Shifting Cues: As seasonal events shift with climate change, how nature responds tells us about the resilience of our ecosystems
Sara Kaiser, Ph.D.
The signs of spring in the Northeast are unmistakable: snow melt, blossoms on flowering trees and plants, the emergence of hibernating animals, and the arrival of migratory birds and butterflies. As spring temperatures warm each year, nature reawakens.
These events in the spring are examples of natural phenomena that follow seasonal patterns. However, climate change is altering seasonal patterns, shifting some events relative to others. The result is a disruption of the predictable cycles under which most species evolved and the possibility of mismatches in timing, such as moths hatching before budburst and birds breeding outside the window of peak food abundance. How will plants and animals respond? What can we learn about their responses to understand the resilience of our ecosystems to ongoing environmental change?
These are the types of questions that inspire my work as a research ecologist at the Cornell Lab of Ornithology. Together with my colleagues from the Smithsonian Migratory Bird Center, Vermont Center for Ecostudies, Dartmouth College, and Wellesley College, we are building on a long-term study of the black-throated blue warbler to understand how migratory songbirds respond to changing seasonality in the Northeast.
We are particularly interested in whether a longer summer growing season in northeastern forests—caused by earlier spring leaf-out dates and later autumn leaf-off dates—leads to an extended breeding season for migratory songbirds. And if so, what that means for the timing of other important late seasonal events for these birds, like molting (replacing all flight feathers) and departing on autumn migration back to their overwintering grounds. Longer growing seasons could benefit migratory bird populations if individuals are able to raise two broods instead of only one. But these efforts may have tradeoffs, like reduced energy and time on the breeding grounds to molt and prepare for migration. Birds could overlap molt and breeding, delay molt and molt faster, or depart later on autumn migration. However, overlapping molt with breeding can lead to reduced parental care, especially during the post-fledging period—the period between leaving the nest and independence—and lower offspring quality and juvenile survival. Molting faster can reduce feather quality and affect foraging, flight performance, survival on migration, and success in finding a mate in the following breeding season. Departing later on autumn migration might affect migratory routes and migration rate—a time period when these birds already have the highest mortality.
Understanding the impacts of changes in seasonality on plant and animal populations and communities requires long-term research. Over the past 40 years, we have studied the black-throated blue warbler at the Hubbard Brook Experimental Forest in New Hampshire, one of the longest running and most intensive studies of a migratory songbird in the world. Every summer, we follow breeding pairs and collect information on their reproduction and survival and monitor changes in behaviors, such as the timing of spring arrival and breeding. Black-throated blue warblers breed in large forest tracts throughout the Northeast and southern Canada, and at higher elevations of the Appalachian Mountains. Most individuals migrate along the Atlantic seaboard, from the Appalachians to the Atlantic coast, and overwinter in forests of the Greater Antilles. The black-throated blue warbler is one of the few migratory songbirds that has been studied throughout their annual cycle. Most of what we have learned about this species’ response to changing seasonality comes from our long-term bird research at Hubbard Brook.
Black-throated blue warblers leave their winter homes in the Caribbean based on the seasonal shift in daylength, a cue unaltered by climate change. To successfully find a mate and raise offspring, they must arrive at their breeding grounds in the north to take advantage of insects such as caterpillars that emerge with seasonal vegetation growth. Males generally arrive to Hubbard Brook in early May, one week in advance of females, and establish and defend their breeding territories from competing males. Once females pair with males, they begin building open cup nests in the forest understory. Females initiate egg laying in mid to late May and lay one egg per day for 3-4 days. On the last day an egg is laid, females alone incubate their clutch of eggs and continue sitting on their eggs for 12 days. When their young hatch, both parents take turns visiting the nest to feed their nestlings, mostly caterpillars, spiders and flying insects, until the offspring fledge the nest 9 days later. Although we know less about the post-fledging period, females and males continue to provide parental care to dependent fledglings for several weeks. At Hubbard Brook, one-third of the pairs in the population will attempt second broods after raising first broods depending on whether insects are abundant late in the breeding season.
The timing of these important seasonal events for migratory songbirds have shifted with climate change.
