2 THB A UeD USB O Nee B Uri alae 
terest in protecting it while it is being constructed. She uses fine material 
firmly attached to the twigs to make the structure sturdy. The exterior is 
composed of tightly woven strips of bark, fine grasses, tiny weed stems, and 
other plant fibers. It is lined with finer materials such as plant downs, 
feathers, and very fine plant fibers. When completed, the nest is a durable 
compact cup, retaining its shape well under adverse weather conditions. A 
few nests are poorly constructed, probably the work of the young, inex- 
perienced females. 
Apparently there is a wide range in the 
dates that the first eggs are laid. My notes 
show the start of egg-laying from May 18th 
to June 18th. This variation is due, to some 
extent, to nest failure and a second start at 
egg-laying. There is also reason to believe 
that some pairs raise two broods in the same 
year, as I have seen indications of mating in 
ae the middle of June. It would, of course, be 
ree Be Byerage size soa, necessary to band the birds (whicheisaqmaas 
cover a cowbird egg. It is an do) to determine with any degree of accuracy 
eae ‘pear tpcres which pairs were starting to nest for the first 
the author) time. However, from the observations I have 
made, it seems reasonable to suppose that the 
start of egg-laying in the first nests varies over a period of a week or 10 
days. 
The eggs are whitish, spotted with shades of brown, with most spots 
concentrated at the large end. Under more normal conditions, four to six 
eggs are laid, but the nests I found contained four or less. This was due 
to the persistent parisitism of the cowbird upsetting the progress of the 
warbler’s nesting cycle. It is a normal condition for the yellow. warbler to 
be one of the hosts of the cowbird, but not to the extent found in the 15 
nests I studied. All except two of these contained one or more cowbird eggs 
at some time during the nesting period, and one nest was parasitized five 
different times. 
I have no figures available on this subject in relation to the yellow 
warbler, but studies of the song sparrow (which is by far the most common 
victim of the cowbird) generally show an average of less than 50 per cent 
of its nests to be parasitized, against 87 per cent for my nests. 
The cowbird starts laying its eggs in the warbler’s nest before its com- 
pletion and often lays in it after its desertion by the young. Unlike most 
other birds that are hosts of the cowbird, the yellow warbler can distinguish 
its own eggs from the parasite’s eggs. So when one or two of the latter 
appear in the nest, the warbler disposes of them by covering them with a 
new addition and lining on the nest. So industrious are they in covering the 
eggs that they rebuild the nest as many as six times if the cowbird is per- 
sistent. They even bury their own when a cowbird’s egg appears among 
them. The ordinary nest is two inches high. A cowbird egg buried in it adds 
one-half inch to it. One nest I studied was 5% inches high, and it was ob- 
vious that it had been rebuilt at least five times. 
