While researching his as-yet-unpublished theory of evolution by means of Natural Selection, Darwin wrote scores of letters to friends, colleagues and complete strangers, asking for their thoughts and observations. Here is a typical Darwin query, written to his second cousin, former university friend, and clergyman, William Darwin Fox, 151 years ago today:
Can you give me any thoroughily well authenticated facts on ever so little variations in nests; I do not mean such cases as the Water owzel habitually having a doomed or open nest—or difference of Sparrow's nest in tree & in hole; but rather any slight difference in degree of perfection of nest of same species in different districts or of any individuals of same species.—
At this stage, Darwin was presumably researching animal instincts. He briefly discussed location-dependent variation in birds' nests in chapter 7 of On the Origin of Species:
As some degree of variation in instincts under a state of nature, and the inheritance of such variations, are indispensable for the action of natural selection, as many instances as possible ought to have been here given; but want of space prevents me. I can only assert, that instincts certainly do vary for instance, the migratory instinct, both in extent and direction, and in its total loss. So it is with the nests of birds, which vary partly in dependence on the situations chosen, and on the nature and temperature of the country inhabited, but often from causes wholly unknown to us: Audubon has given several remarkable cases of differences in nests of the same species in the northern and southern United States.
The water ousel mentioned and misspelt by Darwin is one of my favourite birds, mentioned previously in this blog, the dipper [Cinclus cinclus].