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38 THE CONDOR Vol. XXI Commenting upon the fact that Protocalliphora azurea is recorded by col- lectors and dipterologists as "rare" or ?'very rare" and that specimens of this fly are only to be found in the larger museums and collections, Coutant correctly assumes (loc. cit., pp. 144, 145) "that they are not so rare as is generally sup- posed, but that the adults are peculiar in their habits, flight, etc., and for this reason are rarely taken". I-Ie then goes on to say (loc. cit., p. 145): "Few col- lectors, I imagine, have taken insects very often from the zone of air from fifty to one hundred feet above the ground, in the woods;.yet from the habits of the larvae, this is where we would naturally expect that the adults would occur". This is probably correct, but the lower l?mit, as well as the lateral, will have to be extended considerably. Most of the thirty-nine infested nests taken during the course of my experiments, as well as the two encountered in 1913, were found far distant from forests and all ot? them came from a height of three to fifteen feet above the ground. Turning now to the effect which these blood-sucking larvae of Protocalli- phora azurea have on nestling birds, my observations seem to warrant the follow- ing conclusions: (1) that from 5 to 10 percent of the parasitized nestlings die from loss of blood; (2? that some of the parasitized nestlings which do become full-fledged are so weakened by the loss of blood that they fall an easy prey to rapacious animals; (3) that the larvae of Protocalliphora chrysorrhea are prob- ably more deadly to nestling birds than those of Protocalliphora azurea. The last conclusion seems to be borne out by the case of parasitism recorded by Hen- shaw (loc. cit., pp. 87-88), where there was a fatality of nearly 90 percent instead of one of only 5 to 10 percent.. Much remains to be done along this line of investigation in order to determ- ine how large and universal the damage is which is wrought on our continent by this insect pest. Although the adults of both Protocalliphora azurea and Proto- calliphora chrysorrhea are very rarely taken by collectors (cf. Henshaw, loc. cit., p. 88; Coutant; loc. cit., pp. 144-145), my investigations prove conclusively that the former is not so rare, at least not in certain parts of the country. So far, however, only forty-four birds' nests, infested by the larvae of one or the other of these flies, have been recorded. All forty-four of these infested nests were found at three places, one near Ithaca, N.Y. (Coutant, loc. cit.), two at Welles- ley Hills, Mass. (Henshaw, loc. cit.), and the remaining forty-one in the San Francisco Bay region. It would be highly interesting, and perhaps for the ben- efit of our wild birds, if bird students ?n other parts of.the United States, as well as in Canada and Mexico, would thoroughly investigate this matter in their home districts. Massachusetts Institute of Technology, Cambridge, Massachusetts, May ?, 1918.