7. What new organs are first seen distinctly on the fourth and fifth days respectively of incubation of a hen’s egg? Explain exactly how you would expose them to view, and describe their appearance. 8. What is the period of incubation of a duck’s egg? Observe and describe exactly how the hatching duckling breaks open the shell and escapes from it. 9. Observe and describe a duckling’s first attempt to swim. Is the action instinctive or not? 10. Make other observations upon ducklings, comparing them in as many respects as possible with chicks, and write careful accounts of the results you obtain. 11. How does a fowl’s egg which has been hatched for four days differ from a fresh-laid egg? Describe the new structures which have formed in it, so far as they can be made out by the unaided eye. (1904) 12. Where does the chick begin to form in the egg? Explain the arrangement which brings it as near as possible to the body of the sitting hen. (1905) 13. What is the use of the cloudy masses attached to opposite sides of the yolk of a fowl’s egg? (1906) [Pg 301] CHAPTER XVII. SOME FAMILIAR BRITISH BIRDS. 55. THE THRUSH FAMILY. 1. The song-thrush, mavis, or throstle.—Throstles are to be seen throughout the year. Take every opportunity of observing their habits, and make careful notes of these at the time. Especially attend to the following characters: (a) General appearance.—What is the average size of the bird? What is the length of the tail? At what angle is the tail held? Does the bird move it in any special manner? What is the prevailing colour of the body? Notice the light-coloured and spotted breast. Compare the male and female as regards size, colouration, etc. (b) Habits.—What situations do throstles chiefly inhabit? Do they keep near the edge of the wood, or have you often seen them inside thick woods? Does the bird hop, or walk, when on the ground? Upon what does it feed? Have you ever seen it cracking the shell of a snail? If you find a heap of snail-shells near a stone, watch, at a little distance, for the throstle coming to crack snail-shells on the stone. Does it also eat worms, or fruit? Describe a throstle’s beak. How would you describe the flight—as high or low, swift or slow, straight, in wide curves, or undulating? Does the bird perch on trees? How does it use its toes in perching? (c) Song.—In what months does the throstle sing? Does it sing late at night and early in the morning? How does it compare with other birds [Pg 302] in this respect? In what positions have you seen the bird singing—when flying, perching, or on the ground? Try to write down the syllables which seem to you most like the throstle’s song, and notice how the phrases are repeated. Notice that throstles sing less during August than in autumn. Listen, in September or October, for young thrushes learning to sing, and contrast their efforts with the song of the adult birds.