Mechanisms of deer (Cervidae) impacts on birds: A comprehensive review

Cervidae (deer and their relatives) have increased in numbers and global distribution since the early 20th century, as the result of anthropogenic factors including introduction to new countries and continents, reduced hunting, and suppression of predators. The first literature on how this expansion has affected bird populations appeared in the 1940s, but research on the topic did not accelerate until the 2000s. Abundant Cervidae, whether native or introduced, can exert strong negative effects on bird species that depend on forest understory vegetation below two meters in height by eliminating this strata of the forest through herbivory. However, negative effects of high Cervidae abundance on birds are not limited to effects on forest-floor specialist bird species. Selective browsing of tree saplings alters the future composition of forest canopy, potentially affecting entire avian communities. Cervidae compete with birds for invertebrate food through elimination of leaf habitat. In addition, abundant populations of Cervidae species that graze and browse non-forested habitats can suppress bird communities by eliminating shrublands, especially riparian willow (Salix sp) thickets. The role of Cervidae as predators is another way in which they could potentially be harming birds through destruction of eggs and nestlings. Finally, lead poisoning from bullets used in hunting of Cervidae also has negative effects on birds, including some endangered and charismatic scavenger bird species. In conclusion, global expansion of Cervidae, both native and introduced populations, poses a multi-pronged threat to bird species. These threats should not be overlooked as a major cause of ongoing global bird declines.