Maquetas y prototipos para el sitio web básico

The majority of invertebrate species found in the nests of birds are parasites. Parasites reduce the fitness of the host (Christe et al. 1996), therefore what follows is a discussion of research that explores a relationship between a nesting bird and invertebrate that benefits one member of the interaction to the detriment of the other. I do not discuss instances where a putative parasite is discovered not to have a negative impact on its host as, by the definition used here, these are not parasitic interactions.

Parasites represent a large proportion of extant species and most organisms will encounter them at some stage of their life-cycle (Heeb et al. 2000). Whilst most endoparasites, such as worms, haematozoa and viruses are widely recognised as having a detrimental effect on the health of a bird, ectoparasites, can have an equally profound effect on survival and fitness (Loye and Zuk 1991). Most nestlings share their nest with a suite of ectoparasites (Simon et al. 2003). The impact that a parasite has on its host is termed its virulence and this varies between individuals as well as spatially and temporally (Loye and Zuk 1991;

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Johnson and Albrecht 1993; Heeb et al. 1998; Martin et al. 2001). Further, the type of ectoparasite will also influence its virulence and the nature of its relationship. For example, fleas feed intermittently on chicks in nests, whereas ticks attach for a prolonged period (Heeb et al. 2000).

Ectoparasites can have a detrimental effect on a range of life history parameters. Infestation of chicks by ectoparasites, such as mites and larval dipteran flies, has been demonstrated experimentally to result in slower growth rates (Johnson and Albrecht 1993; Bize et al. 2003). This can lead to reduced weight at fledging (Heeb et al. 2000; Weddle 2000; Berggren 2005; Fessl et al. 2006), poorer body condition (Hurtrez-Bousses et al. 1997) or result in delayed fledging while the chicks achieve the minimum required fledging weight (Bize et al. 2003). Infestation by blowfly larvae Protocalliphora sp., resulted in lowered haematocrit levels in blue tits (Parus caeruleus), which reduced the chick’s ability to thermoregulate (Hurtrez-Boussès et al. 1997; Simon et al. 2004). Sleep can also be affected by parasite infestation (Christe et al. 1996), as chicks divert time from other activities to compensate for the irritation or energy lost to ectoparasites, impinging on sleeping time (Sheldon and Verhulst 1996; Simon et al. 2005). The ultimate cost of ectoparasites to birds can be an increased rate of mortality, and while this is not inevitable, there are many examples of parasite infestation resulting in the death of chicks in nests (see for example Whitworth and Bennett 1992; Brown and Brown 2004; Puchala 2004; Gwinner and Berger 2005; Fessl et al. 2006).

Parents are also subject to fitness costs as a result of parasitic infestation. Nests are a common source of parasites, such as feather lice, for adult birds (Møller and Rozsa 2005). Feather lice can have an impact on flight, metabolism (Møller et al. 2004) and sexual selection through damage to the feather (Hamilton and Zuk 1982; Clayton 1991). Such impacts can result in the late arrival of breeding birds to the breeding grounds, in poor condition (Møller et al. 2004), which can result in a delay in egg laying (Oppliger et al. 1994) and a shortened nesting period (Møller 2005). The presence of ectoparasites in the nest of a bird can also result in more time being spent on nest sanitation duties at the cost of provisioning (Hurtrez-Bousses et al. 2000). Often the impact of parasites is not manifested during the breeding season in which infestation occurs, rather,

the impact is felt in terms of reduced future reproductive output (Brown et al. 1995; Bize et al. 2004). At a population level, parasites influence the evolution of clutch size (Møller 1991; Poiani 1993; Martin et al. 2001) and can result in biased sex ratios (Heeb et al. 1999). Finally, the presence of ectoparasites can result in the death of adult birds (Brown and Brown 2002).

There is little experimental evidence of birds nesting in a manner that parasitises invertebrates, although the regular phenomenon of birds nesting in termite cavities may provide examples of this interaction. These interactions will be discussed further in the discussion of commensal relationships. One documented example of this behaviour, however, involves the presence of caterpillars with stinging hairs in the nests of crested bellbirds (Orieca gutteralis) in Australia (Chisholm 1919; Higgins and Peter 2002). Up to 14 caterpillars, incapacitated but kept alive to ensure that they stay on the nest, have been observed in and on bellbird nests (Chisholm 1918; Chisholm 1919; Leach 1928; Ross 1930). The nature of the relationship between the birds and caterpillars is unclear, however it has been suggested that the caterpillars provide food for the parent and nestling birds (Milligan 1905; White 1918; but see Chisholm 1918). Others have suggested that the caterpillars, provide a measure of protection to the nest contents from their stinging hairs (Chisholm 1918; Chisholm 1919). As yet, no evidence has been presented that confirms the nature of this unusual relationship.

While there is unequivocal evidence of the negative impact of playing host to parasites, little experimental work has examined the positive benefits received by the invertebrate. It would reasonably be argued that the provision of food and shelter to the invertebrate constitutes a benefit to the fitness of that individual. Indeed some research suggests that there are direct benefits to populations of nest invertebrates of an increased brood size (Hurtrez-Bousses et al. 1999) and obligate avian parasites clearly derive benefit from their hosts (Brown et al. 1995). Despite this, detailed studies of the life cycle of many invertebrate nest parasites are lacking (Fessl et al. 2006) and the effect on the invertebrate involved is untested in most studies of avian nest parasitism, even where the data for such evaluations have been collected (see for example Heeb et al. 1998; Simon et al. 2003).

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The literature that concerns invertebrate nest parasites is a discrete subset of the scientific literature that carries an important assumption that is rarely tested. Although there is good scientific evidence that some nest invertebrates reduce the fitness of their hosts, this is not always the case, and the prima facie investigation of ‘birds and parasites’ results in tautologies such as ‘harmful parasites’ (Gwinner and Berger 2005, p. 365). By definition, there can be no other type of parasite than a harmful parasite (Christe et al. 1996). More importantly, rarely is evidence provided that demonstrates the effect of this interaction on the invertebrate. It is assumed, often quite rightly, that the invertebrate is gaining a fitness advantage from the interaction, but the extent, variability within, and mechanism driving this assumption are not clear. Simon et al. (2004, p. 492) acknowledge that the effects of ectoparasites can be subtle and vary between individuals, yet state only that ‘parasites should always have detrimental effects on their hosts’, ignoring half of the definition of parasitism according to community ecology theory (i.e. that putative parasites must also benefit from the interaction). In many cases, this is the deliberate outcome of the experimental protocol, as the taxon of interest is the bird, however, the true nature of the interaction may be misjudged without an evaluation of the impact of the interaction on all members involved.