530 BULLETIN 20 3, UNITED STATES NATIONAL MUSEUM 



ring, but females and young males have an incomplete eye ring in 

 the fall, making their recognition difficult. 



Enemies. — ^This warbler is evidently a rare victim of the cowbird ; 

 I can find only four records of such parasitism. 



Fall. — The autumnal migration route of the mourning warbler is 

 apparently a reversal of the spring route. The bird is an early 

 migrant, leaving its breeding range in July and August, and appear- 

 ing in Central America early in September. It seems rarer in the 

 fall than it probably is, for it is very secretive, skulking through dense 

 thickets and rank herbage; it is mainly silent, also, which helps to 

 make it seldom observed. 



Winter. — Dr. Alexander F. Skutch contributes the following ac- 

 count: "The mourning warbler is an abundant winter resident in 

 the lowlands of Costa Rica, up to an altititude of about 4,000 feet. 

 It is numerous on the lower Caribbean slope, and equally so in the 

 basin of El General on the Pacific slope, where at an elevation of 

 3,000 feet it is still one of the most abundant of wintering warblers. 

 But in the drier northwestern province of Guanacaste, it is rare or 

 absent. Avoiding the woodlands, it frequents low, dense thickets 

 and fields overgrown with tall weeds and rank grass, where it reveals 

 its presence by its constantly repeated sharp call-note, yet is difficult 

 to glimpse. It is solitary during the winter months. 



"During the exceptionally wet year of 1937, a mourning warbler, 

 who lived in a weedy field close by my cabin, sang repeatedly, espe- 

 cially on rainy or darkly overcast and threatening afternoons. I 

 first heard him on February 15, and then at intervals vmtil late in 

 March. As he flitted about through the wet vegetation in search of 

 insects, he would sing a low but full, smoothly flowing, long-continued 

 warble — an exceptionally beautiful song, much like that of the Central 

 American ground-chat. On April 24 of the same year, I heard another 

 mourning warbler sing, but this time only a few detached syllables. 

 This bird then proceeded to eat the protein bodies from the leaf- 

 bases of a young Cecropia tree. These tiny, white, beadlike cor- 

 puscles, produced in numbers on the brown, velvety bases of the long 

 petioles of the broad Cecropia leaves, are the special food of the 

 Azteca ants that dwell in the hollow stems of this fast-growing tree. 

 When the tree is occupied by its usual colony of ants, the corpuscles 

 are removed as fast as they ripen; but if the tree chance to remain 

 untenanted, they accumulate and become a dainty food for a variety 

 of small birds. 



"Although it passes on migration through northern Central 

 America, the mourning warbler has been rarely recorded there. It 

 arrives in Costa Eica in September, usually late in the month, and 



