The diversity of eggshell colours and patterning across different birds is a fascinating example of the extended avian phenotype. The avian egg's background colours range from light green/yellow to brilliant blue and intense maroon, and with the scrawling patterns comparable to the assortment of eggs found in an Easter Bunny's basket. Here we briefly introduce the diversity of eggshell appearance, its perplexingly narrow chemical basis, and clarify how the a priori assessment of assumptions and potential hypotheses can shape new eggshell research. In addition, we highlight some of the recent hypotheses developed outside the visual paradigm that has dictated the most influential hypotheses of eggshell appearance to date. Finally, we indicate priority areas for future comprehensive research on eggshells based on adaptations to ongoing changes in environmental conditions.
AkinsJ.M., HooperM.J., MillerH. and WoodsJ.S. (1993) Porphyrin profiles in the nestling European starling (Sturnus vulgaris)-a potential biomarker of field contaminant exposure. J. Toxicol. Env. Health, 40, 47-59.
2.
ArendtW.J. (2004) A quarter century of variation in color and allometric characteristics of eggs from a rain forest population of the Pearly-eyed Thrasher (Margarops fuscatus). Caribb. J. Sci., 40, 204-217.
3.
AvilésJ.M., StokkeB.G., MoksnesA., RøskaftE. and MøllerA.P. (2006) Nest predation and the evolution of egg appearance in passerine birds in Europe and North America. Evol. Ecol. Res., 8, 493-513.
4.
AvilésJ.M., StokkeB.G., MoksnesA., RøskaftE. and MøllerA.P. (2007) Environmental conditions influence egg color of reed warblers Acrocephalus scirpaceus and their parasite, the common cuckoo. Cuculus canorus Behav. Ecol. Sociobiol., 61, 475-485.
5.
BaicichP.J. and HarrisonC.J.O. (1997) A guide to the nests, eggs, and nestlings of North American birds, 2nd edn. Academic Press, San Diego.
6.
BakkenG.S., VanderbiltV.C., ButtemerW.A. and DawsonW.R. (1978) Avian eggs - thermoregulatory value of very high near-IR reflectance. Science, 200, 321-323.
7.
BandaranayakeW.M. (2006) The nature and role of pigments of marine invertebrates. Nat. Product Rep., 23, 223-255.
8.
BergE.C., McCormackJ.E. and SmithT.B. (2009) Test of an adaptive hypothesis for egg speckling along an elevational gradient in a population of Mexican jays. Aphelocoma ultramarina J. Avian Biol., 40, 448-452.
9.
BertramB.C.R. (1979) Ostriches recognise their own eggs and discard others. Nature, 279, 233-234.
10.
BlumthalerM., AmbachW. and EllingerR. (1997) Increase in solar UV radiation with altitude. J. Photochem. Photobiol., B39, 130-134.
11.
BosqueC. and BosqueM.T. (1995) Nest predation as a selective factor in the evolution of developmental rates in altricial birds. Am. Nat., 145, 234-260.
12.
BuschmannJ.-U.F., MannsM. and GüntürkünO. (2006) “Let There be Light!” pigeon eggs are regularly exposed to light during breeding. Behav. Proc., 73, 62-67.
13.
CasiniS., FossiM.C., LeonzioC. and RenzoniA. (2003) Review: Porphyrins as biomarkers for hazard assessment of bird populations: Destructive and non-destructive use. Ecotoxicology, 12, 297-305.
14.
CasseyP. (2009) Seeing colours in the dark. Curr. Biol., 19, 1083-1084.
15.
CasseyP., PortugalS.J.P., MaurerG., EwenJ.G., BoultonR.L., HauberM.E. and BlackburnT.M. (2010) Variability in avian eggshell colour: A comparative study of museum eggshells. PLoS one, 5, e12054.
16.
CherryM.I. and BennettA.T.D. (2001) Egg colour matching in an African cuckoo, as revealed by UV-visible reflectance spectrophotometry. Proc. R. Soc. B, 268, 565-571.
17.
