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

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СКАЧАТЬ one egg per day while larger species such as swans or geese may have a one- or two-day interval between eggs. Eggs are heavy, and if the female retained them inside her body they would weigh her down, so as soon as an egg has formed it is laid. In the UK, the grey partridge has the largest clutch of any bird, usually 14–15 eggs in total, but they can lay up to 20 eggs. Clutches tend to be larger in species with chicks that can feed themselves soon after hatching, such as many waterfowl, while species whose chicks need more care and attention have smaller clutches. Many seabirds are long-lived and have just one egg per season, investing a lot of time and energy into feeding and raising a single chick.

      Some birds, like great tits, have a single large clutch per year to coincide with a peak in food (caterpillars in the great tit’s case), literally putting all their eggs in one basket. If the parents get the timing wrong or if their food supply fails they will have little breeding success. Other species, such as robins, spread their risk and can squeeze in two, three or even more clutches, especially if one brood fails, but have fewer eggs in each clutch.

      The embryos inside fertilised eggs don’t begin to develop until they are incubated by the parent. Some species, such as barn owls, start incubation as soon as the first egg is laid so the eggs hatch asynchronously over several days. This means that the first chicks are much larger than later ones and may even eat their younger siblings if food is scarce, as we saw on Springwatch with a barn owl chick named Hannibal. Most songbirds delay incubation until the clutch is complete, in which case all the eggs hatch at roughly the same time and the chicks are all a similar size. There may still be a runt of the clutch, because the final egg sometimes has fewer resources put into it, but generally each of these chicks has a more even chance of survival.

      Make Mine a Double

      I ate a double-yolk egg for my lunch. If it had been fertilised, would two chicks have grown/hatched from one egg? Charlotte

      Double-yolk eggs are fairly rare – it’s thought that about one in a thousand commercial hens’ eggs have two yolks, and multiple yolks (triple, quadruple or more) are even rarer.

      They are a result of a malfunction in the chicken’s egg-laying process. Normally, an ovum is formed in the hen’s ovary and develops into the yolk. When it reaches the right size it is released into the oviduct. On its journey through the oviduct, the egg white (albumen) and the shell form around the yolk to create a whole egg, which is then laid. The whole process takes about 24 hours and the laying of one egg usually triggers the ovulation of the next.

      Egg formation and laying are regulated by the hen’s hormones and occasionally things go wrong, especially in young hens. Two or more yolks may be released into the oviduct at the same time and become encased in one shell, resulting in a double-yolker.

      Commercial eggs are unfertilised and do not develop into chicks. However, if domestic hens are kept with a cockerel and the eggs are fertilised then it’s possible that a double-yolker would contain two chick embryos.

      There are a few very rare cases where ‘twin’ chicks have hatched from a single fertilised egg, usually with human assistance. Generally, though, the embryos from a fertilised double-yolked egg would not survive as there simply isn’t enough room in the egg for them to develop. Unlike mammals that have multiple embryos and a nice stretchy uterus, the eggshell cannot expand to accommodate more than one chick. Also, a chick has to be able to rotate within the egg so it can reach the air pocket in the round end, before pecking its way out. This would not be possible if two chicks were squeezed together into one shell. So fertilised double-yolkers are generally doomed but unfertilised ones make a great lunchtime bonus.

      Hard-Headed

      My eight-year-old daughter, Lauren, wants to know why a woodpecker doesn’t break its beak when it hammers on a tree. Jenny

      There are three species of woodpecker in the UK: the largest is the exotic-looking green woodpecker with its red crown and emerald plumage, often seen on the ground feeding on ants’ nests. Green woodpeckers rarely drum, but the black-and-white woodpecker species, the great spotted and lesser spotted, both hammer out their clarion call. During spring, these woodpeckers rapidly drum their beaks against trees, telegraph poles or even metal structures to attract a mate and claim their territory. They choose whichever substance will resonate and amplify their drumming the most.

      Woodpeckers can strike their beaks up to 40 times per second, at a speed of 6 metres per second, and each time their beak hits the trunk it experiences a force of up to 1,000 times gravity. That’s equivalent to a human hitting a wall face first at up to 22km/h, which would cause a nasty concussion, if not worse. So how does the woodpecker not get a headache or break its beak?

      Fortunately, woodpeckers’ beaks and skulls are well adapted to absorb the shockwaves and deal with those forces. Firstly, the brain fits very snugly into the skull so there is little room for it to bump around. Conversely, human brains are suspended in fluid and if we hit our heads the brain sloshes around, bouncing off the inside of the skull, which gives us concussion.

      Secondly, the upper part of the woodpecker’s beak is longer than the bottom half, which helps to disperse the forces. The beak can also be quite flexible so it changes shape as it hits the trunk and absorbs some of the impact. The bones of the skull are very spongy with a fine, mesh-like lining that acts as a shock absorber. Woodpeckers also have a special hyoid bone that loops around under the skull acting like a safety belt for the brain. These birds also have strong, thick neck muscles that support the head and power the drumming. All these adaptations are certainly put to the test during the breeding season, when a male looking for a mate may drum up to 600 times a day.

      Follow Your Nose

      Is it true that if released within a mile of your house, a mouse can navigate back? If so, how? Is it celestial navigation? Kirsty from Lisburn, Northern Ireland

      Many animals have a strong homing instinct, used either during migration or simply to return home at night after a hard day’s foraging. It makes sense that an animal must have a mental map to find its way around its home range or territory. House mice have variable home-range sizes depending on where they live and how much food is available. In a chicken barn with plenty of grain they may live in a tiny area of just four square metres; however, where house mice live out in the wild away from humans their home range may cover as much as one or two square kilometres. Most mice in homes will stay within an area less than 10 metres in diameter.

      House mice are nocturnal and don’t have colour vision. Their other senses more than compensate for their bad sight. Mice have good hearing and can detect ultrasonic sounds. They use their sensitive whiskers to find their way around in the dark. House mice exhibit ‘thigmotaxis’ which is a tendency to follow solid surfaces; for example, they prefer to run around the edge of a room, staying close to the wall, rather than run across open spaces. They use regular paths and runways to move around their patch, whether that’s in a house or field. Mice also leave urine scent trails and spray landmarks around their territory. They are curious and constantly explore their home range using taste, smell and touch to memorise obstacles and where to find food and water.

      As long as food is available, house mice have little cause to leave home, except when they are young and disperse to find their own territory. Some house mice have been recorded travelling up to 2.4 kilometres, but they usually don’t roam that far.

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