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.
So what happens if an animal is taken out of its local area and plonked in a strange new location, such as when a mouse is live-trapped and released well away from home? It seems that mice use various senses to navigate. Blind mice are worse at homing than sighted mice so vision must play a role. Mice are much better at homing if they are less than 135 metres from their original position. House mice don’t tend to range very far so they may not need to be good at homing over large distances, but other species of mice that habitually travel across big areas of landscape are very adept at homing.
Experiments with deer mice (Peromyscus maniculatus) in North America showed that they could find their way home even if they were moved by up to two kilometres. How they navigate is uncertain. Some studies have suggested that certain species of mice might be able to detect the Earth’s magnetic field with an internal compass, similar to migrating birds, while other researchers have found that voles may use the sun to navigate. Other biologists believe that mice may just wander extensively if lost and eventually find their way home by luck rather than judgement. Despite the conflicting evidence, homeowners are still advised to release any mice that they trap well away from their house (or any other houses for that matter!); some pest controllers recommend relocating the mice up to a distance of 3.2 kilometres. The best bet is to try to prevent house mice entering your home by sealing up any access points, especially where food is kept. Bird food kept in a shed or garage can attract rodents so it is wise to keep any nuts, seeds, etc. in sealed tubs or bins that are tough enough to withstand sharp little teeth.
Hello Mum
I believe baby birds imprint on the first thing they see as they emerge from the egg. So how does the cuckoo find a mate when they are raised by surrogate parents that are usually smaller, looking nothing like a likely mate? Nick
There are several types of imprinting in chicks. The behaviour that Nick refers to is called filial imprinting, in which a young bird becomes very attached to a particular object soon after hatching, usually its parent. This is vital for birds such as geese, ducks and other waterfowl and waders that leave the nest shortly after hatching. If a chick is to survive it must follow and stay with its parent. Hatchlings from artificially incubated eggs will latch on to whatever they see during the critical imprinting period, whether it is another animal, human, or even an inanimate object, because the instinct is so strong.
There is evidence that some birds also experience sexual imprinting, learning from their parent’s appearance how future mates should look and sound. In essence, a chick learns to recognise members of its own species so that later in life it can court