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Signals Detected in PRBO's Long-term Data Sets

Fingerprints of Climate Change

Nat Seavy


Warbling Vireo is one of the neotropical migratory bird species arriving earlier in spring at PRBO's Palomarin Field Station. Tom Grey photo.
Our lives are based on events that recur with predictable frequency, from the daily start of our work day to the yearly observance of a holiday or anniversary. We rely on signals that alert us that such events are coming, and if these signals fail, our lives can become disconnected from events unfolding around us.

Birds and other organisms depend on signals too, helping synchronize their behavior with seasonal patterns in the environment. Recently, changes in the timing of biological events have provided the first evidence that climate change is affecting complex ecological interactions. Understanding these changes requires data collected consistently over long periods of time. PRBO's commitment to long-term monitoring enables our biologists now to identify fingerprints of climate change on the seasonal patterns of birds we study.

When Ryan DiGaudio arrived as an intern at PRBO in 1996, he had no idea that he would become focused on how climate change can influence the timing of movements by migratory birds. "We were only beginning to realize that climate change might have such an effect on birds' seasonal patterns," explains Ryan. "Then biologists suddenly recognized that long-term data, collected with standardized methods, were going to be extremely valuable in a way we hadn't expected." In 2003, Ryan completed a master's degree, advised by Terry Root, then at the University of Michigan. For his thesis, he showed that long-term monitoring data, collected at PRBO's Palomarin Field Station, could illuminate changes in the arrival dates of migratory birds that might be responding to climate change.

Understanding the effects of climate change on biological systems has been a major focus of Terry Root's research. Recently, she and Stanford graduate student Dena MacMynowski collaborated with PRBO biologists Grant Ballard and Geoff Geupel to investigate changes in bird arrival times at Palomarin and other sites in California. Building upon Ryan DiGaudio's earlier work, they found that a majority of neotropical migratory birds are arriving earlier in the spring. One consequence could be a mismatch in a bird's nesting cycle with the availability of food for its young. The pattern at Palomarin is consistent with signals that Root and her colleagues have detected in ecological systems throughout the world. More recent analysis of PRBO's long-term data has shown that the story may be complex: some species are arriving earlier in the spring at Palomarin, others later. Nature never gives simple answers!

Sea change

Effects of climate change on seasonal patterns also appear in our oceans. For over 40 years, PRBO biologists have been recording the breeding behavior of seabirds on the Farallon Islands, 27 miles offshore of San Francisco. This work has demonstrated important links between ocean conditions and the timing and success of reproductive efforts by many seabirds. One seasonal event whose timing is crucial is upwelling, when springtime winds bring nutrient-rich waters to the surface, spurring production of food for breeding seabirds. "It appears that upwelling begins about 20 days earlier than it did when we began our monitoring program," notes PRBO biologist Jen Roth. "As yet, we don't know how this will change the ocean food web and how the breeding seabirds will respond."
A Common Murre feeds its chick. Ron LeValley photo.

PRBO marine ecologists have been investigating the timing of nesting by Common Murres, one of the most abundant nesting seabirds on the Farallones. While it appears that Common Murres may be nesting slightly earlier in the spring, this shift is not as rapid as the shift in the ocean transition. Says Jen Roth, "We need to understand how the growing mismatch between ocean upwelling and the timing of Common Murre nesting is going to affect the future of this population." In the past, Common Murres typically fledged more young in years when nesting was initiated early in the season, but today there are signs that the number of young fledged is declining even though the murres are initiating nesting somewhat earlier. "It's possible that food-web disruptions may occur when upwelling happens too early," explains Jen, "and this may compromise the birds' abilities to feed their nestlings later in the year."

These shifts in the timing of migration and reproduction are relatively subtle, and we do not yet understand how they will affect birds' abilities to obtain the resources they need. In other cases, fingerprints of climate change may be dramatic and have immediate, obvious consequences.
Two Adélie Penguins interact in their colony.

In the Antarctic, PRBO biologist Grant Ballard studies the population dynamics of Adélie Penguins. "Over the last 30 years in the Ross Sea, we've seen more and more pack ice generated by strong winds, which is generally good for the ecosystem upon which this species depends for its survival" he explains. "At the same time, farther north, around the Antarctic Peninsula, pack ice is vanishing quickly. Both of these changes are consistent with what climate models project for warming in Antarctica." The problem is twofold: when there is no ice, there are no penguins; where there is too much ice, penguins have difficulty accessing their colonies and foraging locations. With an extremely short season for potential breeding, and with very limited ice-free terrain for their colonies, penguin populations will likely face serious challenges as patterns of ice distribution in the Antarctic change.

Climate change is not new to the Earth or its animals. Using information from glacial ice and lake sediments, climatologists recognize the fingerprints of profound climate change in the past. But climate change today is occurring much more rapidly, and this places a premium on understanding its signals in nature. "We know that we can see the fingerprints of climate change in the data that PRBO collects," observes Mark Herzog, co-director of PRBO's Informatics Division and a contributor to our Climate Change Initiative. "Our challenge now is to use this information to track bird responses to climate change—and use what we learn to conserve birds and their habitats."