The precise function of all aspects of the avian eggshell is largely unknown (Gosler, Barnett & Reynolds 2000, Miksik, Holan & Deyl 1996) but it is known that maintenance of structural viability is part of the function. The base colour of all avian eggshells is white, but the cuticle and outer part of the calcified layer are coloured in many species (Gosler, Barnett & Reynolds 2000, Miksik, Holan & Deyl 1996). The function of the pigment is the subject of much debate. Direct, acute environmental factors such as chemicals and specific metals can alter eggshell pigmentation in the short term (Miksik, Holan & Deyl 1996), but the longer-term development of a highly conserved variation between species is believed to result from an evolutionary process.
The majority of avian eggshells have one of 2 pigments, although internal variations within the egg and light levels can alter these appearances. Eggshells either have a blue or greenish / blue pigment or a brown / red / black pigment (Harrison, Castell 1998). The former is caused by biliverdin, and the latter by protoporphyrines (Soler et al. 2005).
Domesticated or caged birds rarely lay eggs with much pigment (Gosler, Barnett & Reynolds 2000), indicating that there is a critical aspect of living in the wild, even ‘free range’ poultry, which necessitates the development of a distinct pigment.
There are a variety of theories about why bird eggshells have such varying colours. Whilst individual species would have evolved separately biological and physiologically, birds overall could feasibly have evolved specific characteristics as a family (Soler, Moller 1996). Thus it is believed by many that the development of eggshell colouration specific to an individual species is the result of evolutionary adaptation. However the precise reasons for this are not fully understood, but there are a number of hypothesis.
Traditionally it has been believed that avoidance of predation is the single most important factor affecting evolution in terms of eggshell colouration (Bosque 1995). This crypsis (camouflage) is critical for small birds, as the period during which they are present in the nest, either in the egg or as hatchling, represents the longest time during which they have little control over their own safety. For this reason it has long been believed that nest time is minimised, and growth rate maximised, in order for birds to become to maturity, thus relative safety, as rapidly as possible. The colour of the clutch is believed to mimic the environment, thus making it more difficult for predators to see the eggs (Soler et al. 2005).
It was found that eggs from species living in holes were more likely to be brightly coloured blue than those from species that did not live in holes (Soler et al. 2005). This would suggest that there was a need to camouflage the eggs of species living in more exposed areas that would be at risk from predation. However the authors suggested that the reason for the brighter colours was in fact related to increasing visibility and preventing damage to eggs that could not be seen well.
It has been indicated that crypsis is most likely to be operational in ground nesting bird, as opposed to shrub or canopy nesters, although there is evidence for and against both of these (cited by Weidinger 2001). In particular there is evidence that bright colour at the nest (presumably in the format of brightly coloured eggs) did not adversely affect predation rates in the song thrush (Weidinger 2001). This would obviate the need and effort for eggs to be camouflaged. The crypsis hypothesis is weakened where predators search out the nest rather than the eggs themselves (Moreno, Osorno 2003, Stokke, Moksnes & Roskaft 2002). It is also refuted somewhat by species such as the Tree Pipit which have highly variable eggs, that are not camouflaged (Harrison, Castell 1998).
Avian brood parasites are birds that lay eggs in the nest of other species, thus ensuring that the host nest nurtures the offspring until they are able to fend for themselves. It is known that some avian brood parasites have eggshells that exhibit mimetism (mimic the shell of the host species), and the development of eggshells to mimic hosts; and the subsequent responsive adaptations of the hosts; are referred to as a co-evolutionary arms race (Stokke, Moksnes & Roskaft 2002).
One example is the diederik cuckoo, which mimics the weaver bird (Lahti 2005). In addition to receiving the parental care brood parasites such as the diederik also hatch earlier than the weaver bird and remove all other eggs and young. This activity ensures the survival of the cuckoo, without the need for the mother to nurse it.
If an egg is sufficiently unusual, it is more obvious when a parasitic egg is present in the nest. Thus birds such as the African weaver bird have evolved a highly variable egg appearance between birds (interclutch), but a low within-clutch variation (Lahti 2005), which ensures that a parent can detect own eggs but can detect parasitic birds, whether from other village weaver birds or a different species entirely. Indeed, it has been indicated that selection for a low interclutch variation in pigmentation will be weaker than high interclutch variation (Soler, Moller 1996), (thus leading to maximum interclutch variation) so that mothers could easily distinguish between eggs laid by different birds. By contrast selection for low intraclutch variation would be stronger so that there would be minimal variation amongst the eggs laid by a single bird, so that the mother could easily recognise her own eggs.
Given that evolution is survival of the fittest it is logical to assume that an egg from a parasite that resembles the host is more likely to remain undetected, leading to successful nesting and hatching, than one that is obviously different and would be rejected. As it is the mother that will be responsible for laying the eggs in the host nest, it further follows that any genetic variation in eggshell colouration would be a female trait, encoded upon the W chromosome (Gosler, Barnett & Reynolds 2000).
