352 ANNUAL KEPOET SMITHSONIAN INSTITUTION, 19 30 



Dates at which Zarhynchus wagleri began to nest at Barro Colo- 

 rado Laboratory, 1926-1928:8 



1926 Nesting began January 8 



1927 Nesting began January 8 



1928 Nesting began January 2 



This remarkable periodicity indicates the regularity of the annual 

 physiological cycle of the species. While in the main coincident 

 with the dry season the birds' exact nesting period does not appear to 

 be affected by the annual fluctuations in the date when the wet season 

 ends, but rather is governed by those sexual changes which mark the 

 approach of the season of reproduction. They prompt the birds to 

 go, we may say to migrate, to the nesting tree. The extent of the 

 migration we do not know. The birds may spend their lives within 

 a radius of not more than a mile or two from the place of their birth. 

 The significant fact is that the journey to their nesting range is 

 begun in response to a periodically recurring physiological condition, 

 that it is made regularly to a definite place, presumably before 

 visited, and that as such it is fundamentally as true an example of 

 migration as though it were made from the South Temperate to the 

 North Temperate Zone. 



The case is paralleled by the return of tropical sea birds to their 

 nesting grounds situated within the limits of their winter wander- 

 ings, to which I long ago called attention in a paper ^ designed to 

 show that primarily bird migration was, and is, induced by those 

 developments in the sexual organs which precede the season of re- 

 production. Hence it follows that if because of sterility or im- 

 maturity this development does not occur, the bird in which it is 

 lacking may remain in its winter quarters throughout the nesting 

 season.^" 



The members of the laboratory colony do not all begin nesting at 

 the same time. Just as with migrants to the Temperate Zone, there 

 are late arrivals. Thus in 1926 new nests were begun as late as 

 February 11, in 1927 on February 13, and 1928 on February T. 



Short visits are paid to the nest tree some days before nest building 

 actually begins. In the season of 1926 I made no record of such visits. 

 The following year I reached the island on December 22, 1926, and 

 the appended observations were recorded before January 8, 1927, 

 when nest building began. 



* Compare also the nesting dates given beyond for the black-throated hummingbird 

 (Anthracothorax nigricollis) . 



" 1894, " Remarks on the Origin of Bird Migration," The Auk, vol. XI, p. 12. 



^" In mid-July, 1922, I took sjiecimens of both sexes of the following Arctic-breeding 

 shore-birds in southern Ecuador : Sguatarola nquatarola, 'Numenius hudsonicus, Limno- 

 dromus griseus scolopaccug. None had the sexual organs enlarged. See " Distribution of 

 Bird-Life in Ecuador," 1926, Bull. Amer. Mus. Nat. Hist., vol. LV, pp. 192, 194. 



