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Climate change could leave birds and their eggs unprepared for the extreme weather being thrown at them.
While eggs are well-suited to deal with infections and breathing, researchers have suggested that heavy rains and droughts could overwhelm them as the climate warms.
Scientists have uncovered the variety of ways bird eggs have evolved to use water to their advantage.
Eggs from the Museum were among those used to figure out how colour, shape and nest type affected the water resistance, or wettability, of an egg. In turn, this can protect chicks from deadly infections, as well as overheating and asphyxiating.
Dr Marie Attard, the lead researcher of the study, says, 'As the largest analysis ever conducted on eggshell wettability, our study represents a significant advancement in bird reproduction, and our understanding of selective pressures which promote the evolution of specific eggshell traits.
'The plasticity of eggshell traits over the short-term is likely to determine the response of bird populations to rapid and long-lasting changes in their environment, such as global climate change and habitat loss.
'However, we do not know if eggshell characteristics can change quickly enough in response to these external threats. Individual species will either move to more favourable breeding areas, tolerate or adapt to their changed environment, or become extinct.'
The findings, led by British and Canadian researchers, was published in Interface.
The collection of bird eggs at the Museum is the largest in the world, totalling over one million specimens. Held at Tring, the eggs are from over half of all known bird species, including extinct species like the giant auk.
While many of the eggs have detailed information about when and where they were collected, others have lost this information over the passage of time. To research the structure of eggs, the researchers used fragments of these 'data poor' specimens in their measurements.
Taking in 441 species, from hummingbirds to ostriches, droplets were balanced on the shells to see how the water acted.
Marie says, 'We used a machine that drops a specific volume of water onto each egg shell. Each shell was divided into three groups depending on the angle of contact between the droplet and the egg – hydrophilic, hydrophobic, or superhydrophobic.
'Hydrophilic eggs tend to spread water more evenly around the shell, while more hydrophobic eggs form droplets on the surface. We didn't have any superhydrophobic eggshells but it's definitely possible there are superhydrophobic eggshells out there.
'It is believed that eggs laid in mound nests made of litter could be superhydrophobic to deter microbes but we didn't find any.'
This information was combined with other data, such as the lifestyle of the bird and its nesting habits, to build up a picture of why an egg may have evolved in a certain way.
The scientists found that birds living in wet and humid climates generally had hydrophobic egg shells, while those in warm and dry environments had hydrophilic eggs. Hydrophobic shells were also more common in precocial species, where chicks spend more time developing inside the egg.
Other differences also affected how well an egg resisted water, with spottier eggs better at repelling water than plain-coloured eggs.
The differences between the species reflect the environments they live in. For instance, the budgerigar has a very hydrophobic egg shell as it lays its eggs just after heavy rainfall. As well as allowing its chick to breathe, repelling water also traps microbes that might infect the developing bird.
Meanwhile, the brown booby has a hydrophilic shell which causes a thin layer of water to surround the egg. The layer helps reduce heat loss from winds, and the researchers have proposed it may provide additional insulation.
While these eggs are currently well-adapted for their environment, extreme weather scenarios as a result of climate change could hit birds hard.
The impact of extreme weather has already been seen in the extinction of other animal groups. For instance, an abnormally strong El Niño event, with higher temperatures and lower rainfall than expected, has been attributed for the demise of the golden toad in the 1980s.
The researchers hope to find out more about what makes an egg more hydrophilic or hydrophobic, with the possibility chemicals in the eggshell may also affect these properties beyond just the structure. They could also use intact eggs from extinct birds to find out more about how suited the birds were to their environment.
While birds are more suited to move with the changing climate than other animals, the scientists have called for more research to understand just how chick development would be affected in an uncertain future.