Habitat Geology Influences Intraspecific Variation in the Speckling Patterns of Blue Tit Cyanistes caeruleus and Great Tit Parus major Eggs

Abstract. Birdsu0027 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,...