the fox sparrow Passerella iliaca (see 

 Wetmore, 1926, p. 121, map). Also the general migratory pattern 

 appears to be much the same whether the migrant is of presumed 

 Holarctic origin (such as Hirundo rustica erythrogaster) or of Neo- 

 tropical origin (such as Piranga rubra) . 



Thus, while for certain northern birds of limited migration the 

 increasing hours of light per day may be an important factor stimu- 



