Right Place, Wrong Time

Raptors undertake some of the most spectacular migrations in nature—but when and
where they journey is changing as warmer temperatures alter prey behavior.

By Laura Booth. Photographs by Anna Fryjoff-Hung.

“Excuse me,” a newcomer to Hawk Hill gently interrupted me as I scanned the skies over the Golden Gate Bridge, the city of San Francisco, and the sculpted hills of the Marin Headlands.

“What are you looking for?,” she asked.

For migrating birds of prey, of course. On a brisk day in early December, I was looking for raptors, alongside a team of citizen scientists, other interns, and a few full-time staff at the Golden Gate Raptor Observatory’s, or GGRO’s, annual hawk count.

From the summit of Hawk Hill, as our vantage point is called, the scientists and I watched 19 of North America’s raptorial representatives pass by, from bite-sized American Kestrels to majestic Ferruginous Hawks.

This hawk count has taken place every year since 1986. It's a daily event from mid-August to early December. Our schedule depends on the raptors': the peak of the fall migration typically occurs from late September to early October.

But hawk watchers might have to adjust their clocks. Raptors are shifting the timing of their migrations in response to climate change. Predictable migration peaks may become a phenomenon of the past. 

For raptors, migration and nesting recur in cycles cued by their prey: the raptors time their lives by the abundance of food in spring, and its the death or hibernation of that food in winter.

Quite apart from birders’ delight or displeasure, these cycles of death and rebirth are key to maintaining landscapes in the forms we recognize. For example, research in great tits (Parus major) has shown that these pretty, white-cheeked passerines use temperature as a cue to keep their reproductive efforts in synchrony with their prey: caterpillars.

This synchrony is vital to tit populations. But, the temperature cues that birds the tits use to decide when to nest—so that their offspring can fatten up on caterpillars—are changing with climate. If the rhythm’s thrown off, baby birds could starve en masse.

Scientists still don’t know if warming climate will disrupt birds’ delicate rhythms, but the possibility is especially worrying for avian biologists working on species higher in the food chain. Birds of prey, for example, keep prey populations in check and act as indicators of problems lower in the food web.

At the Raptor Research Foundation’s recent meeting in Sacramento, California, the organizers arranged a symposium—the first of its kind—to discuss the effects of climate change on birds of prey.

Researchers agreed that predators are shifting the timing of life events, such as nesting and migration, in response to big, climate-driven changes lower in the food chain.

For example, Dr. Julie Heath had predicted that American Kestrels, a species of falcon in Idaho, would migrate shorter distances later in the season due to warming temperatures, and that’s exactly what she found. Unusually warm winter temperatures mean there's more to eat, and more food means less pressure to migrate in the first place. Heath looked at one population of one species in one location—that's not enough to draw nuanced conclusions about all falcons’ responses to climate change, but it's a start.

But the fine-scale, long-term data required to make informed predictions such as Heath’s are few and far between. Dr. Gerald Niemi and Dr. J.F. Therrien both analyzed multi-decadal data from North American hawk watch sites to assess whether shifts in migration timing have occurred since monitoring began. Niemi found that Sharp-shinned Hawks, Turkey Vultures, American Kestrels, and Merlins had all pushed their migration back by about a week and Golden Eagles by nine to 10 days, whereas Osprey and Bald Eagles seem, confusingly, to have shifted earlier. Likewise, Therrien and his colleagues found a similar pattern: over the last 30 years, 12 species of raptor have delayed their migrations by an average of one day per decade..

Niemi and his colleagues didn’t correlate the migratory changes with temperature or other climate variables. They simply looked for changes in the context of a warming world. Therrien and his co-authors found a correlation between average temperature increases and migration passage date, while acknowledging the role that species-specific life history traits, such as average migration distance, could also be playing a role.

Raptor biologists are quick to cite the impressive resilience of their study subjects, who continue to withstand threats, both natural (low prey availability, ectoparasites) and man-made (pesticide poisoning , collisions with wind turbines).

But the speed of climate change, which is operating on faster-than-evolutionary timescales, appears to pose the most immediate risk to the seasonal rhythms that keep raptors in sync with their prey. Whether the raptors can adapt fast enough to keep pace with the changes aggregating at lower trophic levels —as delays in migration and advancements in nesting suggest— is the question that raptor biologists are still struggling to answer.