Charlie McMurray is an intern at RSPB Scotland working on 'all nature' projects: mammals, amphibians and insects. Here is her latest blog on natterjack toads from RSPB Scotland Mersehead.
Calamitous chorusing

It’s time! April not only brought showers to Scotland, but also lured our charismatic natterjack toads (Epidalea calamita) out of their winter burrows. Natterjacks will happily dig into the earth, unlike common toads, and have different burrows depending on the season. In winter, the toads may be buried into the sand dunes here at Mersehead.

During the breeding season (April to June), they burrow much closer to their breeding pool, minimising their commuting time and maximising their chances to mate.

Males move first, so in late March and early April, these prospective homemovers were probably peering out of their winter hibernacula, considering two things: females and food.

Once they’re snug in their new poolside homes, they start to become more active at night. The males give away their whereabouts quickly because they call loudly from the pools to attract mates. Their calls give a female an impression of their size and, therefore, the suitability of their genes for her offspring. Presumably, ‘bigger is better’ in the world of natterjacks.

Each male wants to be heard, so one call prompts another, and another, and so on. This vocal jostling forms an immersive chorus that carries far and wide. In fact, they’re the loudest amphibian in Europe. At Mersehead, all I have to do is pop my head out of the window to know what they’re up to.

Here’s a clip of our very own amphibious choir. See if you can pick out any individual voices (and the winnowing snipe!). Credit Roseanne Watt.

Calling certainly has one major benefit – the promise of progeny, but it also has significant costs. The noise not only entices females, but unwanted visitors too. Calling en masse provides protection by hiding the toads amongst one another, as zebras do in a herd. In such a herd, you attract more predators than when you’re alone, but the likelihood that you’ll be the unfortunate victim is much smaller.

As with birds, each frog and toad species has its own identifiable call. Typical mating calls are generally produced simply by pushing air back and forth between their lungs and their expandable air sac. The air runs over their vocal chords, creating the sound, and the air sac amplifies it. Natterjacks have just one sac below their mouths, but some frogs have one on each side of their head or none at all.

Attracting mates isn’t the only vocabulary the toads have though. For example, even though the males try to space themselves out in their pools to avoid being confused for a receptive female, mistakes do happen. For such times, they have a specific ‘squeak’. Unwilling females also utter this complaint.

Here’s the big news: Mersehead’s natterjacks are out and about! On the night of Saturday 9th, one optimistic male soloist was heard, but on Thursday 14th, we saw and heard about 30. The chorus isn’t in full swing just yet because the night-time temperatures are a bit low, but it’s a positive sign for this year’s surveys. Each night, we use the chorus to pinpoint where the toads are, so I’m looking forward to seeing some soon.