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 32 THE CONDOR Vol. XXI One of the nestlings died about two days later. Though it seemed plausible that death had been caused by the maggots, a post-mortem examination furnished no absolute proof for this assumption. The other two nestlings s. eemed to be con- siderably retarded in their growth, but finally left the nest. After the departure of the young, only forty-three of the ninety maggots were recovered from the nest, some of'them in the form of pupae, showing that many of them had evi- dently lost their bearings and fallen out of the nest. None of them showed any trace of red. These facts made my former conclusions, that these maggots v, ere blood-sucking parasites, appear doubtful. About this time I was observing two California Linnet nestlings. Despite the fact that there were only two young instead of from three to five, as is usu- ally the case, I noticed that they were growing very slowly and that they lacked the same vigor and liveliness usually exhibited by his species of bird. When I finally succeeded in getting them to take wing, I obtained fifty-four maggots rom their nest,'many of which showed the sought-for red substance similar to that observed in 1913. These maggots were only about two-thirds the size of those taken from the first few 'nests, but to all appearances belonged to the same species. At the laboratory several of the maggots with the red substance in them were decapitated and smears made from this red material which was then ex.am- ined under a high power microscope. The red substance proved to be fresh ver- tebrate blood which of course could only have been obtained from the birds in- habiting the nest. But in order to prove this beyond a doubt, a number of addi- tional experiments were carried out. A few days later I obtained twenty exceedingly small maggots from a Cali- fornia Linnet nest just vacated by its feathery pccupants. Although only about one-fourth grown, every one of them showed traces of fresh .blood. 'All twenty were placed in the nest of a Green-backed Goldfinch containing three young about ten days old and the latter observed from day to day. Nothing extraor- dinary happened, excepting that the three nestlings seemed to be rather weak vhen they left the nest about two a:eeks later. When the nest .was picked apart, all twenty maggots were recovered,. They had meanwhile reached fall growth. None of them however showed any trace of blood, but merely the usual dark brown substance in the posterior end of the intestine. TheSe facts tend to indi- cate that the maggots are voracious feeders during the earlier period of their ex- istence, but that they refrain from taking food after they have attained full growth. As probable as it seemed that the vertebrate blood found in the maggots was avian blood obtained frown the nestlings, this had not yet been absolutely proved, since I had never seen any of the maggots attached to the nestlings. Moreover my experiments up to that time, due to the difficulty of closely observing nest- lings of wild birds in the open, had naturally been. somewhat superficial. In order to remedy this difficulty, I decided to use a brood of tame canaries for subsequent experiments. 'I succeeded in securing a female bird with two nest- lings about a week old, but both young died within a few days, probably because they had taken cold while being transferred to my room. in'or these I substituted gour, nearly full-fledged Green-backed Goldfinches. The old bird seemed to no- tice the deception, but when she saw the four open, hungry mouths, she adopted the strangers as her own. A few days later I selected from some 200 maggots, forty of the most active ones and placed them on the young birds. The maggots