Bird traits and their nutrient impact on terrestrial invertebrate populations

Birds are a large nutrient vector from marine to terrestrial environments where the increased nutrient input typically results in greater primary production and enhanced microbial activity. Associated invertebrate populations however, show large response variability to bird nutrient subsidies. To explain this variable invertebrate response, we performed a meta-analysis (50 articles ranging from polar to tropical regions) where we compared the effect of bird presence on invertebrate populations between: bird taxa, nesting site selection, bird diet and climate regions. In addition, we quantified how different invertebrate taxa and trophic guilds respond to the presence of birds. Invertebrate abundance was on average > 1000% higher by bird presence, but there was little evidence for any specific bird-taxa effects on invertebrate abundance responses. Birds with a mixed diet increased Coleoptera populations the most. Invertebrate responses to bird presence were largest in polar regions but variation remained high. Not all species within communities responded to bird presence, indicating that nutrient limitation is species-specific or ecosystems are affected in different ways by birds. Furthermore, sampling strategies were inconsistent and may impact effect-sizes. Despite the contrasting nature of the different studies, an overall positive invertebrate abundance response was found in the presence of birds, with larger responses observed in polar regions. Standardized sampling approaches would resolve much of the remaining variability. As natural experiments, bird nests and affected areas are a prime spot to study community assembly rules and address issues of anthropogenic disturbance and climate change.