290 THE BIOLOGY OF BIRDS 



rather than drawing the line too closely, makes allowance for 

 the extremes." There is an inertia in instinctive behaviour, 

 until things take a new turn. 



Surprise has sometimes been expressed that a bird should 

 go on sitting on eggs which are not developing. A turtle- 

 dove is a very sensitive creature, knowing when " a profane 

 hand " has, in its absence, touched the eggs or the young, 

 and leaving them in consequence, yet it will sit for eighteen 

 days on addled eggs. But the expression of surprise is due 

 to a misapprehension of the nature of instinctive behaviour. 

 For we may be sure that the brooding dove is not meditating 

 over its eggs or wondering why nothing has happened, it is 

 bound by promptings which must be obeyed, and which will 

 continue to operate until some internal or external change 

 switches off the current. 



A simple adaptation at the nesting time is the suppression 

 of scent in ground-birds like partridges. There is so little 

 scent that a first-rate dog may be taken to within a foot of a 

 sitting partridge without his showing any interest, always 

 provided that the bird is not seen. As birds have no sweat- 

 glands the smell probably comes from the preen-gland or 

 from the cloaca, and it would be very interesting to know 

 how it is suppressed. Tegetmeier, the great authority on 

 pheasants, had a theory which he called " vicarious secretion." 

 He believed that odoriferous particles are normally given off 

 from the bird's skin, but during the brooding, in some way 

 or other, the bye-products are shunted into the food-canal, 

 and got rid of along with the bird's droppings at a safe 

 distance from the nest. But this shunting is obscure. It 

 would be of interest to compare the normal activity of the 

 preen gland with its condition at the breeding season, to see 

 if there is any difference. 



The uses of incubation are : (i) to supply from the body 

 of the brooding bird the warmth which furthers the develop- 

 ment of the embryo within the egg, a function helped by non- 

 conducting materials in the nest ; (2) to hide the eggs and 

 nestlings from the eyes of enemies ; (3) to shelter the young 

 from the heat of the sun, to which many of them are very 



