I found the Mangrove Warbler a rare bird, but my previous experience with this species in Panamá, the Pearl Islands, and in Yucatan is what made me successful. I learned its song and alarm note in 1893. The first morning I went into the mangrove swamps of La Paz I whistled the song of the Yucatan species and the birds answered me; this is the secret of my success, for the species is very secretive in its habits. I found it so difficult to get that I offered fifty cents apiece to the duck hunters and others, including the local taxidermist, but they all failed to get it! By covering eight miles of territory I generally managed to get four or five. Sometimes when I shot one It would fall in the mangroves, with a tide running fast. Under such conditions it generally took a long time to find it, and a great deal of cutting with the machete. Referring to the form found in El Salvador, Dickey and van Rossem (1938) remark: "To add to the difficulties in the path of the collector, the brown and yellow plumage of the males blends perfectly with the dead or dying mangrove leaves which are kept in continual motion by the sea breeze." _Nesting._--Brown sent Colonel Thayer (1909) three nests of the mangrove warbler, only one of which contained a set of three eggs. Of this he says: "The nest with eggs is made (and the others resemble it very much) of light green fern down, cobwebs, and light-colored dried grasses, with a few white feathers plastered on the outside. It is beautifully lined with feathers. It is not so perfectly shaped or so well made as the Yellow Warbler's nest." There are now six beautiful nests of this warbler in the Thayer collection in Cambridge, all collected by Brown near La Paz on dates ranging from May 15 to June 2; all were placed in the red mangroves, either on horizontal branches, mostly near the ends, or in forks; the heights from the ground or water varied from 2 feet to 10 feet. The largest and handsomest nest was 10 feet up on a horizontal branch; it is a very neat, compactly woven cup, made of soft, fine, light buff plant fibers, mixed with plant down, green moss that looks like down (probably the "light green fern down" referred to above or _algae_), a few gray lichens and many whitish flower clusters; it is lined with very fine fibers, apparently from the mangroves, and plenty of feathers; it measures externally 3 inches in diameter and 2-1/2 inches in height; the inner diameter at the top of the in-curved rim is 1-3/4 inches and the cup is near 2 inches deep. The smallest nest measures only 2-1/4 inches in outside diameter. The shallowest nest is only 1-1/2 inches high and 1-1/4 deep inside. These nests are all works of art and quite distinctive; all the materials are smoothly and compactly felted, being tightly plastered together, as if glued on when wet. The light color and compactness suggest certain hummingbirds' nests. Most of the nests seem small for the size of the birds.