Constraints on host choice: why do parasitic birds rarely exploit some common potential hosts?

Abstract

Summary1. Why are some common and apparently suitable resources avoided by potential users? This interesting ecological and evolutionary conundrum is vividly illustrated by obligate brood parasites. Parasitic birds lay their eggs into nests of a wide range of host species, including many rare ones, but do not parasitize some commonly co-occurring potential hosts. 2. Attempts to explain the absence of parasitism in common potential hosts are limited and typically focused on single-factor explanations while ignoring other potential factors. We tested why thrushes Turdus spp. are extremely rarely parasitized by common cuckoos Cuculus canorus despite breeding commonly in sympatry and building the most conspicuous nests among forest-breeding passerines. 3. No single examined factor explained cuckoo avoidance of thrushes. Life-history traits of all six European thrush species and the 10 most frequently used cuckoo hosts in Europe were similar except body ⁄ egg size, nest design and nestling diet. 4. Experiments (n = 1211) in several populations across Europe showed that host defences at egg-laying and incubation stages did not account for the lack of cuckoo parasitism in thrushes. However, cross-fostering experiments disclosed that various factors during the nestling period prevent cuckoos from successfully parasitizing thrushes. Specifically, in some thrush species, the nest cup design forced cuckoo chicks to compete with host chicks with fatal consequences for the parasite. Other species were reluctant to care even for lone cuckoo chicks. 5. Importantly, in an apparently phylogenetically homogenous group of hosts, there were interspecific differences in factors responsible for the absence of cuckoo parasitism. 6. This study highlights the importance of considering multiple potential factors and their interactions for understanding absence of parasitism in potential hosts of parasitic birds. In the present study, comparative and experimental procedures are integrated, which represent a novel approach that should prove useful for the understanding of interspecific ecological relationships in general.

Geographic variation in egg ejection rate by great tits across 2 continents

“…For example, small-sized cavities used by birds as nest sites may prevent parasitism by larger parasitic cuckoos that cannot enter the nest hole, and small hole-nesters prefer smaller cavities as nest sites over larger ones (van Balen et al 1982;Carlson et al 1998). A parallel case of avoidance of parasitism is the close association between potential bird hosts and human habitation that can reduce the risk of brood parasitism because most parasitic cuckoos avoid close proximity to humans, especially in urbanized areas (Grim et al 2011;Liang et al 2013;Møller et al 2016). Host resistance to parasitism is often consistent among populations (e.g., Grim et al 2011).…”

Females that experience threat are better teachers

“…When hosts express defences towards brood parasites, the costs can be high because of recognition errors; hosts may incorrectly identify the eggs and chicks of cuckoos versus their offspring [8][9][10][11]. Therefore, hosts should be selected to modify the expression of defence in relation to the risk of parasitism [7,12].…”

Unusual diet of brood parasitic nestlings and its fitness consequences

“…Lizards also surely require more effort to catch, handle, and transfer than typical invertebrate food items (''loading effect'' sensu Carlson and Moreno 1982). Instead, we suggest that host diet selection may represent a general life history trait (sensu Grim et al 2011) that either facilitates (Brooke and Davies 1989) or prevents (Yang et al 2013) coevolution between parasites and (potential) hosts, even though it most likely did not evolve as a specific antiparasite defense.…”

“…We removed the predictor ''fruit availability'' and instead included a new predictor,''hatching date'' (the rest of the models remained identical to models reported in Appendix Tables 3 and 4). Hatching date was centered on the mean within each year to exclude any confounding effects of between-year variation (following Grim et al 2011). We ran these models for fledging mass, age, and success.…”

“…This variation may be particularly important in cases when nestlings are not fed by their own parents but by members of another species, that is, in the context of interspecific brood parasitism (Davies 2015). Indeed, nestling diet is one of the main factors that seems to govern host selection by parasitic birds (Soler et al 1999, but see Grim et al 2011). Specifically, plant-dominated diets fed by potential hosts to nestlings are fatal to parasitic progeny of both Brownheaded Cowbirds (Molothrus ater; Middleton 1977) and Common Cuckoos (Cuculus canorus; hereafter, cuckoos; Glue and Morgan 1972).…”