14 MARVELS OF FISH LIFE 
the three-spined stickleback. During most of the year 
the male and female fish are coloured alike, being olive- 
green and grey on a silvery background, but about April 
the male fish, in order to attract the opposite sex, becomes 
brilliant in colour. At first there is a mere reddish 
tinge on the under-surface, but as the time approaches 
for him to build his nest his sides and under-surface, 
and even the inside of his mouth, become scarlet, his 
back turns a greenish blue, and his little eyes become a 
brilliant emerald. When he decides to build he first 
scoops out a hollow in the ground, and then arranges 
round this hollow small pieces of roots, weed-stems, 
and other debris, either tugged from the growing vege- 
tation or picked up from the bottom. Wriggling 
through this untidy mass, he binds the pieces together 
with the mucus from his body. 
Next he nips off bits of the same roots and stems, 
and, after chewing them well in his mouth, rams them 
into the bottom of the nest. Having put in three or 
four mouthfuls of material in this manner, he will 
suddenly dart off into the adjacent weeds, talk for a few 
seconds to his intended bride, and then return brighter 
in colour to tackle his work with renewed energy. These 
periodic visits stimulate the secretion with which he 
welds together the building material. Finally the end 
and roof of the nest are completed, and it appears as 
a mound on the ground, surrounded by general debris. 
Working from early dawn to dark without intermission, 
the stickleback completes his task in a couple of days. 
