Springwatch Unsprung: Why Do Robins Have Red Breasts?. Jo Stevens

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СКАЧАТЬ seen flying to the top of our wall with seeds for his fledglings for a few days, then the next day, when they were able to fly down to the ground, he perched at the feeder and dropped seed to them. He then seemed to get fed up with this process because it was too slow, so took to flying at the feeder and crashing into it with both feet up to shake a shower of seed down. The next year, we saw the same behaviour but then not for the next couple of years. So when it reappeared this year we wondered if it could be the same bird, or one of his descendants. Any ideas? Linda

      Linda doesn’t mention whether the sparrow was a tree or house sparrow but either way it’s likely that the clever parenting she witnessed was performed by two different birds. The oldest recorded house sparrow lived for twelve years but their average lifespan is about three years. A tree sparrow is known to have lived for over ten years but typically they only survive for a couple of years. So, while it’s feasible that the original sparrow returned, it is unlikely. Instead, the new feeder-bashing sparrow could be evidence of an interesting phenomenon called cultural learning.

      Cultural learning occurs when a new behaviour, like a novel feeding strategy, is transmitted from one individual to another within a population or from one generation to the next. This is particularly important in social birds such as house sparrows and starlings. Recent research suggests that birds living in large groups, such as sparrows, are better at solving problems because the individuals all have different experiences and skills to draw upon. We’ll never know whether Sue’s feeder-bashing sparrows learned the trick from the original sparrow or worked it out independently. However, if the behaviour had been passed on it doesn’t imply that the original feeder-bashing sparrow intentionally taught the new skill to others; they probably just imitated him.

      The most well-known instance of cultural learning is the intriguing case of the milk thieves. In the good old days, when milk was delivered to the doorstep by a friendly milkman whistling a merry tune, an inquisitive blue tit or great tit discovered that those glass bottles contained fat-rich cream. Originally, milk was delivered in open bottles that birds could easily drink from, but later, when milk bottles were covered with silver foil, blue tits and great tits soon learned to peck through the foil to steal the creamy goodness underneath. The pecking of milk bottle tops rapidly became widespread. It took 25 years for the habit to grow from one observation to being seen in over 400 different locations. Whether each bird worked out how to do this independently or just watched and copied others is open to debate. The behaviour did not spread out in a wave from the original location but popped up sporadically, suggesting that it arose on several occasions. Blue tits and great tits often peel off bark looking for insects and grubs. They are naturally curious and have relatively large brains so it would not have been a huge step to try peeling off milk bottle tops. Of course, there’s no reason why birds would not have copied the behaviour from each other within each location, so it’s likely that a combination of inquisitive individuals and social learning was responsible for the stolen milk. Now that fewer people have milk delivered, this behaviour is rarely seen.

      Run, Rabbit, Run

      Why do rabbits have white, furry bottoms? Does this not make them easier for predators to spot? Lupodod

      Rabbits certainly have their fair share of predators. Fully-grown rabbits are taken by foxes, cats, stoats and polecats while young rabbits are preyed on by badgers, buzzards and weasels. Over 90 per cent of baby rabbits die in the first year of life, most of these in the first three months. Some may die of disease, but a huge number are eaten. This high mortality rate is the main reason why rabbits are so fecund and, well, breed like rabbits! The female can have two or three litters per year, giving birth to about ten young each time. Producing lots of young increases the chance that some will make it through to adulthood and successfully reproduce.

      As they are on the menu of so many predators, rabbits are very vigilant, constantly on the lookout for danger. They have eyes set wide on the sides of their skull which give them a broad field of vision, and sit on their hind legs to see further. With so many enemies around, it might seem counter-intuitive for a prey animal like a rabbit to make itself more visible by having a white rump. However, the white underside of a rabbit’s stubby tail (called a scut) is not that easy to see when the rabbit is grazing or moving slowly. It’s most noticeable when the rabbit is running, probably heading away from danger. This has led to several theories why rabbits have white tails.

      Firstly, it acts as a warning to other rabbits in the warren. If a rabbit senses danger it raises its tail and thumps its foot to alert other members of the warren. Rabbits are mostly active in the low light conditions of dawn and dusk so a white flash would stand out, alerting other rabbits to run. Another theory suggests that by having a white tail, predators are attracted to the ‘right’ end of the rabbit and aim for the rear instead of the head, giving the rabbit a greater chance of escaping.

      Rabbits also have a strict hierarchy and there can be a lot of aggression within the colony. Males, called bucks, will fight for access to females, known as does. The most dominant buck gains mating rights while the does compete for nest sites. Fights between the females can lead to serious injuries and even death. So one theory suggests that the tails are used like a white flag in surrender, to show submission to other rabbits and avoid potentially deadly fights. Dominant rabbits tend to be caught by predators more often than those lower in the hierarchy, perhaps because they are distracted by fights or because chasing off other rabbits makes their white tails more visible.

      The rabbit’s tail probably has several functions, but the costs of having a white, fluffy bottom must be outweighed by the benefits or natural selection would have led to something a bit less conspicuous.

      Pitching Up

      I was driving to Brecon in south Wales last week and saw several colonies of tent-making caterpillars. The colonies were all very large, a metre or so long. Some of the colonies had completely stripped the hedgerow for long stretches and all that was left was twigs and silk. Are these native to the UK (I have seen very much smaller colonies on hawthorn) or are they an invasive species? Rob

      There are quite a few native species of moths and butterflies whose caterpillars make tents, such as the lackey moth (Malacosoma neustria) and marsh fritillary butterfly (Euphydryas aurinia). However, the impressive size and extent of the tents that Rob describes suggests that they were made by one of our eight species of ermine moth that belong to the genus Yponomeuta.

      The adult moths are fairly inconspicuous, with white or grey wings adorned with rows of black spots and a wingspan of 15–26 millimetres, depending on the species. The caterpillars and their webs are much more noticeable, though, especially when there’s a population explosion and their numbers grow to epic proportions. This can occur when there is a lack of predators or parasites (ermine caterpillars are often parasitised by ichneumon wasps) or when weather conditions work in their favour. The caterpillars’ fleecy white tents can cover large areas of trees and hedgerows, looking like giant spider’s webs or something out of a horror film. The truth is much less sinister.

      Each ermine species has a favoured host food plant on which eggs are laid and the caterpillars subsequently munch their way through. For example, the colonies that Rob saw on hawthorn were probably caterpillars of the small or orchard ermine moth (Yponomeuta padella). The adults are on the wing in June/July and the females lay clusters of eggs on their specific food plant. These hatch into tiny larvae that spend the winter burrowed into leaves or stems. They emerge in spring when the buds burst and turn into busy eating machines. A caterpillar grows quickly until it literally needs to burst out of its skin in a process called ecdysis. Once it reaches a certain size, a caterpillar sheds its skin so that it can grow into the СКАЧАТЬ