In twelve species of common perching birds the wing
varied (in from twenty-five to thirty specimens) from 14 to 21 per cent
of the mean length, and the tail from 13.8 to 23.4 per cent. The
variation of the form of the wing can be very easily tested by noting
which feather is longest, which next in length, and so on, the
respective feathers being indicated by the numbers 1, 2, 3, etc.,
commencing with the outer one. As an example of the irregular variation
constantly met with, the following occurred among twenty-five specimens
of Dendroeca coronata. Numbers bracketed imply that the corresponding
feathers were of equal length.[20]
RELATIVE LENGTHS OF PRIMARY WING FEATHERS OF
DENDROECA CORONATA.
---------+-----------+----------+-----------+----------+----------
Longest. | Second in | Third in | Fourth in | Fifth in | Sixth in
| Length. | Length. | Length. | Length. | Length.
---------+-----------+----------+-----------+----------+----------
2 | 3 | 1 | 4 | 5 | 6
3 | 2 | 4 | 1 | 5 | 6
| / 2 | | | |
3 | { | 1 | 5 | 6 | 7
| \ 4 | | | |
2 \ | | | | |
} | 4 | 1 | 5 | 6 | 7
3 / | | | | |
2 \ | | | | |
1 | | | | | |
} | 5 | 6 | 7 | 8 | 9
3 | | | | | |
4 / | | | | |
---------+-----------+----------+-----------+----------+----------
Here we have five very distinct proportionate lengths of the wing
feathers, any one of which is often thought sufficient to characterise a
distinct species of bird; and though this is rather an extreme case, Mr.
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