Long-term temperature records from Hubbard Brook show clear changes in climate since the mid 1950s. Each decade, the mean annual air temperature has increased by 0.25°C, affecting the phenology of deciduous trees. Phenology is the study of periodic events in the life cycles of plants and animals in relation to environmental drivers such as climate. Deciduous tree leaf phenology is mostly governed by temperature. Leaf-out in spring is dictated by warming temperatures while cooling temperatures in the autumn activates leaf-off. Since the research on black-throated blue warblers began in the 1980s, leaf-out dates at the Hubbard Brook Experimental Forest have advanced 2 days per decade (8 days) in response to increasing spring temperatures. Autumn leaf-off has occurred 3 days later per decade (11 days), resulting in a 19-day extension of the growing—or “green”—season.
As the green season lengthens, the availability of food for migratory songbirds to raise their young, early and late in the breeding season, becomes an important question. We have some evidence that warmer spring temperatures are associated with the earlier emergence and spring migration of insects. Likewise, warmer autumn temperatures are associated with additional insect generations late in the growing season and the delayed autumn migration of insects and insect diapause, when insects suspend their development.
In response to changes during spring, we have learned that black-throated blue warblers exhibit flexibility in settlement patterns and the timing of key behaviors early in the breeding season. Among the mountainous topography of the Hubbard Brook Experimental Forest, leaf-out in spring is delayed three days for every 100-m increase in elevation. During warm springs, black-throated blue warblers establish breeding territories at higher elevations but settle at lower elevations in cool springs when leaf-out is delayed. The timing of spring arrival to the breeding grounds and the timing of breeding have become progressively earlier over the last two decades, both behaviors influenced by warming spring temperatures and the timing of spring leaf-out.
Nesting black-throated blue warblers have appropriately tracked yearly variation in spring leaf-out, resulting in more offspring produced each year and population growth. Adjustments in the timing of arrival appears to be less flexible than the timing of breeding. This means that warblers respond to early springs mainly by reducing the interval between arrival and breeding. If the timing of arrival is approaching its limit in flexibility, earlier breeding could come with a cost because birds need time to recover from the fatigue of migration and restore their energy reserves in preparation for breeding.
The timing of breeding of black-throated blue warblers is not mismatched with the timing of peak caterpillar and insect emergence. At Hubbard Brook, the seasonal pattern of insect availability for breeding birds is highly variable throughout the breeding season with a shifting pool of insects with diverse life histories and larval feeding times. Long-term insect sampling has shown no consistent seasonal peak in biomass across years for caterpillars or flying insects.
Seasonal changes in autumn, such as cooling temperatures, shorter days, and the timing of leaf-off, are also important because these cues signal the end of the growing and breeding seasons for most plants and animals in the north. But for black-throated blue warblers and other migratory songbirds, the effects of shifting autumns on the timing of late seasonal events such as molt and autumn migration, is poorly understood. We expect that autumn leaf-off likely provides similar transitional cues as spring leaf-out for migratory songbird populations to shift between phases of their annual cycle. However, autumn has been relatively neglected in climate change research despite evidence of change.
The long-term study of the black-throated blue warbler at Hubbard Brook and associated environmental data on climate, leaf phenology, and insects has taught us about how these birds have adjusted their timing of arrival and onset of breeding to shifting springs. It also provides a strong foundation against which to evaluate shifts in the duration of breeding and onset of molt and migration in response to ongoing change in the autumn. Understanding their responses will be key to assessing the resilience of migratory songbird populations to climate change.
I thank Sarah Garlick, Director of Science Policy and Outreach at the Hubbard Brook Research Foundation, for helpful insights on earlier drafts of this article.
Dr. Sara Kaiser is a Research Ecologist at the Cornell Lab of Ornithology and Director of the Hubbard Brook Field Ornithology Program. Her training includes a Ph.D. from Cornell University in behavioral ecology, an interdisciplinary M.S. from Michigan State University in ecology, evolutionary biology and behavior, and a B.S. from Iowa State University in zoology. She conducted postdoctoral research at the Cornell Lab of Ornithology, Smithsonian Migratory Bird Center, and Smithsonian Center for Conservation Genomics.