Growth Performance of Nestling CuckoosCuculus canorusin Cavity Nesting Hosts

Abstract

Generalist brood parasites, like Common Cuckoos Cuculus canorus, target many host species. Why other sympatric hosts are not used, or actively avoided, remains one of the main gaps in our understanding of parasite-host coevolution. Cavity nesting passerines always represented a textbook example of unsuitable hosts but recent evidence casts multiple doubts on this traditional view. In general, any species can become an unsuitable host for a parasite at laying, incubation, or nestling stages with the last one being much less studied than the others. Therefore we examined Cuckoo chick performance in five cavity nesting host species, including one regular Cuckoo host-the Common Redstart Phoenicurus phoenicurus and four non-hosts: the Pied Flycatcher Ficedula hypoleuca, Spotted Flycatcher Muscicapa striata, Great Tit Parus major, and Coal Tit Periparus ater. Natural nests of non-hosts, as opposed to artificial nest boxes with small entrance holes, are often placed in cavities that show both entrance and inner cavity sizes large enough for female Cuckoos to lay and Cuckoo chicks to fledge. We did not find any evidence for chick discrimination in non-hosts, i.e., no chicks were rejected, attacked, or neglected. Cuckoo chicks grew similarly in nests of all four species of non-hosts, similarly to chicks in host Redstart nests, and generally better than in nests of the most numerous Cuckoo host, the Reed Warbler Acrocephalus scirpaceus. Although Cuckoo chick fledging mass was highly host speciesspecific (i.e., showed high statistical repeatability across various host species), we did not find any evidence for the hypothesis that host body size (mass) positively affects parasite chick growth (fledging mass or age). These findings provide impetus to further study apparently unsuitable hosts and perhaps even reconsider traditional classifications of host suitability in the context of brood parasite-host coevolution.

Is brood parasitism related to host nestling diet and nutrition?

“…cuckoos raised alone) and their fledging age was prolonged by 15%. Cuckoos in mixed broods suffered drastic prefledging mortality, with fledging success decreased to 44% of that of solitary cuckoos (Grim et al 2009a; see also Grim and Samaš 2016). In contrast, in the present study, both plant-and vertebrate-fed cuckoos all fledged (predation and weather effects excluded).…”

“…However, the numbers inevitably reflect the length of our video-recordings: we, just like any other study, did not record nests continuously, but rather used sampling (on average,~10 hr of recordings per nest per whole nestling period; see Methods). Given that cuckoos were fed with unusual items from the age of~1 week onward (Results), they were exposed to these foods for up to~2 weeks (because fledging age is typically~3 weeks; Grim and Samaš 2016). Redstarts provisioned both their own nestlings and cuckoos throughout most of the day (due to its latitude, our study site experiences virtually continuous light during the summer breeding season).…”

Rearing a virulent common cuckoo is not extra costly for its only cavity-nesting host

“…R. Soc. B 285: 20181710 Redstart chicks were measured until the age of 10-14 days and cuckoo chicks until they fledged as handling does not trigger premature fledging [22,32].…”

“…The egg colour of the cuckoo that primarily parasitizes redstarts (hereafter: redstart-cuckoo) is similar to that of its host (egg mimicry): [21], a cuckoo nestling is distinctly heavier (approx. six times) than a host nestling or adult [22] and cuckoo hatchlings evict host progeny [4], thus decreasing the host fitness approximately 8.4 times: this decrease is similar to other hosts [23]. This system also exhibits some unique features: redstart-cuckoos typically do not remove a host egg when laying [23], cavity nests impede successful eviction of the host's progeny and the cuckoo is sometimes forced to grow alongside redstarts (hereafter: mixed brood).…”

“…However, we did not manage to quantify future costs (see Material and methods). For parasitized broods, we predicted longer incubation [26] and nestling periods [22], and lower success at hatching [27] and fledging for the host nestlings [16]. For the nestling stage, we formulated exact quantitative predictions according to allometric relationships among avian growth parameters and avian energetics (see Material and methods).…”