by Ron Johnson
An almost forgotten, overstuffed file cabinet together with weather station thermometers that had been running since the 1880s, exposed the reality of migrating ruby-throats and the mystery of what they might find in the spring welcome-mat of flowers and small insects.
Warming temperatures have brought earlier flowers, insects, and migratory birds in some places, but the timings vary because different organisms respond to different environmental cues. Local temperatures may be a key factor for some plants and insects but migratory birds are influenced by photoperiod (length of daylight) and conditions where they winter and migrate. One question is how the timing fits together for birds and the services they provide such as pollination or pest insect suppression.
Jason Courter and others evaluated the timing of spring migrations for Ruby-throated Hummingbirds using about 40,000 observations reported by networks of “citizen science” volunteers who contributed from across the eastern U.S where the birds are found. Enter the shabby old file cabinet with historical first-arrival dates (1880–1969), which the North American Bird Phenology Program recognized as a scientific fortune in historical, hand-written, post-card records. More recent arrival dates (2001–2010) came from Journey North, hummingbird.net, and an army of present-day citizen scientists who report their observations each year. The large number of observations allowed comparisons of historical to current arrivals from southern to northern portions of the U.S.
Hummingbirds inspire and fascinate us. They also pollinate about 19 species of plants and consume small insects and spiders. Although weighing only 3-4 grams (less than a nickel), ruby-throats migrate from wintering grounds in Central America to breeding areas in eastern North America, typically flying across the Gulf of Mexico each way.
The research found that Ruby-throats arrived earlier in the more recent period throughout the eastern United States but these advances varied by latitude from 11.4 to 18.2 days, with less pronounced changes in more northern states above about 41°N. At mid to southern locations in their North American breeding grounds, warmer winter and spring temperatures correlated with earlier arrivals in the more recent period. Surprisingly, warmer winters and springs at more northern latitudes correlated with later arrivals in relation to conditions. This fascinating result may indicate extended migratory stopovers below about 40°N during these years. Moreover, ruby-throats seem to be responding more to local weather variables during migration in the recent than in the historical period.
The reason for the delay below about 41 N is unclear but may relate to food or foraging opportunities or effects of winter or spring temperatures on plants or other organisms that ruby-throats rely on. For example, we know that some plants, such as our South Carolina peaches, respond best in spring if they have sufficient winter cold to meet dormancy requirements. Some plants that ruby-throats need may also have winter dormancy requirements. Hummingbirds may also have an innate reluctance to venture into more northern areas too early, even though conditions seem suitable, because unexpected northern cold fronts could affect their survival.
The finding of relatively later arrivals in northern areas when winter and spring conditions were actually warmer, signals potential for mismatches between ruby-throats and the flowers or small insects that they rely on. These results also raise questions about other bird species, especially long-distance migrants, and whether climate trends might over time affect pollination or insect pest suppression services.
We are indebted to the early naturalists who recorded migratory bird arrivals onto cards and sent each one through the mail, eventually to reach that old file cabinet. We are also grateful for today’s observers or citizen scientists who report similar observations unfailingly each year, typically with a computer. The foresight of so many people is becoming a database that will hold stories to come of birds in a changing world.
Bibliographic citation: Courter JR, RJ Johnson, KG Hubbard, WC Bridges. 2013. Assessing migration of Ruby-throated Hummingbirds at broad temporal and spatial scales. Auk 130:1-11
The following organizations helped provide the data for this research. Find out how you can contribute by visiting their websites.
For media inquiries, please contact Brian Mullen at firstname.lastname@example.org or 864-656-2063.
Links for further reading:
The Wall Street Journal: “A Science of Signs of Spring”
The Associated Press: “Study: Hummingbirds migrating earlier in spring”
The following major media outlets published the story by The Associated Press:
The New York Times: “Study: Hummingbirds Migrating Earlier in Spring”
U.S. News and World Report: “Study: Hummingbirds migrating earlier in spring”
The Guardian: “Study: Hummingbirds migrating earlier in spring”
The Weather Channel: “Hummingbirds Migrating Earlier in Spring”
The Australian: “Climate change implicated as hummingbirds migrate earlier”
National Public Radio: “Study: Hummingbirds Migrating Earlier In Spring”
The Post and Courier: “Hummingbirds arriving in North America earlier each spring than in the past”
The Seattle Times: “Study: Hummingbirds migrating earlier in spring”
Houston Chronicle: “Study: Hummingbirds migrating earlier in spring”
Yahoo! News: “Study: Hummingbirds migrating earlier in spring”
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