It is supposed by some naturalists that the more immediate cause of the instinct of the cuckoo is that she lays her eggs, not daily, but at intervals of two or three days; so that, if she were to make her own nest and sit on her own eggs, those first laid would have to be left for some time unincubated or there would be eggs and young birds of different ages in the same nest.
Naturalista batzuek uste dute kukuaren instintuaren kausarik hurbilena, kukuak egunero barik hiruzpalau egunik behin errutea dela; era horretan, beraz, baldin bere habia berak egin eta bere arrautzak berak txitatu behar balitu, lehenbizi eginak txitatu gabe utzi beharko lituzke aldi batez, edo bestela habia berean adin ezberdineko arrautzak eta kumeak izanen lirateke.