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A recently published study reveals that climate change may cause birds' eggs to hatch early. In addition to creating warmer temperatures that trigger early embryonic development in birds, climate change also increases the frequency and duration of heat waves. Thus, warming temperatures may lead to asynchronous hatching of individual eggs within a clutch and to increased chick mortality, particularly for birds breeding in the tropics and semi-tropics, and in tropical deserts.
Global warming is threatening increasing numbers of plant and animal species with extinction, particularly because it also interacts with other stressors. For example, many plants are unable shift their geographic ranges fast enough to keep up with the changing climate; coral reefs are highly vulnerable to increasing temperatures and ocean acidity; coastal ecosystems are in danger of being inundated by rising sea levels; and polar ecosystems are already liquifying.
When one considers the myriad species that will be lost to climate change, you might think that the least worrisome are birds because they, at least, can fly away. But because they lay eggs that must be maintained within a strict temperature range in order to hatch, birds too, are very vulnerable to global warming.
Some of the best evidence that birds are being affected by climate change is provided by long-term studies of temperate zone insectivorous birds that breed in North America and Europe. These seasonally breeding birds have a very tight schedule: they must coordinate their reproductive efforts to coincide with a brief window of time when populations of their insect prey are approaching their greatest abundances. But research into seasonally breeding birds of temperate regions do not provide much insight into the life histories and coping strategies of birds living or breeding in other types of habitats, especially birds that reside year round in the tropics and subtropics, or in deserts.
To document how climate change may affect desert dwelling birds, a team of scientists based at Macquarie University in Australia designed a study that specifically examined the effects of warming atmospheric temperatures on embryonic development of wild zebra finches, Taeniopygia guttata. A popular pet, avicultural and research subject, the zebra finch is a small hardy songbird that breeds in arid and semi-arid regions throughout Australia where atmospheric temperatures regularly exceed 36◦C (96.8◦F), which is the lower threshold for optimum embryonic development for birds. Like all birds, zebra finches are physiologically constrained to produce only one egg per day, so they rapidly lay a complete clutch of eggs over a short time interval. Yet, despite this time period, all of the eggs hatch on (roughly) the same day.
“Typically, zebra finches will lay one egg a day for about five days, and will only start to incubate their eggs on the day that the last egg is laid”, said the study's first author, ecologist Simon Griffith, leader of the Avian Behavioural Ecology group at Macquarie University.
Eggs laid earlier in the sequence are left unattended in the nest where they are exposed to ambient atmospheric temperatures. Typically, these temperatures are low enough to prevent embryonic development until incubation commences on the day when the last egg is laid.
“This way, all the eggs incubate and hatch at the same time in the nest, making it easier for the parents to feed and look after the chicks evenly”, said Professor Griffith in a statement.
However, if eggs are exposed to temperatures that are warm enough to trigger embryonic development before incubation begins, they will hatch over a period of several days. The first chick to hatch will be able to monopolise the food provided by its parents, potentially reducing the survival of its younger and smaller brothers and sisters.
"If all chicks hatch at the same time, the competition is much more equal," said Professor Griffith.
Zebra finch nests were regularly exposed to extreme atmospheric heat
To identify the daily maximum atmospheric temperature that wild zebra finch nests are exposed to, Professor Griffith and his team obtained temperature records for their study site, Fowlers Gap Arid Zone Research Station, located in far-western New South Wales, Australia. (In Australia, August through until the following March correspond to spring, summer and early autumn.) These records revealed that the maximum daily atmospheric temperature in the shade regularly exceeded the optimal temperature for embryo development (36–40◦C; grey band in figure 1) during a period of eight and a half years (figure 1):
According to these data, during October, which is the peak month of zebra finch breeding at the study site, maximum daily temperatures of more than 36◦C were reached on 17 days out of 286 days (6%) for which temperature data were available. Remarkably, this developmental threshold was reached on 179 out of 307 days during January (58%). Further, atmospheric temperatures regularly exceeded the temperature that is considered lethal to avian eggs (40.5◦C). Such extreme heat days occurred as follows: October 1%; November 16%; December 13%; January 46%; February 21%; March 3%.
Zebra finch eggs were regularly exposed to extreme heat
To test the prediction that warmer nest temperatures trigger early onset of embryo development in the absence of parental influences, Professor Griffith and his team set up "hot" and "cool" experimental nest chambers that mimicked actual nest conditions. The "hot" experimental nest chambers consisted of a natural nest and a nest-box nest placed on stakes in a location that was exposed to the sun throughout the day, whilst the "cool" chambers consisted of a natural nest and a nest-box nest placed on stakes in a fully shaded location under a tree where they were not exposed to any direct sunlight. These experimental chambers were designed to capture the opposite ends of the natural temperature variation for cool and hot nest locations.
A total of 33 eggs from eight clutches were removed from their nests on the day they were laid, marked with an individual number and replaced with a dummy egg made modelling clay. All removed eggs were placed into one of the four experimental nest chambers. Every other egg within a particular clutch was placed into either a "hot" chamber (solid black line; Figure 4) or a "cool" chamber (solid grey line; Figure 4).
Temperatures in the experimental nest chambers were recorded once each hour by an iButton, which is a tiny data logger, placed next to the eggs. These temperatures were compared to atmospheric temperatures (dashed line; Figure 4), also recorded by iButtons placed nearby but outside these nest chambers.
When the last egg of the clutch was laid, all eggs were returned to their parents, who then incubated their experimentally treated clutch until hatching, a period of 14 days. After the eggs had been incubated by the parents for 10 days, the researchers checked the nests each morning and, if hatchlings were present, the time and date were recorded.
