Does the ‘island rule’ apply to birds? An analysis of morphological variation between insular and mainland birds from the Australian, New Zealand and Antarctic region

The ‘island rule’ states that large animals become smaller and small animals become larger on islands. Morphological shifts on islands have been generalized for all vertebrates as a strategy to better exploit limited resources in constrained areas with low interspecific competition and predation pressures. In the case of birds, most of the studies that validate this rule have focused on passerines, and it is unclear about whether the rule applies to other Orders. Studies suggested insular morphological shift in birds is for greater bill size variation within males and females from the same species, when compared to their mainland counterparts. Increased sexual size dimorphism in island species would represent a strategy for resource exploitation. These insular morphological shifts are thought to be influenced by an island’s environmental and physical characteristics. I tested the validity of the ‘island rule’ within different avian Orders of the Australian, New Zealand and Antarctic region. I compared and analysed existing morphological measurements for insular and mainland related species occurring in the region, linking them with abiotic features of each island using meta-analysis modelling. Overall, the estimated relative insular body size (i.e., SR= island size/mainland size) values showed little and inconsistent differences between mainland and insular closely-related bird species using wing, bill, tarsus and weight measurments as predictors of body size. The ‘island rule’ patterns were partially recognized for bill and tarsus length at the species level, but this weak trend did not prevail at the family or order level. The mean SRs for the assessed traits suggested a minor trend for birds to become larger on islands in spite of the body size of their mainland counterparts. There were no consistent differences between the degree of sexual size dimorphism in islands and mainland species. Island area, distance from mainland, and sea surface temperature were related to small SRs variation. However, because the mean SRs were so close to one, it was not clear if these abiotic features were important moderators of this ratio. Therefore, my findings do not provide enough support to validate the ‘island rule’ for the assessed birds of the Australian, New Zealand and Antarctic region