CherryM.I. and GoslerA.G. (2010) Avian eggshell colouration: new perspectives on adaptive explanations. Biol. J. Linn. Soc., 100, 753-762.
18.
ColemanM.A. and Mc\nabR.A. (1975) Photo acceleration of development in depigmented Japanese quail eggs. Poult. Sci., 54, 1849-1855.
19.
ComfortA. (1951) The pigmentation of molluscan shells. Biol. Rev., 26, 285-301.
20.
ConryP.J. (1988) High nest predation by brown tree snakes on Guam. Condor, 90, 478-482.
21.
DaviesN.B. (2000) Cuckoos, Cowbirds and Other Cheats.T. & A. D. Poyser, London.
22.
DaviesN.B. and BrookeM.D.L. (1989) An experimental study of co-evolution between the cuckoo, Cuculus canorus, and its hosts. I. host egg discrimination. J. Anim. Ecol., 58, 207-224.
23.
De GruijlF.R., Van KranenH.J. and MullendersL.H.F. (2001) UV-induced DNA damage, repair, mutations and oncogenic pathways in skin cancer. J. Photochem. Photobiol., B63, 19-27.
24.
DeemingD.C. (2002) Behaviour patterns during incubation. In: DeemingD.C. (ed.), Avian incubation:behaviour, environment and evolution, pp. 63–87. Oxford University Press,.
25.
DeemingD.C. (2006) Ultrastructural and functional morphology of eggshells supports the idea that dinosaur eggs were incubated buried in a substrate. Palaeontology, 49, 171-185.
26.
DeemingD.C. (2011) A review of the relationship between eggshell colour and water vapour conductance. Avian Biol. Res., 4, 224-230.
27.
EndlerJ.A. and MielkeP.W. (2005) Comparing entire colour patterns as birds see them. Biol. J. Linn. Soc., 86, 405-431.
28.
FischerH. and KöglF. (1923) Zur Kenntnis der natürlichen Porphyrine IV. Über das Ooporphyrin. Zeit. Physiol. Chemie, 131, 241-261.
29.
GorcheinA., LimC.K. and CasseyP. (2009) Extraction and analysis of colourful eggshell pigments using HPLC and HPLC/electrospray ionization tandem mass spectrometry. Biomed. Chromatography, 23, 602–606
30.
GoslerA.G., OliverR., ConnorO.R. and BonserR.H.C. (2011) Protoporphyrin and eggshell strength: preliminary findings from a passerine bird. Avian Biol. Res., 4, 214-223.
31.
GoslerA.G., HighamJ. and ReynoldsS.J. (2005) Why are birds’ eggs speckled? Ecol. Lett., 8, 1105-1113.
32.
GötmarkF. (1992) Blue eggs do not reduce nest predation in the song thrush, Turdus philomelos. Beh. Ecol. Sociobiol., 30, 245-252.
33.
HanleyD. and DoucetS. (2009) Egg colouration in ring-billed gulls (Larus delawarensis): a test of the sexual signaling hypothesis. Beh. Ecol. Sociobiol., 63, 719-729.
34.
HanleyD., DoucetS. and DearbornD.C. (2010) A blackmail hypothesis for the evolution of conspicuous egg colouration in birds. Auk, 127, 453-459.
35.
HartN.S. and HuntD.M. (2007) Avian visual pigments: characteristics, spectral tuning, and evolution. Am. Nat., 169, S7–S26.
36.
HobsonK.A. (1999) Tracing origins and migration of wildlife using stable isotopes: a review. Oecologia, 120, 314-326.
37.
IshikawaS., SuzukiK., FukudaE., AriharaK., YamamotoY., MukaiT. and ItohM. (2010) Photodynamic antimicrobial activity of avian eggshell pigments. FEBS Lett., 584, 770-774.
38.
JagannathA., ShoreR.F., WalkerL.A., FernsP.N. and GoslerA.G. (2008) Eggshell pigmentation indicates pesticide contamination. J. Appl. Ecol., 45, 133-140.
39.
