Table 1.

Hypotheses and predictions with supporting rationale of how eggshell calcium carbonate content in birds relates to life-history strategies and eggshell characteristics. Hypotheses are divided based on Tinbergen's four-question structure [43,44].

level of question/prediction	hypothesis	prediction	rationale and/or proposed mechanism
mechanism	1) Thicker eggshells are achieved through greater deposition of calcite but not matrix during layer formation, resulting in higher relative calcium carbonate content of thicker eggshells.	Species with eggs that have thicker shells also produce shells with higher calcium carbonate content than species with thin-shelled eggs.	The crystalline structure of the shell is believed to be controlled primarily by the organic matrix, which modulates the deposition of calcium from the uterine fluid [45,46]. Selection for thicker eggshell could increase the binding of calcite crystals to the organic matrix during shell formation.
	2) Calcium carbonate content of eggshells is influenced by diet.	Species with diets that are normally higher in calcium invest more calcium in their eggshells.	The majority of calcium needed for egg production must be obtained from their diet during egg formation [34].
	3) Eggshell pigmentation has evolved to compensate for lower calcium carbonate content.	Pigmented eggshells contain less calcium carbonate than immaculate eggshells.	In great tits (Parus major) and Eurasian sparrowhawks (Accipiter nisus) calcium stress and eggshell thinning have been correlated with more pigmented eggshells, suggesting protoporphyrin pigment might be used to strengthen eggs in compensation for lacking calcium [47,48].
			However, in another species (black-headed gulls; Larus ridibundus) the correlation between pigmentation and shell thinning was found to be weak [49].
	4) Species eggshell calcium carbonate content is adjusted to their breeding latitude as a result of calcium availability and selection for thicker shells in colder climates.	Species breeding at higher latitudes (further from the Equator) will have a higher calcium carbonate content in their eggs.	Multiple egg traits are known to vary latitudinally at both an inter- and intraspecies level, believed to be a response to variation in temperature and solar radiation [31,50,51]. There is evidence that thicker eggshell can retain heat longer, which may benefit species breeding at colder latitudes [52], which led to greater calcium carbonate content in these eggs. Additionally, calcium availability in the environment is known to increase in higher latitudes [42].
ontogeny/proximate	5) Precocial species deposit more calcium overall into their eggshell in order to supply the higher demand for embryonic growth without compromising the integrity of the eggshell through excessive thinning.	Eggshell calcium carbonate content is higher in species with precocial modes of development.	Nestlings of precocial species hatch in a more developed state than those of altricial species, in particular they have a more ossified skeleton and muscles and larger brains [53]. This requires greater sequestration of calcium during development, which is supplied by a greater number of mammillary tips of the eggshell [15,53].
	6) Incubation period influences calcium carbonate content.	Species with longer incubation periods will have more calcium carbonate in their eggshell.	Longer incubation period requires less porous eggshells to prevent excessive water loss, and as a result may have denser eggshell produced through greater calcite crystal deposition [54,55], showing an evolutionary relationship between eggshell porosity and incubation length in Alcidae species.
adaptation/ ultimate	7) Calcium carbonate content is influenced by reproductive investment (clutch size).	Calcium carbonate content decreases with increasing clutch size	Patten [42] suggested that the evolution of clutch size is influenced by the availability of calcium in the breeding habitat. This would suggest a strong correlation between clutch size and eggshell calcium content.
	8) A species lifespan influences calcium carbonate content per egg.	Lifespan is negatively correlated with calcium carbonate content.	If calcium foraging is an expensive activity, longer lived species might invest less calcium in eggs per clutch in order to conserve energy for future reproductive attempts compared with species which only have the opportunity to breed a few times over their short lifespan. There is evidence that lifespan influences egg size and clutch size in birds [56].
	9) Eggshell calcium carbonate content is higher in species with eggs that are smaller than predicted for the mass of the incubating parents.	Calcium carbonate content will be predicted by the residual difference between fresh egg weight (as a proxy for egg size) and adult body mass.	Egg traits such as the size, shape and thickness of eggs have evolved in tight concert with adult body mass, as the egg needs to be able to support the weight of the parent during incubation yet remain thin enough to allow the chick to hatch [3,57]. Smaller eggs experience a greater force per unit area of the shell from the mass of the incubating parent and as such could require a higher calcium carbonate content to compensate.
phylogeny/ultimate	10) A large component of variation in eggshell calcium carbonate content is correlated with the species' phylogenetic position.	Calcium carbonate content has a phylogenetic signal close to, but less than, 1 (Pagel's λ) [58].	Many eggshell characteristics have been shown to strongly covary with phylogenetic relatedness in birds [5,30,59]; as such we expect eggshell calcium carbonate content to be similarly correlated to phylogeny.