32 THE CONDOR ' Vol xt 
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 seemed 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 eyi- 
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 were 
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 this species of bird. When 
I finally succeeded in getting them to take wing, I obtained fifty-four maggots 
trom 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 exam- 
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 occupants. 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 
when they left the nest about two weeks 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 from 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. For these I substituted 
four, 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 
