We are now well into the nesting season, with most birds having at least made a start; however, some of the earliest breeders have already raised young and watched them fly. Nest building will have commenced back in January for the trailblazers and amongst them are the Grey Herons. With so much water around, the Cotswold Water Park hosts good numbers of these fish eaters. They nest in trees, usually in colonies – and these can be quite sizeable.
The largest CWP colony usually consists of around 40 nests. This varies with the harshness of the preceeding winter; sub-zero temperatures for weeks on end will freeze the gravel pits, which makes catching fish extremely tricky – so there will be fatalities. A really bad year might see the population fall by 25%. Milder winters will see numbers push up again and more nests will be filled with noisy youngsters, waiting for mum and dad to deliver lunch. Whilst monitoring the nests (to make official counts for the British Trust for Ornithology), I get to see what food has been dropped, under the nests; Roach are the commonest fish and large numbers of Signal Crayfish are also eaten. Scarcer prey items I have seen include a couple of Goldfish!
The greatest danger to the young herons is probably unseasonable gales. They have to be fed for around 7 weeks before they can fly and as they grow the nest eventually becomes too small, so they wander onto adjoining branches, where they can be vulnerable to a sudden gust to an already swaying tree. Once out of the nest they will be found around the shallow margins of neighbouring pits, with adults who will be teaching them how to catch food. After this apprenticeship they will be on their own as Grey Herons are usually solitary hunters; coming together again next January to start the nesting process all over again.