There are 'knowledge gaps' about the Grey Heron population on some of the Scottish Islands – and the British Trust for Ornithology is asking local people to help fill them.

The Grey Heron is amongst Britain’s largest birds and can be seen in a variety of different habitats, including rivers and parks in some of our busiest towns and cities. They usually nest in colonies in trees, building large nests very close to the tree tops where they are not difficult to see – but getting more information on them can sometimes be more tricky.

On the Scottish islands where woodland cover is sometimes sparse, Grey Herons can also be found nesting on the ground. For this reason it can be more difficult to find and monitor heronries, and it is this information that BTO is interested in – where in the Inner and Outer Hebrides do Grey Herons nest?

Read more: Rewilding with top predators

The BTO Heronries Census is one of the longest running bird surveys in the world. Its organiser, Ian Woodward, said: "We would love to fill in the gaps in our knowledge of heronries in these islands but to do so we need the help of those people that live there and those that visit during the early spring. In southern Britain, Grey Herons begin breeding as early as February or early March but further north nesting tends to start slightly later and Grey Herons on the Hebrides may still be breeding well into May.”

BTO's Census requires a simple count of the number of ‘apparently occupied nests’ at a heronry, and the charity would particularly welcome all records of nesting Grey Herons from the Hebrides and other more isolated areas in Scotland. Additionally, it would welcome contributions from residents and visitors willing to check previously known colonies to confirm whether or not Grey Herons are still nesting there.

For more information and how to take part, visit the Heronries Census on www.bto.org