Phillips K.P., Guilfoyle C., Kaufmann J., de Eyto E., Poole R., Coughlan J., Cabot D., Cross T.F., Prodöhl P.A., Reed T.E. & McGinnity P. (2025) Foraging behaviour of nesting Grey Herons inferred from PIT-tagged salmonid fish. ARDEA 113 (1): 23-23

Studies of fish behaviour and ecology create opportunities for studying piscivores that have been underutilised. A leading example is the widespread use by fish biologists of Passive Integrated Transponder (PIT) tagging for identifying individuals, tracking movements, and estimating population sizes. PIT tags have high physical durability and long electronic lifespans. In consequence, if a piscivore swallows a tagged fish, the tag will almost certainly be excreted intact and subsequently detectable at the location of excretion for years. Accessible nesting or roosting colonies of piscivorous birds adjacent to fish PIT-tagging study sites thus present a potentially rich source of information on feeding habits. However, when such PIT tag surveys are conducted, they typically focus on the biology of the prey and not the predator. Here, we make opportunistic use of a PIT-tagging study of young Salmo spp. salmonids in Ireland to investigate the biology of a widespread and familiar avian piscivore, the Grey Heron Ardea cinerea. We scanned the ground underneath an accessible heronry for tags known to have been fitted to their carriers over four days in May 2019 in an intensively monitored river from which emigration is only possible via a fish trap. We found 48 focal tags in the heronry out of 594 tagged fish. Tags in the heronry were significantly biased towards smaller individuals and S. salar. Data on unpredated fish implies that all putative predation events occurred at least 10.8 km from the heronry despite there being foraging habitat less than 150 m from the colony, and that the majority occurred before the end of June in the year of tagging. We discuss our results in the context of the relatively limited information on breeding-season foraging activity for Grey Herons, optimal foraging considerations and ways to refine the use of PIT tagging of fish for studying piscivores.