Nature features

The date of the first booming bitterns varies each year, although there has been a trend towards them starting to boom earlier in recent years. The earliest record, on 31 January 1997, was considered exceptionally early, being several weeks ahead of the norm.

RSPB Minsmere nature reserve

The reason why bitterns are booming earlier is difficult to pin-point, but one likely reason is that the increasing population means that there is now more competition for a mate, so it pays to start earlier. It is thought that early breeding attempts tend to be more successful, and increase opportunities for females to rear a second brood or to re-lay if the nest fails.

It is also likely that the improved quality of the habitat at Minsmere makes it easier for the bitterns to find fish, so they are now stronger and healthier in the spring. At Minsmere, water levels are raised as early as possible after the winter cutting programme to give bitterns more chances of finding fish.

Late winter is an important time for bitterns, which need to feed well to get into breeding condition. The males need to build up their throat muscles to deliver their famous boom, which can be heard several kilometres away. They will almost double their body weight in preparation for the breeding season.

The bittern spotted by Paul Sergent

Minsmere is home to up to 30% of the UK breeding population so it is the best place to see and hear the birds. Last year, there were nine booming males and eight nesting females at Minsmere. They can usually be heard at any time of day from late February until mid May, though they are most vocal in the mornings and evenings.