Among many birds it is usual for all family work to be performed quite irrespective of sex, and the parent who is free takes the task of feeding the one who is occupied with the nest.[37] The male hornbill is a family despot; during the breeding season he walls up his spouse within the trunk of a tree. He feeds her with great care, but he allows her no liberty. As soon as one family is reared many birds at once burden themselves with another. The Californian quail affords an example. In this species the father takes sole charge of the family as soon as the young birds attain the age of three weeks, when the mother begins the labours of rearing a second brood. More curious are the habits of the water hen, among whom the young of the first family assist in the work of feeding their brothers and sisters of the later broods.[38] The labour of feeding the young family is a heavy task in which both parents commonly share. There are no cases of unsuitable feeding of nestlings by careless or ignorant parents. A regular course of nursery dietary is practised, in particular with nidicolous species, where the young are born in a helpless condition; often a special infant food is prepared by a process of regurgitation, or food partly digested and thrown up. Thus baby finches are fed on food made of digested insects; parent parrots also prepare a digested vegetable food; storks break up worms and frogs and pieces of little fishes and mix it with partly digested matter and throw it out on the edge of the nest for the family meals. Young pigeons thrust their beaks into the mouths of their mothers to absorb the so-called pigeon’s-milk, which is really digested food mixed with a secretion from the crop; little cormorants thrust their bills right down the neck of their mother and help themselves to food out of her stomach. The petrels secrete oil from the fish they eat to feed the young: this oil is used also as a weapon of defence both by the parents and the nestlings, who squirt it out from their mouths and nostrils at any unwelcome intruder on the privacy of the nest.[39]