Season and sugar concentration affect bird behaviour at urban sugar-water feeders

Sugar-water bird feeding in residential backyards is increasingly popular, but its effects on wildlife are poorly understood. One concern is whether it results in maladaptive behaviour, such as reliance on artificial food or increased aggression due to increased density of visiting individuals. We studied sugar-water feeder-associated bird behaviour in two cities with different climates. We investigate whether season, city, or sugar concentration influenced bird foraging activity and aggressiveness. We then test whether feeder presence affected backyard bird composition. Birds were most aggressive and used sugar-water feeders most actively in winter, especially the omnivorous native tauhou (Zosterops lateralis). We also found city and seasonal differences in sugar-water feeder usage and aggression. Further, in Auckland, the city with the warmer climate, New Zealand's largest nectarivorous species, tui (Prosthemadera novaeseelandiae), was more likely to be aggressive at feeders with higher sugar concentrations but foraged longer at feeders with lower sugar concentrations. Neither feeder presence nor sugar concentration influenced garden bird species richness or abundance. We discuss the effects of sugar-water feeding on bird behaviour at the global and local scale and suggest future study directions.