These rooks are shown nesting in elm trees. I described the cataclysmic effect of Dutch Elm Disease on Britain’s elm trees (since these books were published) in the previous post in this series.
The wonderful wading bird, the heron (or grey heron) only features in this one picture in the “What to Look for in Spring” book (although it does also feature later in the year). I love the heron, one of Britain’s largest birds, with its dagger beak, what I think of as a graceful, slow flapping flight, sentinel-like approach to fishing, and its loud, harsh cries. In his song, Now westlin winds, Robert Burns described the heron as; "The soaring hern", and my old Observer's Book of Birds describes the heron's flight thus: "When it takes to its great grey wings with dark tips, the unusually slow and languid wing-beats are distinctive."
We used to see herons over our house nearly every day when I was a child, flying between the Peffer Burn Estuary on Aberlady Bay Local Nature Reserve and a major heron colony (or “heronry”) located high up in the branches of a conifer plantation a mile or two inland. I recall the courtship of herons in that heronry, and the demands of their chicks for food, being just as noisy and raucous as that of rooks! The heron in Britain is the subject of the longest-running breeding-season bird monitoring scheme in the world, through the BTO Heronries Census which began in 1928. The best idea here is simply to show the results of this survey, as presented by the British Trust for ornithology on its website:
(Figure from British Trust for Ornithology)
Herons are very susceptible to high mortality during harsh winters and this is clear from the graph above. I think the extremely extended and harsh winter we have just emerged from may also create another low point on this graph. Many bodies of freshwater in the UK have been frozen over for up to two months this winter and, while estuarine and coastal waters have been ice-free, inland heron populations are bound to have suffered. You will see, however, that there has been a general upward trend in heron numbers over the course of the 20th Century, levelling off a little after the turn of the 21st Century. This upward trend is described as "moderate" for the whole Uk and "shallow" for herons in Scotland. It is thought (as reported by the BTO) that the general upward trend may reflect: "reduced persecution, improvements in water quality, the provision of new habitat as new lakes and gravel pits mature, and increased feeding opportunities at freshwater fisheries". Numbers of herons has apparently also increased across Europe since 1980.
It is also stated that: "High rates of nest failure at the chick stage were noted in the late 1960s, but not subsequently", a fact I believe is almost certainly linked to the decline in the concentration of organochlorine pesticide residues measured in herons since that time. This has been studied since then through an annual monitoring contract issued by the Joint Nature Conservation Committee (JNCC, and its predecessor bodies) to the Centre for Ecology and Hydrology (CEH, and ITS predecessors!). The programme was started in the early 1960s, when there were serious concerns over the effects of organochlorine pesticides and organomercury fungicides on a range of bird and mammal species in the UK. High concentrations of these toxic compounds are now well-known to lead to a range of causes of reproductive failure in birds. This early work demonstrated the effects of the organochlorines and eventually contributed to the ban on their use in the UK and abroad. The assessment of a range of organochlorines and also the toxic metal mercury in the livers of herons has revealed significant long-term declines in liver residues of organochlorine pesticides and mercury during the monitoring period. These declines appear now to have largely levelled off. I have copied the relevant trends in these from the JNCC's website below. You can read the original report here.
(From JNCC Report No. 391)
A final issue for the heron in Britain that is worth identifying here as it is directly relevant to Spring-time is that, since 1968, presumably in response to the general warming of the climate and the advance of Spring, the average egg-laying date for herons in Britain is now some 29 days earlier than it was back then, as shown by the BTO here.