KelberA., VorobyevM. and OsorioD. (2003) Animal colour vision: behavioural tests and physiological concepts. Biol. Rev., 78, 81-118.
40.
KellyS.T. and DecapitaM.E. (1982) Cowbird control and its effect on kirtland's warbler reproductive success. Wilson Bull., 94, 363-365.
41.
KennedyG.Y. and VeversH.G. (1976) A survey of avian eggshell pigments. Comp. Biochem. Physiol., B55, 117-123.
42.
KennedyS.W., FoxG.A., TrudeauS., BastienL.J. and JonesS.P. (1998) Highly carboxylated porphyrin concentration: a biochemical marker of PCB exposure in herring gulls. Mar. Environ. Res., 46, 65-69.
43.
KilnerR.M. (2006) The evolution of egg colour and patterning in birds. Biol. Rev., 81, 383-406.
44.
LackD. (1958) The significance of the colour of turdine eggs. Ibis, 100, 145-166.
45.
LahtiD.C. (2008) Population differentiation and rapid evolution of egg color in accordance with solar radiation. Auk, 125, 796-802.
46.
LangM.R. and WellsJ.W. (1987) A review of eggshell pigmentation. World's Poult. Sci. J., 43, 238-246.
47.
LangmoreN.E., StevensM., MaurerG., HeinsohnR., HallM.L., PetersA. and KilnerR.M. (2011) Visual mimicry of host nestlings by cuckoos. Proc. R. Soc. B, Biol. Sci., 278, 2455-2463.
LembergR. (1934) Bile pigments. VI. Biliverdin, uteroverdin and oocyan. Biochem. J., 28, 978-987.
50.
LiebermannC. (1878) Ueber die Färbungen der Vogeleierschalen. Ber. Deutsch. Chem. Ges., 11, 606-610.
51.
Lopez-RullI., MikšíkI. and GilD. (2008) Egg pigmentation reflects female and egg quality in the spotless starling. Sturnus unicolor Beh. Ecol. Sociobiol., 62, 1877-1884.
52.
LovellP.G., TolhurstD.J., PárragaC.A., BaddeleyR., LeonardsU., TrosciankoJ. and TrosciankoT. (2005) Stability of the color-opponent signals under changes of illuminant in natural scenes. J. Opt. Soc. Am., A22, 2060-2071.
53.
MagigeF., MoeB. and RøskaftE. (2008) The white colour of the Ostrich (Struthio camelus) egg is a trade-off between predation and overheating. J. Orn., 149, 323-328.
MartinT.E. and GhalamborC.K. (1999) Males feeding females during incubation. I. Required by microclimate or constrained by nest predation? Am. Nat., 153, 131-139.
56.
MateoR., BeyerW.N., SpannJ.W., HoffmanD.J. and RamisA. (2003) Relationship between oxidative stress, pathology, and behavioral signs of lead poisoning in mallards. J. Toxicol. Env. Health, 66, 1371-1389.
57.
MateoR., CastellsG., GreenA.J., GodoyC. and CristofolC. (2004) Determination of porphyrins and biliverdin in bile and excreta of birds by a single liquid chromatography-UV detection analysis. J. Chromatography B, 810, 305-311.
58.
MaurerG. (2008) Who cares? Males provide most parental care in a monogamous nesting cuckoo. Ethology, 114, 540-547.
59.
MaurerG., PortugalS.J. and CasseyP. (2011a) An embryo's eye view of avian eggshell pigmentation. J. Avian Biol., doi:10.1111/j.1600-0587.2011.05368.x.
60.
MaurerG., PortugalS.J. and CasseyP. (2011b) Speckles of cryptic black headed gull eggs show no mechanical or conductance structural function. J. Zool., doi:10.1111/j.1469-7998.2011.00830.x.
61.
McaldowieA.M. (1886) Observations on the development and the decay of the pigment layer on birds’ eggs. J. Anat. Physiol., 20, 225-237.
62.
MikšíkI., HolánV. and DeylZ. (1996) Avian eggshell pigments and their variability. Comp. Biochem. Physiol., B113, 607-612.
63.
