# Instincts of the Cuckoo explained — The Origin of Species > Chapter companion for The Origin of Species by Charles Darwin. ## What happens Darwin examines the parasitic instincts of the European cuckoo and related species, arguing that such behaviors can be explained by natural selection acting on small, inherited variations. He compares the cuckoo's habits with those of American cuckoos, Australian species, and Molothrus, showing a gradation from imperfect to perfect parasitic instincts. He also discusses the small size of cuckoo eggs, the ejection of foster-siblings by the young cuckoo, and occasional reversion to ancestral nesting, concluding that these instincts arise gradually through advantageous variations. ## Themes in this chapter ### Natural Selection and the Struggle for Existence Darwin applies natural selection to explain the gradual acquisition of parasitic instincts in cuckoos and Molothrus. ### Variation and Inheritance Emphasizes variation in egg size, color, and behavior, and the inheritance of such variations at corresponding ages. ### Adaptation and the Utilitarian Doctrine Argues that cuckoo instincts are adaptive, benefiting the young through increased food and survival, despite appearing odious. ## Character check-ins ### Charles Darwin Author and narrator, presents the argument for natural selection of cuckoo instincts. ### Professor Ramsay Provided observations on Australian cuckoos, including egg size and color variation. ### Mr. Gould Initially believed foster-parents ejected young, later confirmed young cuckoo's own ejection behavior. ### Mr. Hudson Observed Molothrus badius and Molothrus bonariensis, noting gradation in parasitic instincts. ### Professor Owen Noted transitory sharp tooth in young snakes for breaking eggshells, used as analogy. ### Dr. Merrill Reported a young cuckoo in a blue jay's nest, supporting occasional parasitism. ## Key lines > If it had been of an advantage to this species to have laid eggs even smaller than those now laid, so as to have deceived certain foster-parents, or, as is more probable, to have been hatched within a shorter period ... then there is no difficulty in believing that a race or species might have been formed which would have laid smaller and smaller eggs; for these would have been more safely hatched and reared. If smaller eggs gave an advantage—by tricking foster parents or hatching faster—then natural selection could gradually produce a species that lays ever smaller eggs, leading to safer hatching and rearing. _Darwin explains how egg size could evolve through selection._ > I can see no special difficulty in its having gradually acquired, during successive generations, the blind desire, the strength and structure necessary for the work of ejection; for those cuckoos which had such habits and structure best developed would be the most securely reared. There is no problem in believing that the young cuckoo's instinct to eject its foster-siblings, along with the needed strength and body shape, evolved gradually over generations, because individuals with these traits were more likely to survive and be raised successfully. _Darwin defends the gradual evolution of the ejection instinct._ > By a continued process of this nature, I believe that the strange instinct of our cuckoo has been generated. Through repeated cycles of occasional parasitism and inheritance, the remarkable instinct of the European cuckoo was produced. _Darwin summarizes his explanation for the origin of cuckoo parasitism._ ## Links - HTML: https://www.betterreads.online/discover/the_origin_of_species_se/chapters/instincts-of-the-cuckoo - Book: https://www.betterreads.online/discover/the_origin_of_species_se - All chapters: https://www.betterreads.online/discover/the_origin_of_species_se/chapters