Jan., 1919 PROBLEM: DO BIRDS MATE FOR LIFE? bef 
between nesting periods, although it, like the Blue-throated, returns to the same 
nest twig in successive seasons. 
But even if a mate were replaced, might not ihe individual nesting charac- 
teristics be preserved? Does it not seem probable that a female bird normally 
returns to her bridal-nest locality throughout her life time, even though she hab- 
itually forms a new partnership with each successive nesting season? Uniforin- 
ity in number of eggs, shape of eggs, shade of ground color, style of markings, 
kind of spot chosen for nest, individual taste in nest construction, could all be 
satisfied on this hypothesis. Possibly the death of the female would end the 
series, doubtless so in regard to egg characters and in species where the female 
alone builds the nest, but might not the surviving male often bring his new mate 
to the old home, particularly if she had not nested before? How much ‘‘my 
former mate did it this way’’ talk would avail is, of course, problematical, but 
an incident related to the writer by Major Allan Brooks indicates a certain 
tendency along that line. A female Cooper Hawk had been shot from her nest of 
eges. Some days later another female, in adult plumage, was found incubating 
the same eggs, and was likewise shot. What was his surprise later to find a third 
female occupying the nest, this time a bird in the streaked plumage of a sub- 
adult. And as a matter of curiosity she was allowed to, and did, raise the brood. 
Environment must exert an important influence on the nest. This would 
include availability and abundance of nest material normally acceptable within 
reasonable gathering distance, and, as well, the nature of adaptable sites in the 
neighborhood chosen for the nest. For while an adaptable site is no doubt the 
primary requisite to location, in the final analysis food and climatic conditions 
must be the determining factor, or at least the delimiting factor; and, since in 
many species each pair occupies its nest vicinity to the exclusion of all others of 
- its kind, abundant population of such species would soon exhaust normal avail- 
able localities where such localities were limited, and thus crowd some pairs into 
‘“ynusual sites’’. 
Then, given absence of usual nest material, or of usual nest site, or presence 
of attractive but unusual nest material in the locality selected, might not suc- 
cessive different nest builders readily display ‘‘unusual nest’’ characters suffi- 
ciently similar to suggest the same artist, and deceive even the most careful ob- 
server ? 
A striking example of environmental influence which came to the writer’s 
attention is worth recording, though unfortunately no further data were obtained 
in subsequent years. A Black-headed Grosbeak, whose normal nest does not dif- 
ter materially from that of the Rose-breasted, chose a rather open willow thicket 
some distance from other woods, and in the midst of a field of rank, green, 
growing wild oats, which also covered the floor of the thicket. The nest was 
made almost entirely of stalks including heads of the wild oat, still green, and so 
carelessly put together that the projecting bends and ends made a mass as big as 
a hat. (See fig. 10.) 
It would be interesting to record how many successive seasons individually 
distinguishable sets and nests were found. Collecting a male here, a female there, 
and both parents in another place with the characteristic nest, and noting the 
effect on later nests, sets, and locations, would be of great value, and mating 
characteristics of families, perhaps of species, thus be worked out. Truly, Mr. 
Willard has opened up a most fertile field. 
Los Angeles, California, November 1, 1918. 
