62 
Introduction: Variation. 
Lalage leucopygiaUs and Graucalus bicolor correspond in coloration to a consider- 
able extent, male with male, and female with female. These cases call for 
consideration later on. 
3. Seasonal Changes. 
The modifications which birds undergo at certain periods of the year seem 
to depend sometimes upon climatic, sometimes upon sexual conditions. The 
breeding season however is regulated by climatic conditions, the young being 
brought forth at a period when food is abundant; consequently climate should 
be regarded as promoting all periodic variation. Climate alters the appearance 
of the surface of the earth - — causes it to be clothed with a luxuriant vegetation 
or covered with snow and ice, now bringing forth an abundance, and then re- 
moving the supply of food — and organisms are modified to suit these condi- 
tions. In the tropics, as, for instance, in Celebes, where a contrasted summer 
and winter does not exist, but only a fine and a rainy season, strongly marked 
periodic changes in the plumage of the birds are rarely seen. More than 
160 peculiar species are now known from the Celebesian area, and seasonal 
changes are not known to occur in a single one of them, though 
sexual differences are common enough. A few tropical or subtropical Herons 
[Ardeola, Herodias^ Bubulcus), a Cisticola, and perhaps one or two others which 
are resident in Celebes differ when in nuptial and simple plumage, but, in order 
to see seasonal variation in full evidence, it is necessary to look to the northern 
temperate and arctic regions. Here, as is well known , most remarkable con- 
trasts of summer and winter plumage are abundantly represented ; as, for instance, 
the varied dress of the Ptarmigan [Lagopus mutus) in summer, its snow-white 
plumage in winter; the black under surface of the Golden Plover [Charadrius] 
in summer, the whitish of these parts in winter. Many northern forms visit 
Celebes in winter, often in an attire very different from that in which they 
breed in the North; amongst them may be mentioned the Eastern Golden Plover 
[C. fuhus)^ and the Grey Plover [Squatarola) which undergo a similar seasonal 
change; the Stints and Godwits which are suffused with rufous in summer; the 
Glossy Ibis [Plegadis) which has the under parts chestnut in summer, earthy 
brown in winter ; the Phalaropes ; certain Terns [Hgdrochelidon)^ etc. • These 
changes are not of a sexual nature, as the sexes differ little or not at all in 
coloration, and both are subjected to the same seasonal changes; but in many 
— probably in most — cases where there are any secondary sexual differences 
these characters are intensified in the breeding season and new markings are 
sometimes added in the male sex (e. g. the ruff of Machetes^ the black facial 
markings in some species of Aegialitis^ the long tail-feathers of Vidua). It may, 
however, also happen that the sexes are less similar in the winter season than 
when breeding; this seems to be the case to a slight extent with Anthus cervinus. 
