Birds' eggs exhibit a small amount of interspecific diversity in terms of their colors and patterning but considerable variation occurs at the intraspecific level. Here, we examined the contributions of habitat geology, first egg dates, clutch sizes and year in determining intraspecific variation in the intensity of speckling patterns on the eggs of Blue Tits Cyanistes caeruleus and Great Tits Parus major breeding in deciduous woodlands with underlying calcium-rich limestone and calcium-poor gritstone rock types in northwest England. Eggshell speckling patterns varied significantly in relation to habitat geology, with both species laying more heavily speckled eggs in woodlands with gritstone rock types than in woodlands with limestone rock types, even though the weights of laying females and the availability of aerial invertebrates never differed between the two habitat geology types. Meanwhile, there was no variation in eggshell speckling patterns in both species in relation to first egg dates, clutch sizes or year. The eggs of both species were probably more speckled in the calcium-poor gritstone woodland because laying females were compensating for reduced eggshell strength in those woodland areas where there was low calcium availability by depositing more protoporphyrin-based pigments, that constitute the darker speckles and form at locations on the egg where the shell is thinnest, than conspecifics laying eggs in the calcium-rich limestone woodland. We thus conclude that micro-geographic heterogeneity in habitat geology types significantly influence the eggshell patterning of birds' eggs and more broadly our study confirms that environmental factors strongly influence intraspecific variation in avian eggshell patterning.