Which of the boys assumes ownership of the ostrich to the protests of the others? Why does Father agree that his boy has a right to the ostrich? What responsibilities does Father give this boy?

From the text:

`Jack shall retain the ostrich,' said I, `for it is most suited to him; he is a lighter weight than either of you his elder brothers, and Franz is not yet strong enough to manage such a fleet courser. But he is so far to be considered common property, that all may practise on him occasionally; and, in a case of necessity, anyone may mount him.'