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

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СКАЧАТЬ 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 and mate with its own kind. This process would be disastrous for a cuckoo, though, because it is brought up in the nest of another species such as a reed warbler, dunnock or meadow pipit.

      Cuckoos are brood parasites and rely on parents of the host species to bring up their chicks. Female cuckoos wait for a suitable nest to be left unattended before flying down, removing one of the host’s eggs and replacing it with one of her own. She can lay the egg and make a quick getaway in about 10 seconds. When the unsuspecting parents return they incubate the cuckoo egg and raise the chick as their own. The cuckoo chick usually hatches first and ejects all the other eggs or chicks from the nest, ensuring it has no rivals for its surrogate parents’ attention. Meanwhile the cuckoo parents return to Africa and never see their chicks. Instead, the host parents work tirelessly to feed the enormous chick, which can grow to over three times their own size. When the cuckoo finally fledges, it must find its own way to Africa. In the cuckoo’s case, the ability to migrate must be genetically hard-wired in the brain and is innate rather than learned.

      But how does a cuckoo avoid sexual imprinting on the wrong species and know that it is a cuckoo rather than a reed warbler or a dunnock? Biologists aren’t entirely sure how cuckoos learn to recognise a mate and there may be several processes at work. Perhaps some innate elements are later reinforced by mixing with other cuckoos. Brown-headed cowbirds, which use the same breeding strategy as cuckoos, have been shown to use a species-specific ‘password’ to trigger their learning. They make a particular chatter call when they join up after fledging and then learn more about their species. They also look to themselves and reference their own traits, such as ‘that bird looks like me so probably belongs to the same species’. Cuckoos are likely to have a similar system for working out their own identity and recognising which birds are potential mates.

      Awkward Embrace

      We noticed one of our smallish koi carp desperately flipping and trying to free itself from what looked like a black ‘growth’ around its head and gills. We netted the fish and discovered a large toad had it tightly in its grasp. My husband forced the toad off the exhausted fish, which has now recovered in a bucket. The toad jumped back into the pond where there are more fish! We’ve never seen this before. Was the toad trying to ‘mate’ with the fish? Jill, Sussex

      In essence, yes, the koi carp was on the receiving end of an overexcited toad’s amorous attentions! As soon as night-time temperatures reach 5 degrees Celsius or more, usually in February, adult common toads emerge from hibernation. They have just one thing on their mind: mating. They migrate back to their ancestral pond, often travelling several kilometres and crossing dangerous, busy roads. The males often arrive before the females and will lie in wait, either at the pond itself or nearby, ready to pounce on a potential mate. The smaller male will often piggyback on the larger female as she makes her way to the water.

      The male grasps tightly onto the female under the armpits in a special hold called amplexus, Latin for ‘embrace’. During the breeding season the male develops ‘nuptial pads’ on his first three fingers that help him grip onto the female. Males are so intent on breeding that they will often grab toads of the wrong sex or even completely different species like the unfortunate koi carp. Males usually outnumber females at the breeding ponds and competition for mates is high, so it makes sense to grab anything that moves and ask questions later! Actually, males aren’t even that choosy, and have been known to latch on to inanimate objects while in their aroused state.

      Male toads will croak their objections loudly if grabbed by mistake, and usually the grabber will let go within a few seconds. Fish can’t be so vocal, though, and there would have been no audible cue to encourage the confused toad to release Jill’s fish. It was lucky that Jill’s husband spotted the embrace when he did, otherwise the koi carp may have endured a long struggle. Male toads are persistent and will often hang on to the female for several days. One female may attract the attentions of several males, resulting in a writhing orgy of warty skin and interlocked limbs – who’d be a female toad?! Eventually the strongest male will be successful and will fertilise the female’s eggs as she lays them, up to 6,000 eggs in long double strings.

      One study found that 80 per cent of males returned to the pond where they were spawned, so it’s likely that Jill’s toad was a tadpole in her pond, perhaps before it was stocked with fish. Toads can be long-lived and lucky individuals may reach up to 40 years old, so it might be worth keeping an eye out for this overzealous toad for a few years to come.

      Confusing Courtship

      The males and females of some birds, such as robins, look identical. So how does a male robin know he is chatting up a female robin, and not another fella? And why do they both have red breasts? Sue

      Many birds are sexually dimorphic – that is, the male and female look very different. Usually, the males are the more colourful, showy sex and females tend to be more drab. Perhaps the most extreme example of this is the peacock, СКАЧАТЬ