From oil spills to rat-infested nesting sites to fishing nets, seabirds have long faced a wide range of threats to their survival. One study of monitored populations found a 70 percent drop in their numbers since 1950. More recently, climate change has added another challenge for seabirds: As global warming accelerates, they’re increasingly out of sync with their prey.
The timing of many behaviors in the natural world is changing with rising temperatures. Spring blooms are coming earlier, for example. But not all species adjust at the same pace or at all – and that, in itself, can cause problems.
In a new study published in the journal Nature Climate Change, scientists conducted the largest analysis to date of whether seabird breeding seasons are shifting as oceans warm and their prey availability changes. Even though some species are breeding at different times, the study found that seabird breeding seasons haven’t changed much on average.
“Beforehand, we thought some species were breeding earlier and some were getting later, but we weren’t sure exactly what the trends were overall,” said Katharine Keogan, a University of Edinburgh doctoral candidate and lead author of the study. “It was really difficult know what’s happening on a global scale across lots of species.”
For the research, she and 87 coauthors compiled breeding data already collected on 145 bird populations covering 61 species in 64 locations around the world, from 1952 to 2016. The team of collaborators calculated the average time of egg-laying and hatching dates in relation to their location and looked for trends and variation over time. They also compared the data to temperatures of the surrounding waters and looked at other variables that could have an influence, such as bird body size. They concluded that seabirds are generally not varying their own breeding dates.
The concern, however, is that if seabirds can’t adapt, then they won’t find enough food to feed their chicks. Scientists have already found that warming oceans are affecting birds’ traditional food stocks, such as shrimp, squid and small fish. Where once seabirds could rely on prey being available as they breed, rising sea temperatures have caused prey more sensitive to environmental conditions to reproduce at different times than in the past.
That doesn’t mean that some seabirds can’t or won’t change with climatic conditions. Some species of cormorants and gannets tend to be more flexible, Keogan said, whereas some albatrosses have a consistent breeding window that lasts only a couple of days. She believes that birds that forage relatively close to their breeding colonies may be better at adapting, while the species with large foraging ranges, such as albatrosses, are not.
“If they’re thousands of kilometers away, how would they know when to start breeding in response to fluctuating conditions at the breeding colony?” she asked.
Some seabirds’ ability to change could save them. Richard Howells, a doctoral candidate at the University of Liverpool who was not involved with Keogan’s research, has been studying the diets of a North Sea colony of European shag chicks by looking at nearly 30 years of data from 1985 to 2014. His research has linked warming water trends to changes in the shags’ diet.
The shags, a type of cormorant, usually feed on small fish called lesser sandeel. As the numbers and size of sandeel have decreased during the study’s time frame – a change related to fishing and warming waters – the shags responded by varying their diet to include more than 11 different types of prey. The birds’ nesting also varied – one year, they might begin nesting in March, and the next year not until May.
“They are more variable than most species,” said Howells. “I think they’re buffered to the effect of rising sea surface temperature to a degree, but there’s only so much earlier they can breed or only so much they can change their diet.”
On the other hand, these shags are very susceptible to major weather events. This past winter’s huge storm in the United Kingdom, the “Beast from the East,” threw the birds seriously off course. They’ve either disappeared, died or turned up in places where they’ve never been previously sighted, like The Netherlands. None so far have appeared at their nesting site. “That freak couple of days really appears to have changed their behavior,” said Howells.
A lack of information about the prey behavior is a serious impediment to further seabird breeding research. “We know that some fish species are spawning earlier now than in the past, but we don’t know how the actual prey of the birds is responding to temperatures or correlates with temperatures,” said Keogan.
She now wants to investigate on a global scale the impact of breeding times on seabirds’ success at raising chicks to get a better understanding of the consequences of the prey-breeding timing mismatch.
Scientists worry about the consequences if seabirds do fall out of sync with the availability of key forage fish. “We could be on the precipice of a huge global and multispecies catastrophe,” said Howells.
Correction: European shags turned up in The Netherlands for the first time this winter, not Germany. Richard Howells initially misspoke. We regret the error.