LINKED PAPER
Eggshell coloration is an indicator of dietary calcium in Common Pheasants (Phasianus colchicus). Jones, L. R., Black, H. L., Boudreau, M. R., Bracken, R. D., Johnston, N. P. 2024. IBIS. DOI: 10.1111/ibi.13256. VIEW
All birds lay eggs to reproduce, but these eggs can vary dramatically between species in shape, size, and colour. Eggshell coloration has been particularly well-studied, with blue and green hues known to be produced by the pigment biliverdin, while brown, black, red, and yellow hues are produced by the pigment protoporphyrin (Kennedy & Vevers 1976). As well as providing colour, there is evidence that protoporphyrin spots on maculated (spotted) eggshells can strengthen them, but what about non-spotted shells?
In a recent study in Ibis, Landon Jones and colleagues examined eggs of Common Pheasants (Phasianus colchicus), to test the structural-function hypothesis – that protoporphyrin pigments can be used to strengthen eggshell structure when avian diets lack calcium (Gosler et al. 2005) – in unspotted eggs for the first time.
Common Pheasant eggs
Common Pheasants eat a low calcium diet and can lay multiple relatively large clutches of 7-15 eggs per breeding season (Guidice & Ratti 2020). The eggshells of Common Pheasants are unspotted, with an olive-brown ground colour mainly composed of a high protoporphyrin, low biliverdin mixture (Kennedy & Vevers 1976). In this study, eggs were collected from pheasants eating seven diets differing in calcium levels (0.2-4.5%). A 0.2% calcium diet has previously been shown to lead to eggs with thinner shells than those produced by birds on diets of 1.1-4.5% calcium (Jones et al. 2010).
Figure 1. Violin plots for (a) brightness, (b) red–green colour axis and (c) yellow–blue colour axis as a function of calcium diet from eggs produced by Common Pheasants Phasianus colchicus.
Calcium deficiency, eggshell thickness, and colour
The results showed partial support for the structural-function hypothesis, with the eggs from the calcium-deficient diet (0.2% calcium) having lower eggshell thickness and being darkest in colour. However, mean brightness values across calcium groups were less variable than mean values on either colour axis. It is possible that brightness is limited for other reasons, for example, very light and very dark eggs may be more conspicuous to visual predators than intermediate shades (Delacour 1977, Kilner 2006). Despite this, the overall results suggest that eggshell pigments deposited evenly as ground colour, and not just as spots, can strengthen eggshells. If this is the case, it could be advantageous for Common Pheasants, helping them to mitigate calcium limitations during the breeding season. Future studies could investigate the hypothesis further by testing eggshell strength and measuring calcium content.
References
Delacour, J. (1977). Pheasants of the World. Surrey, UK: World Pheasant Association and Saiga Publishing.
Gosler, A.G., Higham, J.P. & James Reynolds, S. (2005). Why are birds’ eggs speckled? Ecology Letters 8: 1105-1113. VIEW
Guidice, J.H. & Ratti, J.R. (2020). Ring-necked Pheasant: Phasianus colchicus. In Billerman, S.M. (ed) Birds of the World. Ithaca, NY, USA: Cornell Laboratory of Ornithology. VIEW
Jones, L.R., Black, H.L., White, C.M., Johnston, N.P., McGee, M.E., Donahue, S.W. & Eggett, D.L. (2010). Effects of calcium-loading on egg production in ring-necked pheasants. Journal of Wildlife Management 74: 1295–1300. VIEW
Kennedy, G.Y. & Vevers, H.G. (1976). A survey of avian eggshell pigments. Comparative Biochemistry and Physiology B 55: 117–123. VIEW
Kilner, R.M. (2006). The evolution of egg colour and patterning in birds. Biological Reviews of the Cambridge Philosophical Society 81: 383–406. VIEW
Image credits
Top right: Jagdfasan (Phasianus colchicus) auf Norderney | Stephan Sprinz | CC BY 4.0 Wikimedia Commons
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