"It's hard to think about 40 C [104◦F] when it is 2 C [35.6◦F] here in Milwaukee, but after doing the conversions I was starting to sweat thinking about those poor birds", said ornithologist Peter Dunn, a professor of biological sciences at the University of Wisconsin-Milwaukee, who was not part of this study.
These data (Figure 4) clearly show that the zebra finches in this study were regularly exposed to atmospheric temperatures of more than 36◦C during six months of the year, when breeding is most likely to occur. These data also show that hot nest chambers spent 2.6 more hours at a normal incubation temperature than did cool nest chambers. Hot chambers also spent 3.7 more hours at lethal incubation temperatures (above 40.5◦C) than did cool chambers. In total, temperatures in the hot chambers exceeded the lethal incubation temperature of 40.5◦C for 18% of the experimental period (35 out of 193 hours), and the temperature peaked at 51◦C (figure 4).
Given that information, you may be amazed to learn that all experimentally treated eggs successfully hatched following their return to the parental nest. (Well, I was surprised!)
"I was surprised to see that some of the eggs in the experiment were briefly exposed to 51 C [124◦F], above the supposed lethal temperature, and they still hatched later", agreed Professor Dunn in email. "It may be that these eggs are able to survive as long as the hot periods are short, and, in this experiment, the temperatures did cool down quite a bit at night (to less than 36 C [97◦F], which is the temperature at which eggs can develop without incubation)."
Overall, the average developmental time was 13 days -- shorter for eggs that had been placed in the "hot" nest chambers. Eggs laid early in the laying sequence developed and hatched sooner, although the time to hatching was similar for eggs placed in either the natural nest chambers or in the nest-box nest chambers.
“In the summer, we found that nests can get very hot, with recorded temperatures of up to 50◦C,” said Professor Griffith. "These birds usually like to incubate their eggs in temperatures between 36-40◦C, and there is evidence suggesting that prolonged temperatures of over 40.5◦C can be lethal to the developing chick embryos.”
Global warming points to a troubled future for birds and other wildlife
"This is an interesting study that identifies a potential effect of climate change on birds and broadens our understanding of the ways in which climate change can affect natural populations of animals", said zoologist Janet Gardner, a research fellow in the Division of Evolution, Ecology and Genetics at the Australian National University, who was not part of the study.
"[M]any papers have looked at how desert birds can withstand scorching temperatures", said Professor Dunn. "[B]ut this is one of the first studies to show how small increases in temperature could lead to potentially lethal effects on developing eggs."
Globally warming is not trivial. According to the Intergovernmental Panel on Climate Change (IPCC), global surface temperatures are projected to increase by at least 1.5◦C, and more likely by 2◦C (and perhaps by twice that), by the end of this century.
"It's hard for many people to think about climate change when you are talking about changes of 2 C, but for some species on the edge of their thermal tolerances this could be critical", said Professor Dunn.
IPCC data predict that global warming will increase the frequency and duration of heat waves, too.
"The work is particularly relevant for birds that live in climates that are already hot, given forecasts for ongoing increases in the frequency and duration of heat waves as the climate changes", said Dr Gardner in email.
In fact, these predicted heat waves are already occurring.
"Australia has been experiencing some dramatic heat waves over the last six years, and some of them have led to the deaths of thousands of birds and hundreds of thousands of flying foxes", said Professor Dunn.
The IPCC also predicts that global warming will further decrease annual precipitation in many mid-latitude and subtropical dry regions -- which include the grasslands and deserts where zebra finches and many other Australian birds live.
"I don't think many people realize that small birds can dehydrate and die within a few hours at 45 C[113o F]", Professor Dunn pointed out in email.
"In any case, I don't think the long-term outlook is good for some of these species that live in desert environments, because they need access to water to avoid dehydration", said Professor Dunn.
But global warming will have other, more subtle, effects as well.
“[Global warming] could play havoc with the family dynamic of these birds, creating situations where a single nest contains chicks of different ages, and even causing some embryos to die if the temperatures remain too hot for a long period,” said Professor Griffith.
"Given that birds have been the subject of long-term monitoring at Fowler’s Gap, it would also be interesting to know if the degree of hatching asynchrony in natural (non-manipulated) nests at the site has increased over the last decade(s) when the climate has been changing rapidly, i.e. whether there is any evidence of change already apparent", said Dr Gardner in email.
The authors also note that warmer temperatures might provide some benefits since parent birds would expend less energy actively incubating their eggs, which raises the question; can birds alter their own behaviours quickly enough to meet these rapidly changing conditions?
"[A]s the authors mention, there are many ways in which parents can influence the effects of environment on their offspring; it remains to be seen whether parents can counter these effects of high temperatures by choosing cooler nest sites or changing the construction of the nest itself, increasing its capacity for insulation for example", said Dr Gardner.
“Future work will need to look at whether these birds, and other birds in general, are capable of adjusting to the alterations caused by climate change”, said Professor Griffith.
The Heat Is On: Climate Change Causes Birds To Hatch Early | @GrrlScientist
Source:
Simon C. Griffith, Mark C. Mainwaring, Enrico Sorato and Christa Beckmann (2016). High atmospheric temperatures and ‘ambient incubation’ drive embryonic development and lead to earlier hatching in a passerine bird, Royal Society Open Science 3: 150371 | doi:10.1098/rsos.150371
Additional reading:
Cynthia Rosenzweig, David Karoly, Marta Vicarelli, Peter Neofotis, Qigang Wu, Gino Casassa, Annette Menzel, Terry L. Root, Nicole Estrella, Bernard Seguin, Piotr Tryjanowski, Chunzhen Liu, Samuel Rawlins & Anton Imeson (2008). Attributing physical and biological impacts to anthropogenic climate change, Nature 453, 353-357 | doi:10.1038/nature06937
