Host specialization and latitude among cuckoos

We tested the prediction that at higher latitudes there will be an increase in the number hosts per cuckoo species. This prediction is confirmed, and the number of hosts exploited per cuckoo species increases with increasing latitude. Although this result is strongly influenced by a single species (the common cuckoo Cuculus canorus) in high latitudes, it is significant and holds even at lower latitudes where this species is absent. This trend may be explained as a case of competitive release and niche expansion by the few cuckoo species that have expanded their ranges north, despite the shortage of hairy caterpillar prey species at high latitudes.