One key fact states that the more diversified a population becomes, the more likely it is to survive as a species. This increases its ability to occupy more territory and gain access to more sustenance, or to spread itself out and become more inconspicuous among predators. Several species of birds demonstrate this type of variation at the embryonic stage. The eggs they lay are coloured or spotted in particular ways, the complete reasons for which have been somewhat elusive to researchers for some time. In fact, birds are the only species that produce pigmented egg shells (Gosler, et al. 2005, p. 1105), and this leads scientists to believe that the pigmentation serves a discoverable purpose. Such reasons as crypsis and the prevention of parasites have been hypothesised. Other hypotheses have been based on sexual selection or on the chemical structure of the eggshell and its influence on eggshell fragility and vulnerability. Hybridization has also been suggested as a factor that influences egg colour. These hypotheses, though varied, have important implications on the evolution of the bird shells and can give insight into the reasons for the various pigmentations that birds' eggs carry.
Safety is a very important issue for birds when it is noted that their eggs are prone to different forms of predation and parasitism. Two types of brood parasitism exist. Conspecific brood parasitism occurs when birds of similar species place their foreign eggs into the nest of a host. Interspecific brood parasitism occurs when birds of other species infiltrate the nest and place their own eggs in the clutch. This can be very dangerous to the existence of a particular avian species, since the some parasites are known to be vicious and ruthless. The most notorious of these parasites are cuckoos, and their parasitism is dangerous as they often hatch before the genuine brood and expel authentic eggs from the nest, terminating that attempt of the species to reproduce.
This is truer of some species than of other, depending on the type of predation suffered by each. The great tit (Parus major), for example, does not expel parasites from its nest, and this appears to be contingent on the fact that it is not a host to the European cuckoo parasite (Gosler, Higham and Reynolds, 2005. p. 1105-6). Village weavers, on the other hand, do remove foreign eggs from their nests, and have therefore to learn the appearance of their eggs (Collias, 1993, p. 684). The implication of this fact is that the weaver eggs must have progressively developed a distinctive appearance in order to facilitate recognition.
It has been observed that West African village weavers' spotted eggs have noticeable intraspecific differences, and hypotheses have been formed concerning the reasons for this. The first spots on eggs might have been achieve by one incidence of genetic mutation in the species, but this kind of spotting in weavers is now commonplace. Relying on the reasoning of the previous paragraph, it was predicted that within the weavers' clutches the diversity of colouration would be minimised in the absence of interspecific parasites and maximised in their presence. Researcher David Lahti found opportunity for an experiment involving these West African weavers in the fact that the species had