The avian beak and associated traits have long been popular targets for studies of adaptive trait evolution. The beak is the main food-processing tool of a bird. Hence, its size and shape are expected to be chiselled by natural selection to fit the feeding ecology of its carrier. One celebrated example of adaptive beak evolution is the radiation and associated diversification of feeding niches and beak shapes among Darwin’s finches [1, 2]. Long-term field studies of these finches have provided strong evidence that beak morphology evolves rapidly in response to changing ecological conditions, such as food type, food availability and interspecific competition [2–4]. Morphological evolution may be constrained by various factors, however . For instance, the size and shape of morphological traits in animals may be strongly constrained by body size and may exhibit more or less fixed allometric size relationships across species [6, 7]. Yet, the avian beak appears to be a trait that often exhibits high levels of evolvability [8–11].
The house sparrow Passer domesticus is a granivorous, widely distributed, human associated bird species [12, 13]. The origin of its commensal relationship with humans has been debated for some time [14–16]. However, in a recent paper Sætre et al.  used population genetic data, as well as previously published fossil and ecological data, to suggest that the house sparrow became associated with early human agricultural societies that rose in the Middle East about 10,000 years ago and then experienced a massive size and range expansion from about 4,000 years ago as agricultural civilizations spread through the Palearctic and Oriental regions. Although the house sparrow comprises several phenotypically distinct subspecies, these are virtually identical at neutral genetic markers, suggesting that phenotypic differentiation is of very recent origin .
One of the subspecies has apparently maintained the ancestral ecology of the species, however. The subspecies P. d. bactrianus is not associated with humans but breeds in natural or seminatural habitats, such as riverine scrub, hedgerows and trees near pastures and grassland, often far from human habitation [17, 18]. It is similar to other house sparrows at neutral genetic markers, however, consistent with the hypothesis that the ecological difference is associated with a recent ecological transition of the commensal house sparrows . In autumn P. d. bactrianus move southwards in large flocks from their breeding grounds in Central Asia to reach their wintering grounds in south-east Iran and western parts of the Indian sub-continent [19–21]. Significant for this study, P. d. bactrianus feed mainly on wild grass seeds whereas the commensal subspecies prefer seeds from cultivated crops, such as wheat and oats . The evolution of bony structures is possibly less constrained by weight in sedentary than in migratory birds because they are less dependent on endured flight. Hence, the abandonment of migratory behaviour in commensal house sparrows may have facilitated any evolutionary trend towards increased size and robustness of the beak and skull structures.
The abandonment of migratory behaviour in commensal house sparrows is likely to be an adaptation to the year-round supply of food provided by sedentary human societies through storage of cereals, spilling and feeding of domestic animals . Further, the transition from a diet of mainly wild seeds to one consisting mainly of grains from cultivated cereals  is likely to have imposed selection pressures on beak and skull morphology of the sparrows. Domestication of plants is associated with an increase in seed size [22–25], but domesticated cereals also differ from their wild relatives in several other traits, such as seed hardness and texture . Moreover, domesticated cereal seeds remain encapsulated in a tough rachis that holds the seed together, whereas in wild cereals the rachis fragmentizes at ripening . These trait differences would most certainly affect seed processing in a granivorous bird and hence the optimal size and shape of its beak and skull. In Darwin’s finches handling of larger and harder seeds is associated with larger, deeper beaks and a correlated increase in robustness of the muscle and jaw architecture of the birds .
Here, we apply geometric morphometrics to study variation in the beak and skull of commensal and non-commensal house sparrows from the same overall geographic region, namely Iran. The diversity of house sparrow subspecies is high in Iran (five recognized subspecies, including the non-commensal P. d. bactrianus) . Moreover, climatic and environmental conditions vary extensively regionally, from the moist and fertile temperate shore of the Caspian Sea in the north, through dry steppes and deserts in central parts, to tropical habitats in the south of the country. Climatic conditions are likely to affect agricultural practice, the choice of which crop species to cultivate and seed characteristics, all of which may affect optimal beak and skull morphology . However, cultivated crops differ from their wild ancestors in consistent ways , suggesting that adaptations to commensalism per se may have larger effects on morphological evolution than such local climatic variation. If adaptations to cultivated cereals have played a significant role in the recent morphological evolution of the house sparrow, we expect to find relatively larger divergence between commensal and non-commensal sparrows in beak and skull morphology than among the various commensal house sparrow subspecies, despite extensive environmental variation. We further predict that the commensal types should share (derived) skeletal features. Finally, we specifically predict that commensal house sparrows would have larger, more robust beaks and skull features associated with a more robust muscle and jaw architecture compared to the non-commensal P. d. bactrianus, as adaptations to process the larger and tougher seeds of cultivated cereal crops. At the proximate level, accelerated development (heterochrony) is a well-characterized ontological process that one could predict would yield the necessary changes in robustness.