When Timing is Everything: Migratory Bird Phenology in a Changing Climate
The timing of key life events (phenology) is a critical part of nearly every important ecological relationship. Nowhere is this more evident than in the annual cycle of migratory birds: bird migration, breeding, and nesting are timed every spring to coincide with the peak availability of critical food sources in a delicate synchronization that occurs across large latitudinal gradients and diverse habitats. This synchrony between birds and key resources helps to ensure that birds survive migration and successfully reproduce.
Of the many ways in which climate change affects wildlife, changes in phenology is one of the most ubiquitous. As temperatures warm and precipitation patterns change, many species of plants, insects, and birds have advanced important phenological events. Plants are putting out leaves earlier, insects are emerging sooner, and many birds have advanced the timing of their migration. These changes have been observed for many decades and across different habitat types, although impacts vary between species.
While the phenology of birds, their habitats, and their food sources are all generally advancing in response to climate change, they are not always doing so at the same rate. Birds and the species with which they interact (including their food sources, predators, and competitors) are not equally responsive to warming; climate change therefore has the potential to desynchronize these critical relationships. Generally, studies have found that species higher in the food chain (like birds) have advanced their phenology less than species lower in the food chain (like plants and insects). The great tit, for example, is a European songbird that relies on a short burst of caterpillar availability each spring to feed its young. Over the past decades, temperatures have warmed and caterpillars are consistently emerging earlier. The great tit, however, has not advanced its egg-laying date as fast as the caterpillar has advanced its peak biomass date, and so many young nestlings are born too late to benefit from the short caterpillar supply. This type of phenological mismatch could have serious demographic consequences for migratory birds, and could ultimately cause a decline in population levels (although many questions remain about the magnitude and ultimate impact of these changes). As with many aspects of climate change, phenological shifts will not be universally negative for all species: some species, for example, may benefit from an extended breeding season.
Developing effective management responses to phenological shifts is challenging, because managers have no direct control over phenology. Yet habitat managers can make modifications to maintain diverse habitats in the face of phenological changes. For example, by taking management actions that favor a range of plants with different phenologies, managers can “hedge their bets” and prepare for multiple potential responses to change. Maintaining diverse yet connected habitats can help migratory birds take advantage of different resources and phenological responses.
Citizen scientists and biologists can help track the phenology of birds by signing up to participate in the USGS-led Nature’s Notebook, which uses phenological information to help improve our understanding of how variations in climate affect the natural world, including birds.
As part of a broader effort to develop a management-relevant research program on climate change and migratory birds, NCCWSC is working directly with wildlife and land management agencies (including federal and state agencies, as well as non-governmental organizations that manage important bird habitat) to identify important research questions. Climate-induced changes in phenology have important implications for a range of management activities, including habitat and vegetation management, invasive species control, and prioritization of species based on differing vulnerability to climate impacts. Ultimately, management strategies informed by phenological research can provide for more effective migratory bird conservation in a changing climate.
Madeleine Rubenstein is a biologist with the U.S. Geological Survey’s National Climate Change and Wildlife Science Center. Her research examines how climate change affects migratory birds, with an emphasis on understanding and responding to the information needs of wildlife and habitat managers.
Photo: Hermit thrush (migratory songbird) with nestlings. Tom Martin, USGS