Rained out: Arctic nesting birds struggle with climate change | @GrrlScientist
As global warming disrupts local climate patterns around the world, even Arctic breeding birds are affected. But unlike tropical and subtropical birds, which are harmed by rising temperatures, Arctic birds are harmed by increasing rainfall.
I’ve written before about how global warming harms birds. In that study, the researchers documented that a small local temperature rise was great enough to trigger embryonic development before the mother started incubation, which can cause the eggs to hatch early. Since birds can only lay one egg per day, early embryonic development translates into asynchronous hatching. Asynchronous hatching puts a tremendous strain on the adult birds because they end up raising a nest full of chicks of different ages, where the first-hatched nestlings have a competitive edge over their younger and smaller brothers and sisters. Because first-hatched chicks are bigger and stronger, they effectively monopolise the food provided by their parents.
But the birds in that study live in arid and semi-arid regions of Australia where it is already plenty hot, so it makes sense that as global warming causes ambient temperatures to increase by a few degrees, that would be sufficient to cause bird embryos to start developing early. But what about birds that breed in polar regions? Is climate change harming them, too? If so, how?
An international team of scientists recently published a comparative two-year study that sheds some light on these questions. In their study, they tracked variations in nestling growth rates for two songbirds that breed in the Arctic; the Lapland longspur, Calcarius lapponicus, which is an Arctic breeding specialist, and Gambel’s white-crowned sparrow, Zonotrichia leucophrys gambelii, which is a generalist breeder.
Lapland longspurs are migratory New World buntings that breed exclusively in the High Arctic of Alaska, Canada, and, more recently, throughout Eurasia. When breeding, Lapland longspurs build an open-cup nest on the ground on tussock-tundra located near the tree line. Although they primarily eat seeds, when they are breeding, Lapland longspurs satisfy their dramatically increased energetic demands by eating arthropods — insects and spiders — which they also feed to their rapidly growing chicks.
Gambel’s white-crowned sparrow is a medium-sized migratory songbird that is slightly smaller than the longspurs. They winter in the southern United States and breed in brushy habitats found in a variety of geographic locations, even as far north as the Arctic Circle. These personable birds construct open-cup nests close to the ground in bushes or small trees. White-crowned sparrows are mainly seed-eaters, but like Lapland longspurs, they also switch their diet to arthropods during the demanding breeding season.
Graduate student Jonathan Pérez, of the Animal Behaviour Graduate Group at theUniversity of California, Davis, and his colleagues tested their predictions that clutches laid earlier in the breeding season would do better, and that the nestlings of the Arctic-specialist longspurs would grow faster than the generalist sparrows. Additionally, because the weather was significantly different between the two study years, Mr Pérez and his colleagues also tested their prediction that nestlings would generally grow faster in warmer, drier conditions. To test these hypotheses, the researchers compared data for these two birds collected during the 2013 and 2014 breeding seasons.
Nestling growth rates were significantly different between breeding seasons
The study site was located on the North Slope of Alaska’s Brooks Range, where the researchers tracked 110 white-crowned sparrow nestlings and 136 Lapland longspur nestlings from a total of 58 nests from four sites over two consecutive breeding seasons.
Mr Pérez and his colleagues documented daily nestling mass gain and, as predicted, they found that nestlings of both species that hatched in 2013 (dotted lines; figure 1) gained significantly more mass, and fledged sooner, than those that hatched in 2014 (solid lines; figure 1):
As you can see in the above figure, the Arctic-specialist longspur nestlings (figure 1B) did grow faster than the slightly smaller generalist white-crowned sparrows (figure 1A) in both seasons. Further, the team also found that nestlings that died before leaving the nest had lower growth rates than those that fledged.
Increased rainfall (not temperature) was most harmful to nestlings
Was weather causing these differences in nestling growth rates between the two breeding seasons? Meteorological data reveal that 2014 (solid lines; figure 2) was a much cooler year overall than was 2013 (dotted lines; figure 2):
Analysis of these data revealed the generalist sparrow nestlings’ growth rate (solid line; figure 3A) was unaffected by maximum daily air temperatures, whereas higher maximum daily air temperatures were associated with increased growth rates for the Arctic-specialist longspur nestlings (dotted line; figure 3A). In contrast, the minimum daily air temperature had no effect on growth rates for either species.
The weather was astonishingly cooperative during this study: both breeding seasons had the same number of rainy days — 14 — and there was no difference in average daily rainfall during the nesting period. However, 2014 was a much wetter year overall: the total rainfall during the nesting period was 73% greater in 2014 (68.8 mm) than in 2013 (39.8 mm). Yet when these data for both years were analysed and compared, the research team was surprised to find that the increased rainfall in 2014 was associated with a significant slowing of the daily growth rates for both species (figure 3B):
But why was rainfall associated with reduced nestling mass gains in 2014?
