Why Are Bird Eggs Coloured? Two Pigments Doing Several Jobs

Two pigments account for essentially all eggshell colour. One is a breakdown product of haemoglobin and produces brown and reddish tones. The other is related to the compounds in bile and produces blue and green. Applying the blue-green pigment throughout the shell as it forms gives a uniformly coloured egg, while applying the brown pigment late, as the egg passes through the final section of the oviduct, produces spots, blotches and streaks whose pattern depends on how fast the egg was moving and whether it was rotating. That mechanism explains why markings are frequently concentrated at one end and why the pattern varies between eggs from the same bird. White eggs simply lack both pigments. The system is economical, since two compounds and a variable application produce the entire observed range.

Several functions have been proposed and each is supported in some species:

• •Camouflage, which is well supported for ground-nesting birds whose eggs match the substrate closely
• •Strengthening, since the pigments appear to affect shell structure and heavily pigmented eggs may have thinner shells where calcium is scarce
• •Protection from ultraviolet light, since pigment absorbs radiation that would harm the embryo
• •Temperature regulation, since darker eggs absorb more heat, which matters for exposed nests in cold places
• •Signalling quality to a mate, with brighter blue eggs proposed as an indicator that the female is in good condition
• •Individual recognition, which matters enormously where brood parasites are present

Where one species lays eggs in the nests of another, colour becomes the battleground and the resulting dynamics are among the best-documented examples of coevolution. Parasitic species evolve eggs matching those of their hosts, in some cases with distinct lineages specialising on different hosts and each matching its own target closely. Hosts respond by evolving more distinctive and more variable eggs, making mismatches easier to detect, and by becoming better at recognising and ejecting foreign eggs. Parasites respond with closer mimicry. Experiments placing artificial eggs of varying similarity into nests have mapped how precisely hosts discriminate, and the answer differs sharply between populations, with those long exposed to parasitism rejecting far more readily than naive ones. The system is a rare case where an evolutionary contest can be observed in progress rather than inferred.

Egg shape varies as much as colour and has its own contested explanation. The traditional account held that pointed eggs roll in a tight circle rather than away, which suits cliff-nesting birds, and that observation is correct for the species concerned while failing to explain the wider pattern. A large comparative study across many species found that shape correlates most strongly with flight ability, with stronger fliers laying more elongated and asymmetric eggs, and proposed that a streamlined body shaped by flight constrains the egg that can pass through it. Clutch size matters too, since pointed eggs pack more efficiently into a nest with the narrow ends inward, which is visible in species laying four eggs in a neat arrangement. Shell membrane properties rather than the shell itself appear to determine the shape during formation, which was a surprising finding.

The distribution of white eggs is informative about what colour is for. Birds nesting in holes, burrows and covered nests overwhelmingly lay white or pale eggs, which follows if camouflage is a main function, since an egg nobody can see needs none. White eggs may also be easier for a parent to see in a dark cavity, which reduces breakage. Producing pigment costs something, so a species not needing it stops. Open-nesting birds by contrast have coloured and marked eggs almost universally. Several groups have moved between nesting types and their egg colour has followed, which is the kind of correlated change that supports the functional explanation. That said, the pattern has exceptions and several proposed functions operate simultaneously, so the honest position is that colour serves different purposes in different lineages.

Two pigments produce the whole range, one from haemoglobin breakdown giving browns and one related to bile giving blues, with markings applied late as the egg passes through and moves. Camouflage, shell strength, ultraviolet protection, temperature and signalling have all been supported in different species. Hole-nesting birds lay white eggs almost universally, which fits camouflage being a main function.