PARENTAL CARE AND NEST-MAKING 313 



fixed. If the arrangement is disturbed by some intrusion, 

 the mother-bird puts it right again. Non-brooder though 

 she is, she is very punctiHous in her parental care. After 

 each egg-laying the mound is, of course, carefully closed up. 

 (5) Eventually it is left more or less alone, though the birds 

 keep an eye on it all the time. The heat of the sun promotes 

 fermentation among the leaves — Bacteria helping Birds — 

 and the temperature sometimes rises to over ninety degrees 

 Fahrenheit. If the weather is very warm the temperature 

 of the mound is apt to rise too high, but the birds attend to 

 this by opening up and loosening the material around the 

 egg-chamber. It is to this extraordinary carefulness that 

 they owe their name " Thermometer Bird." They are also 

 careful to secure good ventilation, for the embryos would die 

 if there were not plenty of air in the nest. The fact is that 

 they do die of suffocation if some accident befalls their 

 parents and the hill becomes sodden. (6) The young birds 

 have a prolonged development of about forty-five days 

 within the egg, and they are very vigorous when they are 

 hatched. They wrestle upwards out of the egg-chamber 

 and out of the mound, and make for the bush with all haste. 

 If anything should stop them, say fatigue, in the process of 

 wrestling out of the mound, they die. For if an instinctive 

 activity is coercively stopped, it does not usually recom- 

 mence. If the birds get safely out they are still in consider- 

 able danger from enemies, until they are able to get off the 

 ground on to the trees, which must be a great rehef. 



This is only a glimpse of the mound-bird's story, but we 

 have said enough to show that a bird which does not incubate 

 may nevertheless show a wonderful subtlety of parental care. 



There are different kinds of mound-birds and different 

 expressions of the parental care. In some cases a great 

 mound of vegetation is made and used jointly by several 

 mothers. In all cases there is the associated adaptation on 

 the part of the young birds that they are precociously 

 vigorous when hatched. Some, as we have mentioned, are 

 able to fly away the day they are hatched. This illustrates 

 what may be called a *' temporal variation," in other words, 