It is important to consider the role of environmental determinants in eggshell colouration (Grim 2005). As put by Gosler et al “a large environmental component to phenotypic pattern variation in a host species might increase intraspecies variation, hampering the ability of a brood parasite to mimic that host’s eggs” (Gosler, Barnett & Reynolds 2000).
Mimetism is a geographical concept as, for instance, there are no North American birds that actually utilise egg mimicry (Stokke, Moksnes & Roskaft 2002). There are brood parasites, such as the cowbird, but these use many different hosts, rather than a single species. In this way there is no time, or arguably need, to develop the eggshell characteristics as there are insufficient instances of brood parasitism of a single species for it to require protection. Thus hosts often accept the cowbird eggs, so it has no need to adopt mimetism to ensure adoption. Indeed, it has been found that where there is no specific species targeting by brood parasites, there is little correlation between interclutch and intraclutch variation and the rejection rate of nonmimetic parasitic eggs (Stokke, Moksnes & Roskaft 2002).
Moreno and colleagues proposed that the very fact that many eggshells are brightly coloured could have evolved for a reason related to sexual selection and attraction (Moreno, Osorno 2003). They describe the pigmentation of eggshells as being a signalling method by which females can convey information to mating males.
It is known that biliverdin, responsible for the greenish blue pigment in eggshells, is a strong antioxidant. Therefore a female who is able to lay eggs containing this pigment is believed to have a higher capacity to control free radicals (Soler et al. 2005), as they are able to remove antioxidants from their circulation via deposition in eggshells (Moreno, Osorno 2003). Thus the female would be indicating to the male, post mating; that she is in high physical condition and it is worth the male investing time caring for the young as they too should be in a good physical condition. This hypothesis is referred to as a post mating sexually selected signal governing the evolutionary change in eggshell colouration.
Soler et al found that where females had to compete for male attention in birds with polygynous habits, eggshells had more blue-green colour than those where mating relationships were essentially monogamous (Soler et al. 2005).
The fact that steroid hormones can affect the accumulation of pigment in the shell gland in quail (Miksik, Holan & Deyl 1996), lends support to the idea that sexual selection has a role in eggshell pigmentation. In addition the finding that old age female pied flycatchers laid eggs with less depth of pigmentation when compared to younger females (Moreno et al. 2005) also supports the idea that a greater degree of pigmentation indicates better health. However it should be noted that the majority of research into this hypothesis has been carried out by the same or allied teams, and has yet to be ratified by independent individuals.
Whilst there is evidence for each of the main evolutionary hypothesis underlying the pigmentation of avian eggshell colouration there is by no means a consensus view. Indeed it is not even agreed that pigmentation has an evolutionary cause. Recently Gosler et al have essentially dismissed an evolutionary role, instead presenting evidence for pigmentation being a compensatory effect related to egg shell thickness (Gosler, Higham & Reynolds 2005). Their structural-functional hypothesis (as opposed to the signalling hypotheses which relate to evolution) indicate that, in passerines (accounting for around 60% of birds) calcium availability predicted both eggshell thickness and degree of pigmentation. They further suggest that the protoporphyrins responsible for pigmentation are actually part of the eggshell matrix, thus their presence indicates a structural strengthening role. However, even this research team do admit that the incorporation of protoporphyrins in the eggshell may have arisen due to an evolutionary adaptation. The logic of this is unassailable, as eggs that have shells thick enough to protect their contents will obviously survive to hatch and breed, whereas those with weakened shells are much less likely to do so.
The mere fact that an egg of a brood parasite closely resembles that of the host cannot be taken as definitive proof that evolution has taken place. It is entirely possible that similar environmental conditions have affected both host and parasite in a similar way, or that eggshell pigments would have changed over time regardless (Grim 2005). Given that eggshell pigmentation is known to be affected by environmental conditions such as chemicals and hormones, it would not be unreasonable to conclude that a similar condition is affecting both birds in a similar way. The fact that there is adaptation amongst, for instance, birds parasitised by the cuckoo family, could easily be explained by their similar environments.
Likewise despite its traditional support, the lack of a consistent relationship between eggshell colour and degree of predation argues against the hypothesis of crypsis. Whilst it is undeniable that some eggs do closely resemble their environment, thus are less likely to be seen by predators, the fact that others do not, yet are not subject to a higher rate of predation highlights the flaws in this hypothesis.
Finally the sexual selection hypothesis is relatively new and has yet to receive much attention. The initial evidence seems sensible, but there has simply been insufficient information provided thus far to form a conclusive opinion.
In conclusion, there are a variety of evolutionary methods by which eggshell colouration may have developed. There is evidence for and against each of these, as well as against the whole concept of evolution acting at all. However it is likely that evolution does have a part to play, although the precise method by which it acts is unclear. It is likely that a different form of evolutionary selection is operational depending on individual species and the environment in which it is living, as well as the other species located in the vicinity.
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