Another hypothesis is the laying damage hypothesis, which postulates that the eggshells are adapted to damage the eggs of the host when the former is being laid, and prevent the parasite's eggs from being damaged when the host lays its eggs. In support of this hypothesis, eggs of the shiny cowbird parasitizing the house wren and the chalk-browed mockingbird and the brown-headed cowbird parasitizing the house wren and the red-winged blackbird damaged the host's eggs when dropped, and sustained little damage when host eggs were dropped on them. Most avian brood parasites have very short egg incubation periods and rapid nestling growth. In many brood parasites, such as cuckoos and honeyguides, this short egg incubation period is due to internal incubation periods up to 24 hours longer in cuckoos than hosts. Some non-parasitic cuckoos also have longer internal incubation periods, suggesting that this longer internal incubation period was not an adaptation following brood parasitism, but predisposed birds to become brood parasites. This is likely facilitated by a heavier yolk in the egg providing more nutrients. Being larger than the hosts at growth is a further adaptation to being a brood parasite. There is a question as to why the majority of the hosts of brood parasites care for the nestlings of their parasites. Not only do these brood parasites usually differ significantly in size and appearance, but it is also highly probable that they reduce the reproductive success of their hosts. The "mafia hypothesis" evolved through studies in an attempt to answer this question. This hypothesis revolves around host manipulations induced by behaviors of the brood parasite. Upon the detection and rejection of a brood parasite's egg, the host's nest is destroyed and nestlings injured or killed. This threatening response indirectly enhances selective pressures favoring aggressive parasite behavior that may result in positive feedback between mafia-like parasites and compliant host behaviors. There are two avian species that have been speculated to portray this mafia-like behavior: the brown-headed cowbird of North America, "Molothrus ater", and the great spotted cuckoo of Europe, "Clamator glandarius". The great spotted cuckoo lays the majority of its eggs in the nests of the European magpie, "Pica pica". It repeatedly visits the nests that it has parasitised, a precondition for the mafia hypothesis. An experiment was run by Soler "et al." from April to July 1990 – 1992 in the high-altitude plateau Hoya de Guadix, Spain.