Does predation pressure drive heronry birds to nest in the urban landscape?

Predation is an important selective force that determines breeding success in all animals. Animals adopt a range of antipredatory strategies to overcome predation pressure in the breeding site, which is also a major predation ground. Birds mitigate this pressure through parental care, colonial or group nesting, and selecting habitats that are predicted to hold less number and diversity of predators. Heronry birds' increasing use of urban habitats for breeding has been suggested as a measure to avoid predation pressure, despite quantitative support for this argument is lacking. We tested this hypothesis by comparing diversity, abundance, and frequency of predation attempts of birds of prey on heronries of wild (mangrove forest) and urban habitats. The house crow (Corvus splendens), brahminy kite (Haliastur indus), and black kite (Milvus migrans) were the major birds of preys in mangrove forests and urban areas. The predators partitioned the nest resources; the kites preyed the chicks, and the crows preyed the eggs. The abundance of avian predators and the frequency of successful predation of eggs and chicks were significantly lower in urban heronries compared with mangroves. Our results suggest that predation might be a reason for the birds to switch to urban habitats for breeding.