Limits to host colonisation and speciation in a radiation of parasitic finches

<p><span><span><span><span><span><span><span><span><span><span><span>Parasite lineages vary widely in species richness. In some clades, speciation is linked to the colonisation of new hosts. This is the case in the indigobirds and whydahs (<i>Vidua</i>), brood-parasitic finches whose nestlings mimic the phenotypes of their specific hosts. To understand the factors limiting host colonisation, and therefore speciation, we simulated the colonisation of a host using cross-fostering experiments in the field. Despite DNA barcoding suggesting that host species feed their chicks similar diets, nestling <i>Vidua </i>had low survival in their new host environment. Nestling <i>Vidua</i> did not alter their begging calls plastically to match those of the new hosts, and were fed less compared to both host chicks and to <i>Vidua</i> chicks in their natural host nests. This suggests that a key hurdle in colonising new hosts is obtaining the right amount rather than the right type of food from host parents. This highlights the importance of mimetic nestling phenotypes in soliciting feeding from foster parents and may explain why successful colonisations tend to be of hosts closely-related to the ancestral one. That non-mimetic chicks are fed less but not actively rejected by host parents suggests how selection from hosts can be sufficiently intense to cause parasite adaptation, yet sufficiently relaxed that parasitic chicks can sometimes survive in and colonise new host environments even if they lack accurate mimetic phenotypes. The difficulties of soliciting sufficient food from novel foster parents, together with habitat filters, likely limit the colonisation of new hosts, and therefore speciation, in this parasite radiation.</span></span></span></span></span></span></span></span></span></span></span></p>