Table 2.

Summary of bird species functional traits used.

Trait	Source	Scale	Description
Body size	Literature45	Continuous	Average individual bird body size (including males and females)
HWI	Literature31	Continuous	Hand-wing index, an estimate of wing shape used as a proxy for flight ability in birds
Habitat specificity	Literature45	Continuous	The sum of the species’ habitat type preference
Clutch size	Literature45	Continuous	The number of eggs produced during a single breeding event
Trophic level	Literature45	Categorical	Including three trophic categories: carnivores, herbivores, and omnivores
Territoriality	Literature31,46	Categorical	Exclusive, defended access to habitat and food resources throughout the year, including three levels: strong, weak, and none
Flocking tendency	Our observational data and Literature45	Categorical	The measurements of bird flocking behavior: strictly social, occasionally social or strictly solitary