Cuckoos, cowbirds and hosts: adaptations, trade-offs and constraints

Abstract

The interactions between brood parasitic birds and their host species provide one of the best model systems for coevolution. Despite being intensively studied, the parasite-host system provides ample opportunities to test new predictions from both coevolutionary theory as well as life-history theory in general. I identify four main areas that might be especially fruitful: cuckoo female gentes as alternative reproductive strategies, non-random and nonlinear risks of brood parasitism for host individuals, host parental quality and targeted brood parasitism, and differences and similarities between predation risk and parasitism risk. Rather than being a rare and intriguing system to study coevolutionary processes, I believe that avian brood parasites and their hosts are much more important as extreme cases in the evolution of life-history strategies. They provide unique examples of trade-offs and situations where constraints are either completely removed or particularly severe.

Figure 1

Flow diagram of the most likely evolutionary pathways between cuckoo breeding strategies and prey size (a) and egg size (b). The presumed ancestral state is shaded in light grey, whereas the common current state in parasitic cuckoos is the boldly lined box. Solid arrows represent significant evolutionary pathways (p<0.05) and dashed arrows represent trends (p<0.1). Modified after Krüger & Davies (2002) and based on the phylogeny of Aragon et al. (1999).

Figure 2

Scheme of the coevolutionary arms race between a brood parasite and its host.

Figure 3

(a) Relationship between parasitism frequency and host reproductive success for acceptor and rejector strategies. The model assumes that there are costs of acceptance and also costs of rejecting a parasitic egg. In the absence of brood parasitism, acceptors have the highest reproductive success but at high parasitism frequencies the costs of acceptance are higher than the costs of rejecting and rejectors have a higher reproductive success. (b) Schematic relationship between lifetime parasitism frequency and host fitness. The current strategy defines a given degree of egg rejection, which can vary from 0 to 1. If a new strategy would have higher fitness (i), this would point towards evolutionary lag in the current strategy but if a new strategy would have lower fitness (ii), this would point towards evolutionary equilibrium in the current strategy. Top panel (a) modified after Takasu (1998) and Winfree (1999).

Figure 4

Scheme showing the relationship between a host trait and its fitness value for both predation and parasitism risk. If a trait affects host fitness similarly under both predation and parasitism, the resulting selection pressure will be directional (a), but if predation and parasitism risk are affected in opposite directions, a balancing selection pressure is the result (b).

Figure 5

(a) Nest attentiveness in Yellow Warbler nests parasitized by Brown-headed Cowbirds. Nests with higher attentiveness suffered a lower cost of brood parasitism because fewer host eggs were removed (p=0.018), fig. 1d from Tewksbury et al. (2002). (b) Comparison between depredated and successful Yellow Warbler nests in relation to the incubation feeding rate. A higher incubation feeding rate is associated with a higher nest predation rate (p=0.020), fig. 3d from Tewksbury et al. (2002). Differences between successful (fledging at least one host chick), parasitized and predated nests of the Cape Bulbul Pycnonotus capensis from a study in South Africa (Krüger 2004), documenting a directional selection pressure on nest architecture such as cup depth (d), but a balancing selection pressure on nest height (c). Differences in both cup depth (F_2,159=23.008, p<0.001) and nest height (F_2,159=9.760, p<0.001) are highly significant.