While it might feel as if spring has only just arrived in East Anglia, for some of our birds the season is already well-advanced.
The county’s nesting grey herons already have chicks in the nest and the adult birds are busy provisioning their growing young with fish, amphibians, larger insects, small mammals and occasional waterbirds. Many of the county’s herons breed at traditional sites, known as heronries, where double-figure counts of active nests can be fairly typical. The birds often select tall trees, placing their bulky and somewhat untidy nests high above the ground. At a few sites the herons make their nests within reedbeds, the thick reed growth providing cover from prying eyes.
Heron numbers are monitored during the breeding season, with volunteers from the British Trust for Ornithology’s (BTO’s) Heronries Census making counts of active nests. The figures from this survey, which has been running since 1928 and which is now one of the longest-running bird surveys in the world, have charted the heron’s changing fortunes. Heron numbers have generally enjoyed a long-term increase but the BTO’s data reveal the detrimental impact of severe winter weather, after which breeding numbers can decline dramatically. Cold weather and frozen lakes and rivers limit access to food and many birds starve if the conditions do not improve. Over recent years we have seen a more sustained decline in heron numbers. While it is too early to say whether this gives cause for concern, it might be that recent poor winter weather and early spring gales are causing problems for the species.
Sitting at the top of its food chain, the grey heron provides a good measure of the state of the wider freshwater environment – some herons do feed in the saline conditions of our coastal waters – and if heron populations are doing well then it is an encouraging sign. Pollutants, like heavy metals and pesticide residues, which can get into our waterbodies, may impact on heron survival and breeding success, so it is important that we continue to monitor both their numbers and their breeding attempts.
The grey heron is a familiar species to most of those who spend some time near lakes and rivers, but it is interesting to note that a changing climate has seen it joined by other heron species, the most noticeable of which has been the little egret. This small white heron, with black legs and yellow feet, has now started breeding alongside grey herons at some of the county’s heronries. Other heron colonists have also arrived, though breeding in reedbeds rather than on tree-tops. Both purple heron and great white egret – two of the more recent colonists – are now reported fairly regularly from Suffolk and there is the potential for them to become established as more widespread breeding species.