6 _ INTRODUCTION. 
an incomplete nest, you must judge for yourself how soon it 
will be finished. A pair of our smaller birds, in the latter part 
of May or in June, ordinarily spend from five to ten days in 
building one, and sometimes end their work sufficiently in ad- 
vance to allow the female vacation for a day or even two. 
Earlier in the season, other birds are generally occupied two or 
three weeks. Woodpeckers are very uncertain in this respect, 
and it is often difficult to decide when their nests should be 
broken into to obtain the eggs, unless one can watch them 
closely at their work (carried on chiefly in the morning) and 
observe the final cessation of chips. The creepers, nuthatches, 
Chickadees, and certain wrens customarily lay their eggs in de- 
serted woodpeckers’ holes or other cavities, which they line 
with warm materials, though the Chickadees occasionally exca- 
vate for themselves with great and long-continued labor. 
After the first egg has been Iaid, one is generally added on 
each succeeding day (apparently most often in the morning) 
until the complement is made,® before which time the nest 
should not be visited, except in cases of necessity. Most 
birds lay four or five egos (occasionally three or six) in a set, 
commonly fewer in that of a second brood than before. Many 
wrens, titmice, and. kingfishers often lay more ; the former even 
ten, or very rarely twelve. Gallinaceous birds are also prolific, 
and two cr three hen-birds are said sometimes to lay in the 
same nest. Humming-birds, eagles, and pigeons, usually lay 
two eggs in a set, as do also old birds of other species, partic- 
ularly among the hawks and owls. Many sea-birds have only 
one. If anest be found with the same number of eggs for two 
8To this law the chief exceptions are the birds of prey, and the cuckoos, but 
among the smaller land-birds the average rate of laying is one a day. Thus among 
different species the time for laying four eggs varies from three to even seven days, 
generally being four, 
