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The ‘Birkdale nightingale’, ‘Bootle organ’ and ‘Thursley thrush’ are all regularly used colloquial monikers that, in certain regions, have replaced the more commonly accepted name of our smallest and rarest British toad, the natterjack. The reason why such a seemingly inconspicuous and rare toad should been given so many local nicknames is purely down to its incredible ratchetlike call, which marks the highlight of its breeding season, and is also a tremendous spectacle for anybody with a penchant for toadspotting.

Natterjack toads

WHEN

April to mid-May

WHERE

Ainsdale NNR, Merseyside; Caerlaverock Nature Reserve (WWT), near Dumfries

‘Do not park here! The solid yellow line is the key identification feature of the natterjack toad.

David Woodfall

This diminutive relation of our common toad is entirely restricted to Europe, with its heartland being the Iberian Peninsula, and becoming progressively rarer further north. In Britain the natterjack was widely if locally distributed around southern and western coastal locations, but healthy populations can now only be found along the northwest English coast around Merseyside and in southwest Scotland centred on the Solway Firth.

To the untrained eye, natterjacks are similar to the common toad, but their size of no more than 75 millimetres, dry brownish to olive-green warty skin and yellow stripe, which runs the length of their backbone like a ‘no-parking here’ line, easily distinguishes this wonderful and mercurial little toad. In addition, natterjacks have short hindlimbs, giving them the ability to run at surprising speeds over short distances. Unlike common toads, where the females are generally larger, there is little difference in the size of the natterjack sexes, even when the females are bursting with eggs and ready to spawn.

Natterjacks in Britain are now found almost exclusively among sand dunes and the periphery of salt marsh close to our coastlines; they will always be below 100 metres above sea level. Befitting a species that is most abundant in Spain, it is no coincidence that this is one of the warmest habitats in Britain. These sandy spots are also perfect for a species that is a compulsive burrower, meaning that natterjack toads are easily able to dig down to escape from the extremes of temperature. The short, dense vegetation attracts lots of invertebrate prey, and, as sandy places are also well drained, natterjacks have evolved to breed in ephemeral freshwater pools and ponds in the dune slacks. A typical breeding site will often be no more than a small, sandy, shallow and unvegetated pool with a maximum depth of 30 to 50 centimetres that will often have completely dried out by the height of summer.

In common with all our native amphibians and reptiles, the natterjack is a species that opts out of the coldest autumn and winter months by hibernating. In the case of natterjacks, this time is spent underground in self-excavated burrows either alone or with other toads, with the largest number – an astonishing 44 – recorded by the celebrated herpetologist Trevor Beebee.

On good nights you might get the impression that you had dropped in at the Okavango delta or the Amazon basin at dusk rather than it being just a misty night on Merseyside!

After this period of torpor, the toads then emerge into the light in March or early April once the air temperatures have warmed up sufficiently to sunbathe.

Adults leaving their hibernacula, or winter residence, have usually only two things on their mind, and the first of these is food. The toads emerge to start foraging at dusk and may move several hundred metres from their burrow to feed in the dune slacks before returning to the same burrow before dawn. The natterjack is much more athletic than the common toad and hunts actively by running down its prey over short distances. The long sticky tongue then shoots out at lightning speed to ensnare the unfortunate ant, bug, beetle or fly. Smaller prey is swallowed immediately while larger prey often takes a while to gulp down and can be disposed of by the toad’s ingenious ability to retract its eyeballs back into its head, thereby applying pressure to the roof of the mouth and, hence, helping to crush and swallow the food.

While natterjacks will eat virtually anything that will fit into their mouths, the adults themselves have comparatively few predators due to the nature of their skin, as the larger warts contain parotid glands that secrete a poison when molested. This deters most predators, with the exception of some members of the crow family, which have learnt to disembowel them leaving the skin behind, and grass snakes, which seem less susceptible to the poisons. A secondary defence for the natterjack when faced with a grass snake is to puff itself out like a little balloon and stand high on all four legs to give the impression that it is larger and more menacing than it really is.

Having fed, the toads are then keen to move on to the details of mating. The first stage involves the short migration back from their winter quarters to the breeding site of choice and this generally occurs in late March or early April on evenings where the temperature is above 8°C. The males are the first to arrive at the pond and initially tend to occupy burrows close to the water. It is easy to tell when the breeding activity starts in earnest as the natterjack has the distinction of being Europe’s loudest amphibian and its calls can easily be heard from a mile away on a warm, still night. After sunset the males emerge from their burrows to take up evenly spaced positions in the pond margins. They then adopt a stance with their forelimbs straightened to keep their head and, more importantly, their vocal sac clear of the water.

The sight and sound of a male in full voice is an unforgettable experience. Its strident call is produced by an inhalation of air through a couple of slits in the bottom of the mouth, which is then shuttled backwards and forwards between the lungs and the vocal sac. This results in an enormous inflation of the vocal sac so the toad’s actual head becomes three times its normal size. As the normally purplish or bluish throat distends, in the light of a torch it appears ghostly white and translucent and is very reminiscent of a child’s bubblegum bubble. Normally, a male will be stimulated into calling by the churring of other surrounding males but even passing vehicles and planes can initiate a chorus. The call sounds like a ‘rrrrRIP’ that lasts for about a second before a slight upturn at the end, and individual males will usually call continuously for around a minute before taking a short rest and winding up again.

Calling males vastly outnumber the females on any given night, as the males may well stay for the entire breeding season, and a visitation by the female could be confined to just a few hours on one night of the year. As the ratio is skewed towards amorous males, much time is often wasted in chasing, grabbing and releasing other males in a case of mistaken identity. This error is soon sorted out by the grabbed male, who makes a small croak to let the other male know it is ‘riding the wrong chariot’; he is then immediately released. Females do not often arrive until after dark and are usually grabbed by the first males who chance upon them. In each case the male then proceeds to lie on the back of the female and keep hold by using his forelimbs to tightly clasp around her armpits in a grip called ‘amplexus’, which is strengthened by rough nuptial pads on the digits of the toad’s forelimbs.

This coupling process is a lot more gentlemanly than with common toads, where an unmated female can become covered by a writhing mass of males to the extent that she may even occasionally drown. With natterjacks, however, there seems little territorial behaviour or aggression, and, once a male has attached himself to a female, it seems often to be taken as read that the female is no longer available and the couple are left unmolested as the female selects a suitable shallow spot to begin spawning.

Most spawning takes place at night but can occasionally be seen in broad daylight after particularly busy nights. The egg-laying itself is a protracted process taking around three hours, with the female ejecting eggs in a long string, in between periods of rest; the male then fertilises these externally. The natterjack females will produce between 3,000 and 4,000 eggs in this way; the strings are easily distinguished from those of the common toad after a period of 24 hours, as the eggs develop into a single row as opposed to the double row of their common counterpart. As soon as the spawning is finished, the male swims away to look for more mating opportunities, while the female leaves the water, spent, her breeding season over and wanders off to feed.

During sunlight hours, natterjack tadpoles are visible in large numbers at the water’s surface or along the pond edges. This contrasts with frog tadpoles, which are much more shy and retiring. The tadpoles must grow quickly as they will all die if the pond dries out completely. Those that survive the high summer droughts and the jaws of predatory insects – the minority – metamorphosise, and head to the land and a brave new world in which they must fatten up in preparation for hibernation.