One of the reasons house sparrows are so adept at exploiting diverse diets might involve plasticity in the release of digestive enzymes ( Brzek et al., 2009). Adult house sparrows have a fairly opportunistic diet throughout much of the year, especially in cities and suburbs where human refuse is plentiful ( Summers-Smith, 1988). Later, following fledging, they favor grains, especially outside urban areas ( Anderson, 2006). Nestling house sparrows are fed an insect-based diet for the first three days after hatching. Body size of birds also increased with latitude, and perhaps most interestingly, all of these geographic trends in biological traits arose rapidly in the introduced populations ( Johnston and Selander, 1964 Selander and Johnston, 1967 Johnston and Selander, 1971). Introduced populations in North America were discovered to have pale coloration in hot, arid climates, but darker coloration in cooler, humid climates ( Johnston and Selander, 1964). Most well-known through the pioneering work of Richard F Johnston and Robert K Selander, plumage color and aspects of body size (wing, tail and tarsus length, skeletal characteristics, and body mass) were found to vary within and between native and introduced populations ( Selander and Johnston, 1967 Johnston and Selander, 1964 Johnston and Selander, 1971 Johnston, 1969 Johnston, 1973). Importantly, many morphological characteristics also vary geographically. The currently favored hypothesis for badge size is that it serves some role in mate choice, as females tend to choose males with large badges, and badge size is positively correlated with male sexual behaviors ( Veiga, 1996). Recently, however, the largest meta-analysis to date revealed that badge size is at best an unreliable signal of dominance status ( Sánchez-Tójar et al., 2018). Large badge size has been thought to convey an individual’s propensity to win in male-male competitive interactions the logic was that possessing information a priori about a competitor could save both the badge-holder and his opponents from wasted energy and risk of injury ( Rohwer, 1975). Arguably, this badge is one of the factors that made this species a model in behavioral ecology ( Sánchez-Tójar et al., 2018). The most conspicuous morphological difference between male and female sparrows is the large black throat badge of males. Image Credits: All images taken by Janneke Case in Tampa, Florida, United States, in 2019. ( D) Male house sparrow provisioning nestlings. This topic remains contentious ( Box 1), but these declines have been attributed to a multitude of factors, including infectious disease, pollution, pesticide use, predator dynamics, new building methodologies and more efficient grain harvesting and storage ( Shaw et al., 2008 Summers-Smith, 2003 Singh et al., 2013 Bell et al., 2010 Dadam et al., 2019). Though still widespread, significant declines have been reported in the native range of the species since the 1970s. A reliance on humans is evident from their colonization of Northern Europe, Eastern Europe and Central Asia in the early 1800s, as agriculture spread and urbanization increased ( Summers-Smith, 1963). However, they also continue to increase their geographic range by exploiting ongoing and accelerating anthropogenic change ( Ravinet et al., 2018 Saetre et al., 2012). Becoming commensal some 10,000 years ago, house sparrows are now strongly associated with habitats that have been modified by humans. House sparrows are native to parts of Asia, North Africa and most of Europe, (with the exception of Italy which is occupied by the Italian sparrow P. Native distribution and natural range expansions Here, we explore the natural history of house sparrows and the contributions that these birds have made to basic biology and beyond. Their ubiquity and close association with humans have undoubtedly led to their detailed study across biological and even sociological disciplines. It is for this relationship with people that they received their species identifier domesticus, which derives from the Latin domus or 'house', from Carl Linnaeus in 1758 ( Jobling, 2009 Anderson, 2006). Considered anthrodependent, some populations have gone extinct locally without human presence ( Ravinet et al., 2018 Summers-Smith, 1988). House sparrows can be found living and breeding in climactically extreme environments from deserts in southern California to cities above the Arctic circle, where they are found almost exclusively in close proximity to human habitation ( Hanson et al., 2020b). The species is one of the most widely distributed and common birds in the world, represented by 12 different subspecies ( Summers-Smith, 2009). House sparrows are small, sexually dimorphic birds in the family Passeridae.
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