The impact of a catastrophic flood on species and size composition of the diet of fish-eating birds

Floods in June 2013 affected significant portions of the Czech Republic (total damages amounted ~600 millions of euro). This study examines the impact of catastrophic flood on the species composition and size of fish prey in the diet of the common kingfisher (Alcedo atthis), a fish-eating bird, nesting and hunting on Botič stream (Prague, Czech Republic) in 2013. Hundred and forty years water (flow 74.5 m³ s⁻¹) caused considerable damage to property and it is likely that the character and size composition of biota, especially fish, changed. This should be reflected naturally in the diet of resident kingfishers. The diet of kingfishers before and after the flood were investigated from the mass of regurgitated pellets, which were collected from the nest tunnel and chamber immediately after the successful breeding period before and after the flood event. Before the flood (normal situation; flow 0.4–1.5 m³ s⁻¹), the average length of fish caught was 6.5 cm LT (total length), average weight 2.6 g, and the index of food diversity was 1.58. After the flood, the average length of fish caught was 7.5 cm LT, weight 4.1 g, and the index of the food diversity was 1.36. It was evident that after the flood kingfishers were forced to hunt significantly larger prey. Six fish species (Gobio gobio, Squalius cephalus, Perca fluviatilis, Scardinius erythrophthalmus, Rutilus rutilus, Pseudorasbora parva) which were hunted both before and after the flood composed 96.5 and 99.8% of the catch (by numbers). Surprisingly, the impact of floods may not always be reflected in the species composition of the diet of fish-eating birds, it mostly depends on the presence of fish broadly occurring in the stream, natural stability of the fish stock and on the composition of the fish assemblage in the upstream catchment area.