“[O]ne memory that sticks out about the 2014 season that had us particularly excited to analyze this data set was the fact that it started raining part way into the nestling period and most days there would be at least a drizzle going on”, said Mr Pérez in email. He said that, prior to this abrupt weather change, they had been putting the nestlings in a lined container and then measuring them, one by one, before returning them to their nest.
“But since it was so wet out we were worried about the nestlings getting wet and cold from our handling of them”, said Mr Pérez. “So our solution was to turn a large clear garbage bag into a big plastic tent, but it was only big enough to fit over my head while sitting and leave a small work area in front. So I would be hunched over under this sheet with the nestlings and our measurement gear trying to keep everything dry including the nestlings while the wind is trying to tear everything off us and the poor person assisting is sitting in the rain with a Rite in the Rain [waterproof] notebook.”
Because 2014 was a much wetter year overall than 2013, increased rainfall must have affected some critically important aspect of these birds’ lives — something that strongly influences daily nestling growth rates. The researchers suspected that “something” could be food availability.
Increased rainfall was associated with smaller arthropod populations
During the nestling phase for both breeding seasons, the researchers measured the abundance of terrestrial and aerial arthropods every week using pitfall traps and sweep nets. The captured arthropods were frozen then dried and sorted, and their dry mass measured and recorded to the nearest milligram (figure 4):
The data show that the peak in pitfall arthropod biomass occurred roughly one week (one sampling interval) later in 2014 relative to 2013 (lower panels; figure 4). This corresponded to a peak in the activity and density of terrestrial arthropods — mostly wolf spiders (family Lycosidae) and ground beetles (family Carabidae). The same pattern was seen for the sweep-net biomass peak, which also occurred about one week later in 2014 relative to 2013 (upper panels; figure 4). Sweep net data generally correspond to the emergence and activity of flying insects — particularly of Alaska’s state bird, the mosquito.
Global Warming is predicted to increase rainfall in the Arctic
Although most people think that the warmer temperatures associated with global warming should be generally beneficial to living things, climate change also triggers a suite of unpredictable events and consequences. For example, scientific models show climate change is more likely to increase the likelihood that severe storm events will occur at unanticipated times of the year — as is already happening. For this reason, at least some researchers use “global climate disruption” as a more descriptive term for climate change.
“[A]s interannual variation and frequency of unexpected storm events increases with climate change, we may begin to see unexpected responses in nesting birds that may not match what we would expect from general patterns of environmental change”, Mr Pérez pointed out.
Additionally, the study reveals that challenging conditions force parents to make difficult trade-offs between their own survival and that of their offspring. Because climate models predict more frequent storms and increased precipitation, this will escalate the uncertainties faced by breeding birds — particularly for those breeding in extreme environments like the Arctic, where the cost of making a mistake will be greatly magnified.
“[I]ncreases in storm events at unexpected times of year may lead to really bad years if negative conditions end up coinciding [with] when nestlings are in the nest”, said Mr Pérez.
“Other years may end up being great”, added Mr Pérez.
“Species at range edges are sentinels of climate change because they often experience high environmental variability and harshness”, said Daniel Ardia, an expert on the role of environmental variation in bird behaviour and physiology and an Associate Professor of Biology at Franklin and Marshall College, who was not involved with this study.
“Pérez and his co-authors reveal the direct effects of weather variation on nestling growth, an important determinant of fitness, showing how climate variability might have strong negative effects on populations,” said Professor Ardia in a statement.
“What makes the study so compelling is that they were able to link weather variability to food supply showing the causal link between predicted weather variation and reproduction”, said Professor Ardia.
As both studies show, the effects of global climate change are myriad, often subtle, and may not be straightforward. As we saw in the study that I wrote about previously, global warming can trigger an unexpected response in tropical and subtropical birds where extreme temperatures can cause their eggs hatch early, thereby leading to unequal competition for food between nestlings. In this study, we see that extreme weather associated with climate change is causing increased rainfall, which then reduces the availability of food for nestlings in the Arctic. For these reasons, when discussing an average global two-degree temperature increase, it’s not just the average change that matters, but it’s the extremes that come with it we especially need to think about.
Jonathan H. Pérez, Jesse S. Krause, Helen E. Chmura, Shae Bowman, Michaela McGuigan, Ashley L. Asmus, Simone L. Meddle, Kathleen E. Hunt, Laura Gough, Natalie T. Boelman, and John C. Wingfield (2016). Nestling growth rates in relation to food abundance and weather in the Arctic, The Auk: Ornithological Advances 133:261–272 | doi:10.1642/AUK-15–111.1
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Originally published at Forbes on 24 March 2016.