634 BULLETIN 2 03, UNITED STATES NATIONAL MUSEUM 



the winter months. At times, in the highlands, a single individual 

 of the species will attach itself to a mixed flock of warblers and other 

 small birds. On the central plateau of Costa Eica, I would sometimes 

 find a single Wilson's warbler keeping company among the coffee 

 bushes with a pair of the pretty, chestnut-headed Delattre's warblers 

 {B asileuterus delattrii)^ a resident species which remains paired and 

 maintains a territory throughout the year. But perhaps more often 

 the blackcaps pass the winter quite alone. On October 12, 1933, 1 came 

 upon two male Wilson's warblers fighting earnestly on the ground in 

 the garden of the house in which I dwelt on the Sierra de Tecpan. 

 While I did not witness the beginning of the conflict and can only 

 surmise its cause, it seems probable that this was a struggle for the 

 possession of the garden in which it was staged. They separated a 

 few moments after I came upon them ; and I did not hear them sing, 

 as migratory warblers will sometimes do under similar circumstances. 

 As the date for their departure approaches, these long-solitary 

 warblers tend to draw together in flocks for their northward flight. 



"On April 27, 1933, I heard a Wilson's warbler singing in the 

 Guatemalan highlands. Five days later, I found another caroling far 

 more whole-heartedly, repeating several times over his simple but 

 happy little lay, a rapid chipping gradually ascending in pitch. These 

 songs hearalded their northward departure. By the latter part of 

 April, few Wilson's warblers remain in Costa Rica, although they have 

 been recorded as late as May 5. In Guatemala they linger somewhat 

 later, a few individuals tarrying until past the middle of May. Long 

 before they depart from Central America as a whole, these warblers 

 appear to withdraw from the lower altitudes at which they are not 

 uncommon earlier in the year, probably merely ascending the moun- 

 tains to higher levels. Thus, in the lower Motagua Valley in Guate- 

 mala, at about 500 feet above sea level, I found them fairly abundant 

 in January and February, but did not record them later. In the 

 Pejivalle Valley of Costa Rica at 2,000 feet, I found a number in 

 February, 1934, but failed to see a single one when I revisited the 

 locality in April, 1941. In the Basin of El General, on the Pacific 

 side of the same country, they may be exceedingly numerous at an 

 altitude of 3,000 feet from their fall arrival until the following March, 

 when they rapidly grow more silent and become fewer. In dry years 

 they are seen in April only as rare transients, while in abnormally 

 wet years they may remain somewhat more numerous. This local 

 movement, well in advance of the main northward migration, is 

 paralleled by that of the Tennessee warbler; it appears to be caused 

 by increasing dryness or higher temperatures, or by the two in combi- 

 nation. 



"Early dates of fall arrival in Central America are : Guatemala — 

 passim (Griscom), September 11; Sierra de Tecpan, 8,500 feet, Sep- 



