_Specimens examined._--Total number, 48 (17 males, 19 females, 12 unsexed), as follows: Mariana Islands, USNM--Guam, 21 (Jan. 29, May 20, June 21, July 20, 29)--Rota, 1 (Oct. 27); AMNH--Guam, 18 (Jan. 22, 29, Feb. 15, July 10, Aug. 11, 12)--Saipan, 8 (Sept. 17). _Remarks._--The taxonomic relationships of the species and subspecies of the genus _Collocalia_ are not fully known. The many different name combinations applied to the five kinds named from Micronesia are evidence of the lack of agreement among previous writers as to the correct systematic positions of the kinds. The genus is widely distributed in southeastern Asia and adjacent islands and is divisible into a number of species and subspecies. This diversity is apparently influenced by the restriction of the birds to local habitats caused, as Stresemann (1931b:83) states, by the necessity of staying by their nesting areas which are in caves. Stresemann also points out that the birds are thus dependent on "narrowly limited ecological conditions." The birds are confined to certain areas and are, therefore, isolated from other populations. Most of the volcanic islands of Micronesia have numerous caves which are suitable to the swiftlets for nesting. _C. inexpectata_ evolved in the Malayan region and apparently spread to Micronesia via the Philippines to Palau and to the Marianas. The two subspecies of _C. inexpectata_ in Micronesia resemble closely those to the westward but are smaller. I am following Peters (1940:224) in the treatment of these, and although some future reviser may rearrange these species and subspecies, it appears to me that the Micronesian swiftlets fall into the two natural groups (_C. inexpectata_ and _C. inquieta_) now recognized, even though their parent stocks in Malaysia, in my opinion, are inadequately known. At Guam and Rota, the NAMRU2 party found swiftlets concentrated at cliff areas, flying about in large groups. Away from the cliffs fewer were seen and singles were observed in woodland openings, over fields, and in the coconut groves. On May 18, 1945, a colony of nesting birds was found approximately two miles east of Agaña on Guam. This colony was in a coral sink-hole which was approximately 75 feet deep and 60 feet in diameter. The nests were grouped in clusters of 5 to 25 or more, on underhanging ledges, sheltered from the light. The nests, which were fastened securely to the irregular ledges, were knocked down by shots from our collecting guns. Approximately 250 nests were found; no eggs were observed, the nests containing young birds. The young were in various stages of development; some were with little feather growth, others were completely feathered. Nests examined contained only one young each. The pile of guano below each cluster of nests was large; an estimate made at the time indicated that there were 10 or more tons in each pile. Guano deposits in large quantities were found also in caves at Amantes Point, Guam.