MikšíkI., HolánV. and DeylZ. (1994) Quantification and variability of eggshell pigment content. Comp. Biochem. Physiol., A109, 769-772.
64.
MoralesJ., KimS.-Y., LobatoE., MerinoS., TomásG., Martínez- De La PuenteJ. and MorenoJ. (2010a) On the heritability of blue-green eggshell colouration. J. Evol. Biol., 23, 1783-1791.
65.
MoralesJ., TorresR. and VelandoA. (2010b) Parental conflict and blue egg colouration in a seabird. Naturwissenschaften, 97, 173-180.
66.
MoralesJ., VelandoA. and MorenoJ. (2008) Pigment allocation to eggs decreases plasma antioxidants in a songbird. Beh. Ecol. Sociobiol., 63, 227-233.
67.
MoralesJ., VelandoA. and TorresR. (2011) Biliverdin-based egg colouration is enhanced by carotenoid supplementation. Beh. Ecol. Sociobiol., 65, 197-203.
68.
MorenoJ., LobatoE., MoralesJ., MerinoS., TomásG., Martínez-De La PuenteJ., SanzJ.J., MateoR. and SolerJ.J. (2006) Experimental evidence that egg color indicates female condition at laying in a songbird. Behav. Ecol., 17, 651-655.
69.
MorenoJ. and OsornoJ.L. (2003) Avian egg colour and sexual selection: does eggshell pigmentation reflect female condition and genetic quality? Ecol. Lett., 6, 803-806.
70.
OlsonC.R., VleckC.M. and VleckD. (2006) Periodic cooling of bird eggs reduces embryonic growth efficiency. Physiol. Biochem. Zool., 79, 927-936.
71.
ParsonsJ. (1970) Relationship between egg size and post-hatching chick mortality in the Herring Gull (Larus argentatus). Nature, 228, 1221-1222.
72.
PatentD.H. (1977) Reptiles and how they reproduce.Holiday House Inc., New York.
73.
PikeT.W. (2011) Using digital cameras to investigate animal colouration: estimating sensor sensitivity functions. Beh. Ecol. Sociobiol., 65, 849-858.
74.
PoundsJ.A., FogdenM.P.L. and CampbellJ.H. (1999) Biological response to climate change on a tropical mountain. Nature, 398, 611-615.
75.
RatcliffeD.A. (1967) Decrease in eggshell weight in certain birds of prey. Nature, 215, 208-210.
76.
ReynoldsS.J., MartinG.R. and CasseyP. (2009) Is sexual selection blurring the functional significance of eggshell colouration hypotheses? Anim. Behav., 78, 209-215.
77.
RiehlC. (2011) Paternal investment and the ‘sexually selected hypothesis’ for the evolution of eggshell coloration: revisiting the assumptions. Auk, 128, 175-179.
78.
RozenboimI., BiranI., UniZ., RobinzonB. and HalevyO. (1999) The effect of monochromatic light on broiler growth and development. Poult. Sci., 78, 135-138.
79.
SanzJ.J. and García-NavasV. (2009) Eggshell pigmentation pattern in relation to breeding performance of blue tits. Cyanistes caeruleus J. Anim. Ecol., 78, 31-41.
80.
SchönwetterM. (1960-1992) Handbuch der Oologie.Akademie Verlag.
81.
ShafeyT.M., Al-BatshanH.A., GhannamM.M. and Al-AyedM.S. (2005) Effect of intensity of eggshell pigment and illuminated incubation on hatchability of brown eggs. Brit. Poult. Sci., 46, 190-198.
82.
ShafeyT.M. and Al-MohsenT.H. (2002) Embryonic growth, hatching time and hatchability performance of meat breeder eggs incubated under continuous green light. Asian–Austr. J. Anim. Sci., 15, 1702-1707.
83.
ShafeyT.M., Al-MohsenT.H., Al-SobayelA.A., AlhassanM.J. and GhannamM.M. (2002) Effects of eggshell pigmentation and egg size on the spectral properties and characteristics of eggshell of meat and layer breeder eggs. Asian–Austr. J. Anim. Sci., 15, 297-302.
