Use of Citizen Science to Determine Prey Partitioning Between Two Coastal Raptors

In order to effectively conserve a species, it is often important to understand the ecological niche it occupies, and the resources required to sustain it. For predators, prey is typically the most important resource, and predators either outcompete for or partition resources to secure long-term food security. Raptors have been shown to utilise both strategies. However, demonstrating competition and prey partitioning between raptors is difficult. Traditional diet collection methods can be prohibitively time-consuming over large spatial areas and biased towards breeding individuals and nest sites. Use of citizen science, in particular images and videos recorded from social media, potentially overcomes these issues. This theory was tested by investigating the dietary breadth, diversity, overlap and competition between two Australian raptors, the Eastern Osprey (Pandion haliaetus cristatus) and White-bellied Sea-Eagle (Haliaeetus leucogaster) (WBSE), in New South Wales and Queensland (Australia). These raptors were suspected of occupying the same realised niche and partitioning the fish in their diets to reduce direct competition and conflict. The fish component of both raptor diets was shown to be moderately broad and diverse, with greater evenness in the diet of WBSE, and an Eastern Osprey preference for mullet (Mugilidae). There was a moderate dietary overlap, with Morisita's Index values of between 0.59 and 0.61. Competition between the raptors was assessed to be low due to the high abundance of shared prey, and limited overlap in other prey. Prey partitioning appeared to be determined by resource availability. Highly abundant prey is shared, whilst less abundant prey is partitioned according to hunting style, with WBSE swooping to collect fish from the water surface, and Eastern Osprey diving to access deeper dwelling fish. This study represents the first attempt to study a complex dietary relationship in raptor prey partitioning using diets sourced from social media. The data provided adequate detail to demonstrate dietary partitioning and identify potential causes.