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Understanding How a Dynamic Bird Population Responds to Changing Weather Patterns

Song Sparrows, Climate, and Change

Nat Seavy with Kristen Dybala

Song Sparrow. Photo by Len Blumin.
When biologists use distribution models to predict where birds might move under future climatic conditions, they assume that where a species occurs today represents the entire range of climatic conditions in which the population can persist or grow. What if species are actually able to tolerate a wider range of conditions and are limited instead by habitat loss or competition from other species? Then our projections about future bird distributions could be inaccurate. One way to reduce this uncertainty is to examine how climatic conditions influence individual birds' nesting success and survival. This knowledge can complement distribution modeling in important ways.

Discerning the influence of climate on bird survival, reproduction, and movement calls for detailed data collected over long time periods. An outstanding source of such data comes from PRBO's Palomarin Field Station, where biologists have documented local birds' survival and reproductive success since 1966. Today, Kristen Dybala, a graduate student at University of California, Davis, is using this information to closely examine how climate change can influence coastal Song Sparrow populations.
Monitoring nesting birds near Palomarin Field Station. PRBO photo.

PRBO has already shown that climate does affect this species at Palomarin. Using the historical data, PRBO quantitative ecologist Nadav Nur worked with research associate Mary Chase to show that more Song Sparrow young are produced in cool, wet years than in warm, dry years. In wet years, sparrows may have a longer nesting season and, apparently, lose fewer nests to predators.

Now Kristen Dybala, with the use of tiny radio tags, is tracking juvenile Song Sparrows to examine how climate influences the survival of individual young birds after they leave the nest. She is also using the historical data to examine the survival of adults through the breeding season, fall, and winter. "The goal," says Kristen, " is to discern the present-day role of climate in determining whether the Song Sparrow population is growing—and how this population may respond as climatic conditions change."
Kristen Dybala. PRBO photo.

Results of Kristen's study will refine our view of just how climate determines where a population can grow. By knowing how nest success and survival will change above a certain temperature, or below a certain amount of rainfall, we can better understand the full range of conditions the population might withstand. This information can help make projections from distribution models much more accurate.

Currently, climate models suggest an unfamiliar future at Palomarin, with much warmer temperatures but relatively little change in annual rainfall. "The problem is that we've never seen a year with that combination of rainfall and temperature, so when we project how the reproductive success of Song Sparrows will change, we're making a prediction for a set of conditions that we've never observed," explains Kristen. Situations like this, where future conditions fall outside the range of present ones, represent some of the greatest challenges for projecting the effects of climate change.

Distribution models represent a "top-down" approach that yields broad generalizations across large spatial extents (regions, entire states, continents, or even the world). Detailed studies of reproductive success and survival represent a "bottom-up" approach, examining how local populations will respond to climate change. By combining these two approaches, we can reduce the uncertainty in our model projections and ensure that we are providing the best information to guide future conservation and management.