84.
ShafeyT.M., GhannamM.M., Al-BatshanH.A. and Al-AyedM.S. (2004) Effect of pigment intensity and region of eggshell on the spectral transmission of light that passes the eggshell of chickens. Int. J. Poult. Sci., 3, 228-233.
85.
SieffermanL., NavaraK. and HillG. (2006) Egg colouration is correlated with female condition in eastern bluebirds (Sialia sialis). Beh. Ecol. Sociobiol., 59, 651-656.
86.
SkutchA.F. (1976) Parent birds and their young.University of Texas Press.
87.
SolerJ.J., MorenoJ., AvilésJ.M. and MøllerA.P. (2005) Blue and green egg-color intensity is associated with parental effort and mating system in passerines: Support for the sexual selection hypothesis. Evolution, 59, 636-644.
88.
SorbyH.C. (1875) On the colouring-matters of the shells of birds’ eggs. Proc. Zool. Soc. Lond., 23, 351-365.
89.
StevensM., ParragaC.A., CuthillI.C., PartridgeJ.C. and TrosciankoT.S. (2007) Using digital photography to study animal colouration. Biol. J. Linn. Soc., 90, 211-237.
90.
StoddardM.C. and StevensM. (2010) Pattern mimicry of host eggs by the common cuckoo, as seen through a bird's eye. Proc. R. Soc. B, 277, 1387-1393.
91.
StokkeB.G., MoksnesA. and RøskaftE. (2002) Obligate brood parasites as selective agents for evolution of egg appearance in passerine birds. Evolution, 56, 199-205.
92.
TschanzB. (1959) Zur Brutbiologie der Trottellumme (Uria aalge aalge Pont.). Behaviour, 14, 1-100.
93.
UnderwoodT.J. and SealyS.G. (2002) Adaptive significance of egg colouration. In: DeemingD.C. (ed.), Avian incubation: behaviour, environment and evolution, pp. 280–298. Oxford University Press.
94.
VerbeekN.A.M. (1988) Differential predation of eggs in clutches of Glaucous-winged Gulls. Larus glaucescens Ibis, 130, 512-518.
95.
VeteranyL., HluchyS. and VeteranyovaA. (2004) The influence of ultra-violet radiation on chicken hatching. J. Env. Sci. Health, A39, 2333-2339.
96.
VorobyevM. and OsorioD. (1998) Receptor noise as a determinant of colour thresholds. Proc. R. Soc. B, 265, 351-358.
97.
WallaceA.R. (1889) Darwinism: An exposition of the theory of natural selection with some of its applications.Macmillan, London.
98.
WangX.T., ZhaoC.J., LiJ.Y., XuG.Y., LianL.S., WuC.X. and DengX.M. (2009) Comparison of the total amount of eggshell pigments in Dongxiang brown-shelled eggs and Dongxiang blue-shelled eggs. Poult. Sci., 88, 1735-39.
99.
WangZ.P., LiuR.F., WangA.R., LiJ.Y. and DengX.M. (2011) Expression and activity analysis reveal that heme oxygenase (decycling) 1 is associated with blue egg formation. Poult. Sci., 90, 836-841.
100.
WeidingerK. (2001) Does egg colour affect predation rate on open passerine nests? Beh. Ecol. Sociobiol., 49, 456-464.
101.
WeidingerK. (2004) Relative effects of nest size and site on the risk of predation in open nesting passerines. J. Avian Biol., 35, 515-523.
102.
WestmorelandD. and BestL.B. (1986) Incubation continuity and the advantage of cryptic egg colouration to Mourning Doves. Wilson Bull., 98, 297–300
103.
WestmorelandD. and KiltieR.A. (1996) Egg crypsis and clutch survival in three species of blackbirds (Icteridae). Biol. J. Linn. Soc., 58, 159-172.
104.
WestmorelandD. and KiltieR.A. (2007) Egg colouration and selection for crypsis in open-nesting blackbirds. J. Avian Biol., 38, 682-689.
