This is Michael Walter's latest report:
Statistics are bone-achingly dull, but the interpretation of rows of figures can lead to all manner of fascinating insights or leave us scratching our heads as to what is going on. A case in point concerns the butterfly population at Blean Woods, which is monitored in two areas of the reserve by different observers. This involves weekly walks along a set route every spring and summer, counting the numbers of each butterfly species. Summaries for both routes have now been produced, and while looking through them yesterday I was struck by a number of differences between the two sets of data, one of the more interesting of which concerned the brimstone. Brimstone is the old name for the bright yellow element sulphur (hence the biblical phrase “fire and brimstone”),and is very aptly applied to this large insect, whose males are bright lemon-yellow, while the females are much paler, almost verging on white. It was a regular visitor to both transects up until 1998, when something happened that caused the population to plummet almost to extinction on the reserve. But, whereas the population in the central area began a recovery in 2013, it has remained stubbornly rare in the north-east area. Yet, remarkably, these two transects are only a quarter of a mile apart. Evidently, one or more features, which are not necessarily visible to us, set these two areas of the wood apart. Perhaps one day we’ll discover what precipitated the collapse, and why the butterfly should have recovered at differing rates in two nearby areas. Like the more familiar peacock, it appears in the spring and late summer, not because it has two generations but because it overwinters as an adult. So the brimstones we see in August are freshly emerged from their pupae or chrysalises, while those that brighten a sunny spring day have just woken from hibernation. The unusual, wavy outline of its closed wings mean that, when hanging upside down from vegetation in winter, it can escape detection by predators thanks to its uncanny resemblance to a yellowing ivy leaf.
Jackdaws breed in tree-holes in the wood, but feed in the surrounding farmland, and so tend to vacate the reserve once their young have fledged, but a group congregated in the reserve recently, excitedly calling out the first syllable of their name. That call has a remarkable textural quality, such that the sound often fools me into looking up for a flock, only to realise that a solitary individual is responsible for all the chatter as it flies over. The birds were invisible amongst all the foliage, but I felt certain it really was a group rather than a single clever ventriloquist. However, I was left without any idea as to what had prompted this babblingcouncil of birds in the treetops. We talk freely about some species, such as jackdaws, being sociable, but without really understanding what we mean by that. Why do they prefer to assemble in flocks, and what information is being exchanged in the calls that, to the ill-tuned human ear, sound more like a cacophony than any kind of language.
Brimstone butterfly - photo from Butterfly Conservation
Jackdaw photo by Dave Smith