g2 Introduction: Variation. 



Lalage leucopygialis and Graucalus hkolor 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 uj^on 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 

 IGO 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 Cistkola, 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 [Charadrins) 

 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. fulviis), 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 

 Avhen breeding; this seems to be the case to a slight extent with Anthus cervinus. 



