Nature 2018 april

Harbingers of Spring?

Though spring is here this does not guarantee that temperatures are going to be rising. Night time temperatures are particularly unreliable. But from March and on into April what triggers some creatures to go into overdrive is the gradual increase in day length.

Despite barely positive mercury readings, moths are one of the first signs that Spring has sprung. Conveniently, one of the first to emerge from their chrysalis is the March Moth. The females are in fact wingless and attract their drab brown suitors to a tree trunk lair: like most moths, using a chemical attractant or pheromone. Another is the Orange Underwing, which flies by night and day. As is oft the case with moths the name is a succinct descriptor. A largish, greyblack moth that is perfectly drawn to hide out amongst dead leaves or bark. Once disturbed from its daytime siesta it will show a flash of sunset yellow which I have found still causes me an automatic step backwards. These early moths provide a vital source of prey for bats emerging from hibernation and the caterpillars are food for those feisty blue tits we see hanging upside down inspecting the catkins.

As soon as there is a run of warmer, damp nights when temperatures reach 7°C this marks the beginning of the mass migration of one of our most important amphibians. The Common Toad emerges from hibernation in late March and travels to their spawning pond. The same ponds are frequented each year regardless how difficult they are to access, due to roads etc. Compared to frogs they are squat and bulbous in shape. Courtship can be quite ferocious, leading to occasional fatalities. Once pairs have become settled eggs are laid encased in a gelatinous string perhaps 15 feet long with up to 6000 eggs. Two weeks later the toad spawn hatches and brownish-black tadpoles emerge. As frogs are able to breed in colder conditions than toads, by the time April arrives frog tadpoles are much further advanced. The risk frogs run is that in a particularly cold early spring, like this year, egg development can slow or stop. Once hatched, tadpoles will not survive a prolonged period of cold.

A recent report of research conducted by the RSPB, the British Trust for Ornithology and several of the UK’s nature conservation bodies describes the impact of climate change, in particular temperature change and level of rainfall, over the last 50 years. The study looked at the impact on our resident bird populations as well as the dates when migratory birds arrive in the UK each spring. Earlier arrival of migrating species or breeding dates for resident flocks impact on the survival rates of young chicks due to the risk of insufficient supplies of food. This has been most dramatic for four of our most frequent garden visitors: the Robin, Dunnock, Nuthatch and Goldfinch. The report also warns that in a world of changing climatic conditions there is an impact on long-range migrators, notably the Swallow. Compared to 50 years ago swallows are now arriving in the UK fifteen days earlier. If weather conditions are favourable this can boost breeding success but breeding may be delayed if impacted on by unpredictable supplies of their primary food: winged insects. Another consequence of milder winters has been a reduction in UK visiting populations of waterbirds: ducks, geese, swans and waders. With a warmer climate in northern European lakes and rivers waterfowl are finding new habitats that can sustain larger gatherings over the winter months. By contrast, Mediterranean and North African species, such as the Little Egret, which was unknown in the UK until 1996, is now a frequent sight on the Tring Reservoirs and even rivers, such as the Chess.

In April, by which time the first Brimstone butterfly will have already visited your garden, look out for a flash of silver and blue dancing across a lawn or down a flower bed. This is most likely a Holly Blue. If it settles you will see the underwings are a powder blue which is peppered with black spots. The butterfly is dimorphic. In other words the colouration of males and females differs with the former’s upperwings uniformly blue whilst the latter have a ‘sooty’ margin. These early arrivals have overwintered as pupae. No surprise to hear the females lay their eggs on holly which abounds in this area. The annual population of this blue has increased by over 150% in recent years, perhaps benefiting from milder winters and reductions in pesticides. Nevertheless, annual populations can be widely fluctuating. It used to be believed this variation was solely dependent on the severity of the winter. More recently, it was noticed the highs and lows in the population followed approximately a six-year cycle. Such patterns suggest an alternate reason, one that has a biological rather than environmental cause. It turns out this is down to a tiny parasitic wasp that lays a single egg in the body of the caterpillar. Such is the relationship between the caterpillar and the wasp grub that the grub does not cause the caterpillar to wither away and die. Instead it allows the caterpillar to pupate. However, instead of a beautiful blue butterfly emerging in spring, out pops the tiny adult wasp.

End Note: With the late flourish of blizzards at the beginning of March this year, marking the start of the meteorological spring, it was delightful to see the snowdrops in their full glory poking through the blanket of snow. I was reminded by some writings on the Snowdrop by Richard Mabey. Mabey, who lived near Tring until he moved to Norfolk, recounted that snowdrops in the British Isles are a mixture of ‘wild’ so called native varieties and naturalised imports from the Continent. Until the Middle Ages there were probably just a few isolated populations in upland areas. The widespread distribution we are familiar with today was a result of the snowdrop’s importance to religious symbolism. In Medieval Europe the flower was venerated during Candlemas due to its flowering coinciding with the Feast Day. As a consequence it is believed it was brought from the mountainous regions of Europe by monks visiting monasteries across this country. Having planted them in monastery gardens, over time they escaped and gradually became naturalised. Much closer to home, Mabey also recounts that between 1860 and 1883 the vicar of St Mary the Virgin Drayton Beauchamp, who was also a botanist and plantsman, developed a green cultivar of snowdrop, which has since been given official status, and a pink variety which has not been given similar status although it is said to have been seen local to the village. Possibly an interesting excuse for a ramble that way one day – so any news on this would be much appreciated.