Eggshell strength in cuckoos and cowbirds

Brooker, Michael; Brooker, Lesley

doi:10.1111/j.1474-919x.1991.tb04589.x

Cited by 62 publications

(53 citation statements)

References 29 publications

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“…Significant deviations from these values would arise if puncture attacks were biased towards one or the other egg type [35], and/or if eggs frequently resisted puncture when attacked. In the case of parasite eggs, the latter at least is expected to be at play, given that cowbird eggs are structurally adapted to reduce their vulnerability to breakage, being rounder and thicker-shelled than those of their hosts and non-parasitic relatives [36][37][38][39][40][41].…”

Section: Methodsmentioning

confidence: 99%

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

2011

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Despite the costs to avian parents of rearing brood parasitic offspring, many species do not reject foreign eggs from their nests. We show that where multiple parasitism occurs, rejection itself can be costly, by increasing the risk of host egg loss during subsequent parasite attacks. Chalk-browed mockingbirds (Mimus saturninus) are heavily parasitized by shiny cowbirds (Molothrus bonariensis), which also puncture eggs in host nests. Mockingbirds struggle to prevent cowbirds puncturing and laying, but seldom remove cowbird eggs once laid. We filmed cowbird visits to nests with manipulated clutch compositions and found that mockingbird eggs were more likely to escape puncture the more cowbird eggs accompanied them in the clutch. A Monte Carlo simulation of this 'dilution effect', comparing virtual hosts that systematically either reject or accept parasite eggs, shows that acceptors enjoy higher egg survivorship than rejecters in host populations where multiple parasitism occurs. For mockingbirds or other hosts in which host nestlings fare well in parasitized broods, this benefit might be sufficient to offset the fitness cost of rearing parasite chicks, making egg acceptance evolutionarily stable. Thus, counterintuitively, high intensities of parasitism might decrease or even reverse selection pressure for host defence via egg rejection.

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Section: Methodsmentioning

confidence: 99%

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

2011

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“…Accordingly, cuckoo eggshell thickness across host-races varies positively with the rejection rate of its particular hosts [36]. Increased parasite egg strengths have also been hypothesized to decrease the probability of damage during rapid laying by the female parasite herself [37] or by other, competitor parasites, laying in the same host nest [38] during multiple parasitism [39]; but experimental support is lacking for the latter two functions of greater parasite eggshell strength [18].…”

Section: Introductionmentioning

confidence: 99%

Alternative mechanisms of increased eggshell hardness of avian brood parasites relative to host species

Igic

Braganza

Hyland

et al. 2011

J. R. Soc. Interface.

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Obligate brood parasitic birds lay their eggs in nests of other species and parasite eggs typically have evolved greater structural strength relative to host eggs. Increased mechanical strength of the parasite eggshell is an adaptation that can interfere with puncture ejection behaviours of discriminating hosts. We investigated whether hardness of eggshells is related to differences between physical and chemical traits from three different races of the parasitic common cuckoo Cuculus canorus, and their respective hosts. Using tools developed for materials science, we discovered a novel correlate of increased strength of parasite eggs: the common cuckoo's egg exhibits a greater microhardness, especially in the inner region of the shell matrix, relative to its host and sympatric non-host species. We then tested predictions of four potential mechanisms of shell strength: (i) increased relative thickness overall, (ii) greater proportion of the structurally harder shell layers, (iii) higher concentration of inorganic components in the shell matrix, and (iv) elevated deposition of a high density compound, MgCO 3 , in the shell matrix. We confirmed support only for hypothesis (i). Eggshell characteristics did not differ between parasite eggs sampled from different host nests in distant geographical sites, suggesting an evolutionarily shared microstructural mechanism of stronger parasite eggshells across diverse host-races of brood parasitic cuckoos.

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“…The much smaller egg of the Indian subspecies could very likely be grasped by the two main babbler hosts. In addition, the eggshell of Clamator cuckoos is particularly strong, even for cuckoo eggs which are known to have particularly strong eggshells [29]; but see also [16]. This makes egg ejection through egg puncture much harder [29] and/or more costly [3].…”

Section: Results (A) Breeding Biology Of Jacobin Cuckoo and Cape Bulbulmentioning

confidence: 99%

Brood parasitism selects for no defence in a cuckoo host

Krüger

2011

Proc. R. Soc. B.

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In coevolutionary arms races, like between cuckoos and their hosts, it is easy to understand why the host is under selection favouring anti-parasitism behaviour, such as egg rejection, which can lead to parasites evolving remarkable adaptations to 'trick' their host, such as mimetic eggs. But what about cases where the cuckoo egg is not mimetic and where the host does not act against it? Classically, such apparently non-adaptive behaviour is put down to evolutionary lag: given enough time, egg mimicry and parasite avoidance strategies will evolve. An alternative is that absence of egg mimicry and of anti-parasite behaviour is stable. Such stability is at first sight highly paradoxical. I show, using both field and experimental data to parametrize a simulation model, that the absence of defence behaviour by Cape bulbuls (Pycnonotus capensis) against parasitic eggs of the Jacobin cuckoo (Clamator jacobinus) is optimal behaviour. The cuckoo has evolved massive eggs (double the size of bulbul eggs) with thick shells, making it very hard or impossible for the host to eject the cuckoo egg. The host could still avoid brood parasitism by nest desertion. However, higher predation and parasitism risks later in the season makes desertion more costly than accepting the cuckoo egg, a strategy aided by the fact that many cuckoo eggs are incorrectly timed, so do not hatch in time and hence do not reduce host fitness to zero. Selection will therefore prevent the continuation of any coevolutionary arms race. Non-mimetic eggs and absence of defence strategies against cuckoo eggs will be the stable, if at first sight paradoxical, result.

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Eggshell strength in cuckoos and cowbirds

Cited by 62 publications

References 29 publications

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

Brood parasite eggs enhance egg survivorship in a multiply parasitized host

Alternative mechanisms of increased eggshell hardness of avian brood parasites relative to host species

Brood parasitism selects for no defence in a cuckoo host

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