Are tits really unsuitable hosts for the common Cuckoo?

Abstract

Avian brood parasites exploit hosts that have accessible nests and a soft insect diet. Com-mon Cuckoo (Cuculus canorus) hosts were traditionally classified as suitable if both pa-rameters were fulfilled or unsuitable if one, or both, were not. In line with this view, hole- nesting tits (Paridae) have become a text-book example of unsuitable Cuckoo hosts. Our extensive literature search for Cuckoo eggs hatched and chicks raised by hosts revealed 16 Cuckoo nestlings in Great Tit (Parus major) nests, 2 nestlings and 2 fledglings in Blue Fits (Cyanistes caeruleus), and 1 nestling in a Crested Tit (Lophophanes cristatus) nest. Our own data from natural observations and cross-fostering experiments concur with lit-erature data that Great Tits arc able to rear Cuckoo chicks to fledging. The natural obser-vations involve the first known eases where a bird species bccamc parasitized as a by-product of nest usurpation (take-over). Surprisingly, Cuckoo chicks raised by Great Tits , grew better than Cuckoo chicks raised by common hosts, even alongside host own chicks. The frequency of Cuckoo parasitism in tits may be underestimated by studying tits in arti-ficial nest-boxes with small cntranccs that prevent Cuckoos from laying and/or fledging. Results support a view that host suitability is not a catcgorical parameter (host suitable or unsuitable) but a continuous phenomenon. Understanding the diversity of parameters that determine host selection by Cuckoos is limited, bccausc studies on Cuckoo chick diet, growth, and survival in most hosts arc rare. Therefore any data arc valuable and provide indispensable material for future meta-analyses.