You’ve heard the phrase, “Nothing in biology makes sense except in the light of evolution,” an insightful phrase penned in 1972 by Theodosius Dobzhansky. I would like to add a second part to that phrase, and it goes like this: “… and, nothing in evolution makes sense except in the light of co-evolution.” This would hardly be an exaggeration, and it can hardly be better exemplified than with examples from migratory birds. Migratory birds have to be adapted to at least three different ecological settings. They breed in one area, migrate (and often spend considerable time) through another area, and winter in a third area. In each area they must feed and avoid predators, and in the nesting area, they must have make and protect nests, and the feeding is even more critical because they are growing their chicks.
Changes in one or more of these zones can change the viability of a bird species’ strategy, and even influence the bird’s survival chances in other areas. In the case of one subspecies of the Red Knot, a migratory bird, changes in the breeding ground caused by anthropogenic global warming have caused changes in the morphology of the bird, which in turn have caused changes in the birds’ ability to survive in the migratory zone.
The Red Knot (Calidris canutus) is a shorebird, a kind of sandpiper, with a global distribution. The subspecies C. c. canutus breeds in the Taymyr Penninsula and migrates to Western Europe then western and southern Africa.
The birds arrive in Arctic in the spring, where the adults, and later, their chicks, feed on insects. The insects are abundant after the snow melt. So, the birds arrive, ideally, just in time to take advantage of the abundant insects. The young then grow, and migrate south where they stay an entire year before doing their own migratory thing. While in Europe, these birds feed on bivalves that hide in mud, and on plants. The long beak of this bird facilitates their foraging on the bivalves. Having a long beak is good, because the shallower and more readily accessible bivalves are smaller, less abundant, and slightly toxic, especially to young birds (adults may have somehow adjusted to the toxin), but deeper bivalves are less toxic and more abundant.
Climate change has caused the timing of the snowmelt in the Arctic to change, with the snow melting off on average a half a day per year earlier over the last three or do decades.
This has caused the insects to emerge earlier. Also, the earlier snowmelt has affected the insects so they are less abundant, and their body sizes are smaller.
However, the Red Knots continue to arrive at about the same time every year, so the abundance and quality of the insect food source is measurably reduced. This, in turn, has stunted the growth of the young. The smaller young have smaller beaks. So, when they arrive in Europe, they have access mainly to the shallower, somewhat toxic bivalves, and plant material, and can’t get as easily at the deeper, more abundant, and non toxic bivalves down deeper in the mud. This, in turn, causes a lower survival rate for these young birds.
The population of these Red Knots has been declining, and this may be the reason.
Meanwhile, the adults that have lived for a few years are seen to have longer (normal length) beaks. It is likely that some Red Knots either grow more quickly or have some other way of addressing the problem of food supply. It is possible that natural selection is changing this bird population to manage these changes in climate.
This may be a good thing long term, but it is hard to say. The Arctic has indeed been warming more than the rest of the planet, and this is likely to continue. It is not easy to predict how Arctic insect populations will change under these changing conditions. If insects end up emerging over a more prolonged period of time, or if some other aspect of the ecology changes that causes the insects to be exploited more efficiently by a competitor of the Red Knot — or less efficiently — or if some other change in ecology happens in the wintering grounds or the flyways, then this could get even more complicated.
Climate change has happened in the past, and it is certainly true that many populations of birds and other critters have adapted, in an evolutionary sense, to these changes. Just as likely, species or subspecies have gone extinct. Every population of migratory bird probably has a very interesting (and often harrowing) story behind how they arrived at their current co-evolutionary relationship to the world around them. The problem with the currently changing climate, changing because of the human release of copious amounts of greenhouse gasses into the atmosphere, is that this change is happening at a rate that has rarely, if ever, been seen since birds evolved to begin with.
This is not entirely unexpected. JP Myers and Robert Lester predicted, in the 1992 book Global Warming and Biological Diversity that asynchrony of insect emergence and shorebird migration would cause population declines in shorebirds like the red knot.
Caption to the figure at the top of the post:
Fig. 3. Prey choice and prey availability at the Mauritanian wintering grounds. (A) Analysis of stable isotopes of blood samples shows that juvenile red knots (n = 676 birds) largely ignored the most abundant but mildly toxic prey, Loripes. However, with an increase in age, adult red knots (n = 1664) added substantial amounts of Loripes to their diet, but only if they had long bills. Plotted are means ± SE. (B) This bill length–dependent diet shift may be explained by the depth distribution of Loripes. The majority of these bivalves live between 30 and 40 mm below the seafloor, which is precisely the range of the bill lengths. The other two food sources, Dosinia bivalves and Zostera rhizomes, are found at shallower depths and are accessible to all red knots. Bars indicate medians, boxes indicate 25th to 75th percentiles, and whiskers indicate ranges.
The research is reported in this paper:
Body shrinkage due to Arctic warming reduces red knot fitness in tropical wintering range. BY JAN A. VAN GILS, SIMEON LISOVSKI, TAMAR LOK, W?ODZIMIERZ MEISSNER, AGNIESZKA O?AROWSKA, JIMMY DE FOUW, ELDAR RAKHIMBERDIEV, MIKHAIL Y. SOLOVIEV, THEUNIS PIERSMA, MARCEL KLAASSEN
SCIENCE13 MAY 2016 : 819-821
Reductions in body size are increasingly being identified as a response to climate warming. Here we present evidence for a case of such body shrinkage, potentially due to malnutrition in early life. We show that an avian long-distance migrant (red knot, Calidris canutus canutus), which is experiencing globally unrivaled warming rates at its high-Arctic breeding grounds, produces smaller offspring with shorter bills during summers with early snowmelt. This has consequences half a world away at their tropical wintering grounds, where shorter-billed individuals have reduced survival rates. This is associated with these molluscivores eating fewer deeply buried bivalve prey and more shallowly buried seagrass rhizomes. We suggest that seasonal migrants can experience reduced fitness at one end of their range as a result of a changing climate at the other end.