February 8, 2012
Weather changes not just from season to season, but also from year to year. Where I live in Minnesota, we had only a few days of frost before the year’s end, and January, normally the coldest month of the year, was relatively balmy. But in another year we might have days on end of sub-zero weather during the winter. It is hard for a person to detect climate change at this scale, even though global temperature measurements clearly show that the planet has warmed.
But every now and then something comes along that demonstrates a longer term trend that we can see and measure more directly. For instance, the USDA recently released a new version of its “Plant Hardiness Zone Map.” If you are a gardener in the United States, you probably already know about this map; its zones are used to determine what kinds of plants can be grown outdoors in your area, the estimated dates of the last killing frost in the spring and the first killing frost in the fall. This is at least the second time in my memory that this map has been redrawn with all the zones moved to the north, reflecting a warming planet in a way that every gardener can observe and understand.
Not all global climate changes are simple warming, however. Global warming causes changes in ocean and atmospheric circulation as well. Westerly winds in the southern Pacific Ocean have shifted south towards the pole and have become more intense. A recent study in Science shows that the foraging patterns of breeding Wandering Albatross (Diomedea exulans) on the Crozet Islands has been changed by global warming in a way that seems to benefit them now, but that will likely harm them in the future.
Albatross are members of the bird order Procellariiformes, also known as the “tubenoses” because of the tube-like “nostrils” on their beaks. There are about 170 species of this kind of bird, including the petrels, shearwaters, storm petrels, diving petrels, and albatrosses. It is commonly said that the ocean is the last great frontier on earth, and this is probably true. It should not come as a surprise, then, that the Procellariiformes are among the “last great frontiers” of birding and bird research. Since the tubenoses spend almost all of their time at sea, they are hard to study. They come to land only to breed, and even then, usually on remote islands. They are so committed to being in the air over the ocean or floating on the surface of the sea that most members of this order are unable to walk at all. One group of tubenoses has the capacity to shoot a stream of noxious liquid (from its gut) at potential predators, which is an interesting adaptation to being unable to stand up and peck at intruders attempting to eat one’s egg or chick. (See this post for more information on tubenoses and a review of an excellent recent book on the tubenoses of North America.)
Life-long mated pairs of albatross settle in a nesting area during breeding season to lay and incubate eggs, hatch them and care for the young. The nesting sites are communal, so it is impossible for a pair of nesting birds to leave their egg or chick alone while they go out to find food—fellow albatross in the same colony view unguarded eggs or chicks as free snacks. The demand for food increases as the chick grows and requires more and more seafood every day, but the time available for foraging remains at 50 percent of normal because the two parents have to split the duty of guarding the nest and looking for food. In addition, dozens or perhaps hundreds of albatross from a given colony are foraging in the same general area, because they are all tending to nests at the same time. This probably diminishes the total amount of food that is available.
For all these reasons, foraging during nesting is a stress point in the life history of albatross. The birds forage by soaring around over the ocean, using wind as their main form of propulsion, literally sniffing out food sources (they have excellent smelling abilities). Therefore, the pattern of oceanic winds should matter a lot to their survival, especially during breeding season.
Which brings us back to changes in wind patterns due to global warming. The study by Henri Weimerskirch, Maite Louzao, Sophie de Grissac and Karine Delord is destined to become a classic because it touches on a sequence of logically connected observations to tell a compelling story. For my part, I’m going to use this in a classroom to demonstrate interesting science at my next opportunity. Let’s go over it step by step.
Albatross breeding is clearly difficult, and failure is likely common. One indicator of this is the fact that wandering albatross lay only one egg per season. Most coastal and terrestrial birds lay more than one, and in many species the number they lay varies from year to year depending on conditions. If wandering albatross lay only one egg, ever, there is a sort of underlying biological expectation of a low success rate.
For most birds, size matters. Within the normal range for a species, individual birds grow larger when conditions are good, and those birds do better in periods of difficulty because a large body stores more reserves and provides for more effective competition with other birds. A bird can grow large and bring lots of food back to the nest only if foraging is good, and the amount of food a bird obtains in a day is a combination of time (how long one forages) and the amount of food available in the environment.
The amount of food an albatross can obtain depends in part on the total area of the ocean that is searched each day, which in turn depends on how fast the bird flies. Since the albatross soars on the wind most of the time, this means that everything depends on factors such as the speed and direction of the wind. The study we are looking at today combines all of these things in an elegant exposé of the link between climate and the difficult job of producing baby albatrosses.
The wandering albatross travel enormous distances from their breeding grounds, often going more than 1,000 miles before returning to the nest to relieve their mate from guard duty. Males forage more widely and more to the south than females, who prefer northern waters. During this time, the birds use the wind as their primary form of locomotion. The researchers have shown that the winds in this region have increased in strength by a measurable amount, owing to shifts related to global warming. The average wind speed has gone up by about 10 percent from the 1990s to the present day. This allows the birds to move from foraging area to foraging area more swiftly than otherwise possible.
The total amount of time it takes both male and female albatross to complete a full journey of a given distance has decreased by between 20 percent and 40 percent from the 1990s to the present, and the speed at which the birds are observed to fly has gone up about the same for females, though the observed speed increase for males is not statistically significant. This is direct evidence that the amount of time spent foraging is less under present conditions than it was in the recent past, and it can be inferred that this is caused by the correlated increases in wind speed.
During the same period of time, the birds have gotten bigger. In 1990 the average female was about 7,500 grams and by 2010 females were about 8,500 grams. Males increased by about the same percentage, going from the mid-9,000 range to about 10,500 grams. These differences in mass are not reflected in the overall dimensions of the bird, just their weight. This indicates that during periods when the birds are on average smaller, many are underfed.
Breeding success for albatross varies considerably. The chance of successfully launching a baby albatross from the nest for the 350 pairs studied ranges from about 50 percent to just over 80 percent depending on the year (I’m leaving out one really bad year when the success rate was only 25 percent). During the past 40 years, over which it is thought the wind patterns have changed as described above, the “moving average” of breeding success (taking a few years together into account to dampen natural variation) has changed from about 65 percent to about 75 percent. These birds indeed seem to be benefiting from changes in wind pattern caused by global warming.
Most changes in weather, patterns of wind and rain and other effects of global warming are negative, as any review of the literature on this topic over the past decade will show. The benefits being experienced by these birds is unusual. But it may also be temporary. The researchers who produced this result say that the shift of winds towards the poles that brought higher energy patterns to these islands is likely to continue. As wind speeds increase, the benefit the birds will receive will at first level off then start to decrease, as overly windy conditions are bad for the albatross. The shift of westerly winds to the south of the islands will probably decrease the viability of foraging over the next few decades because it will make it easier for the birds to get to places with lower quality forage and thus decrease the rate of obtaining food. So, if the current changes in wind patterns are a gravy train for the Crozet Island wandering albatross, the train may eventually leave the station without them.
Weimerskirch, H., Louzao, M., de Grissac, S., & Delord, K. (2012). Changes in Wind Pattern Alter Albatross Distribution and Life-History Traits Science, 335 (6065), 211-214 DOI: 10.1126/science.1210270
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