A guillemot incubating its egg on a cliff face on Skomer Island, Wales. Photo credit: Tim Birkhead.

A guillemot incubating its egg on a cliff face on Skomer Island, Wales. Photo credit: Tim Birkhead.

Common guillemots really do put all their eggs in one basket; or at least one egg. Jostled by their neighbours and perched high on rain-lashed cliff ledges, pairs of guillemot parents tend their precious individual egg directly on the rock. ‘Breeding at high density is the guillemots’ way of protecting their eggs and chicks from predators such as gulls and ravens, but it means that colonies become rather unhygienic’, says Duncan Jackson from the University of Sheffield, UK, describing how the eggs become smeared with the neighbours’ faeces, potentially smothering the developing embryos within. So when Timothy Birkhead, also from Sheffield, heard press reports that muck does not stick to self-cleaning guillemot eggs, he was sceptical. ‘After studying guillemots on Skomer Island, Wales, for 40 years I knew that their eggs were invariably covered in it’, says Birkhead.

Determined to dispel the self-cleaning modern myth, Birkhead and Jamie Thompson obtained a licence giving them permission to abseil down to the bird's precarious cliff face breeding ledges to photograph the eggs and collect a few. Back in Sheffield, they carefully cleaned the fragile shells before cutting them into 1 cm2 portions and gluing a fragment across the top of a glass vial filled with carbon dioxide to measure how much gas diffuses through the shell. Next, they daubed filth that had been collected from the nesting site over the shell and re-measured the vial's carbon dioxide level. Sure enough, less gas was able to diffuse across the mucky shells.

Wondering how many of the shell pores that allow gas into and out of the egg become clogged when covered in muck, Jackson retrieved the fragments and filmed them with X-rays to build a detailed view of the shell and its layer of dirt. After reconstructing both, it was apparent that the shells that had the most clogged pores tended to have the lowest gas permeability.

But it was still unclear whether the shells were capable of repelling water to wash away dirt. To test the shell's self-cleaning powers, Jackson gently tilted portions of the egg and sprinkled vacuum cleaner dust over them before dripping on droplets of water to see if they rolled away and washed the dirt off. ‘If the eggshell's surface still contained debris suspended in water droplets after our simple trial then the shell could not be self-cleaning’, says Jackson. However, despite their impromptu drenching, each fragment remained awash with a puddle of dusty water. The eggs were not self-cleaning, in contrast to the famously self-cleaning cauliflower and broccoli leaves that Jackson tested, which were immaculate after the simulated rain.

Having confirmed that the eggs are not self-cleaning, Birkhead and Nicola Hemmings suggested that Jackson test whether the waterproof outer layer of the shell – known as shell accessory material – may somehow prevent the essential pores from becoming blocked. Stripping the complex outer layer – including calcium carbonate, fats, sugars, proteins and pigment – from the surface of the egg, Jackson then daubed muck on the exposed shell and filmed the surface with X-rays. The stripped soiled egg was in much worse shape than the shell that retained its outer coating, with more than three times as many pores clogged with dirt. ‘It looks like shell accessory material limits the amount of debris that can enter and block pores, which means that gases can travel through the guillemot's eggshell even when the egg is dirty’, says Jackson, although it is not clear how the outer layer prevents blockages. But one thing is certain: press reports that guillemot eggs are self-cleaning were a little premature.

Jackson
,
D.
,
Thompson
,
J. E.
,
Hemmings
,
N.
and
Birkhead
,
T. R.
(
2018
).
Common guillemot (Uria aalge) eggs are not self-cleaning
.
J. Exp. Biol.
221
,
jeb188466
.