Like many birds, Great Knots undertake an astonishing annual migration. They travel over 6000 miles from their non-breeding grounds in Northwest Australia to breeding grounds in Eastern Siberia. A crucial stage in their migration is a stopover on the East Asian coast where fat reserves are replenished ready for the final leg of their journey.
In recent years human impacts have resulted in the disappearance of many East Asian stop-over sites traditionally used by Great Knots; most notably the destruction of a site in South Korea that once hosted over 300,000 migrating shorebirds but where, after a major land reclamation project, less than 5000 birds now arrive.
The remaining major stop-over site used by Great Knots is the Yalu Jiang estuarine wetland in China, where around 44,000 birds spend two months during their northward migration. An international team of researchers, led by Shou-Dong Zhang & Zhijun Ma from Fudan University and Jan van Gils & Theunis Piersma from the University of Groningen, tracked Great Knot populations at this staging post from 2011 to 2017.
The Yalu Jiang wetland is a protected area and little of the habitat has been lost in recent decades. Despite this protection, the site has still suffered from human activities. In 2011 and 2012 the vast majority of prey eaten by Great Knots was the small mollusc Potamocorbula laevis. However over the last few years the density of P.laevis at Yalu Jaing wetland declined by an astonishing 98%, due to a combination of parasitic infection and pollution from nearby ports and sea-cucumber farms.
Great Knots have adapted to this decline in their preferred prey species by targeting other molluscs. However, these species are both larger and harder-shelled than P.laevis, making them more difficult to digest. Great knots swallow shelled prey whole, which are then broken up in the muscular gizzard, shell fragments being excreted in the faeces. The harder the shell, the more force – and therefore more time – is needed to break it up in the gizzard. The force required to break up shells from prey eaten in 2016-2017 was around 10 times greater than was needed in 2011. This extra digestion time creates a bottleneck during which Great Knots cannot eat any more prey items, reducing overall food intake.
Great Knots do have a response to this: changing their own physiology. Like many migratory birds, Great Knots change the size of their digestive organs during migration, reducing their size during long-distance flights (reducing weight-load) and increasing them during fuelling stops. The researchers found that Great Knots appear to be adapting their digestive organs in response to their lower quality diet. Average gizzard mass of individuals at Yalu Jiang wetland increased by 15% over the course of the study, equating to a 32% increase in shell processing capacity.
Does this mean Great Knots have been able to successfully adapt to the deterioration in their environment? Sadly, it appears not. Their increased digestive capacity did not compensate for the reduction in prey quality. They still suffered from digestive bottlenecks, resulting in Great Knots in 2016-2017 consuming prey at a rate over 70% slower than in 2011-2012. Theoretically they could compensate for this by foraging for longer; however, the researchers calculate great knots in 2016-2017 would have needed to forage for 13-16 hours a day to achieve the body mass gain necessary to migrate on time, compared to less than 5 hours foraging for birds in 2011-2012.
Unsurprisingly then, the number of Great Knots arriving at Yalu Jing wetlands declined by a third between 2011-2012 and 2016-2017. The researchers doubt this is because birds are switching to better staging grounds, given the apparent lack of high-quality alternative sites. Rather, the implication is that the lack of food intake and resulting decrease in weight gain is having obvious effects on the subsequent survival of migrating Great Knots.
This study shows the value of long-term monitoring of wildlife populations; possessing baseline measurements of food intake during ‘the good times’ allows us to properly assess the effects of changing food availability on migrating birds. It is also a reminder that the challenges facing migrating birds as the habitats they depend on deteriorate are likely to go beyond even their remarkable adaptive abilities.