The American Revolution’s first “shots heard round the world” were fired on April 18, 1775, in Concord, Massachusetts. It’s also where the first “budburst” of shrub and tree leaves has advanced from early-May to mid-April, raising concerns that the egg-laying schedules of many migratory birds no longer match the optimal availability of caterpillars to feed their young.
Henry David Thoreau, a Concord philosopher and amateur scientist, was the first to record the leaf-out dates of Concord’s shrubs and trees in the 1850s. Scientists at Boston University (BU) are repeating Thoreau’s observations and are surprised at the early leaf-emergence schedules of lowbush blueberry, red maple, red oak and 41 other plants.
“Over the past 160 years, the mean date of leaf emergence has shifted from May 8 in Thoreau’s time to April 20 in recent years,” wrote BU biologists Richard Primack and Amanda Gallinat in the March-April 2016 issue of American Scientist. “Eighteen days may not seem like a whole lot, but it can make a big difference for other plants and animals that depend on the appearance of these leaves, and whose schedules may not have shifted in the same way.”
Primack said that on average, birds migrating to the Concord woods during Thoreau’s life arrived four days before emergence of leaves, but now arrive 10 days after the leaves emerge. He said the mismatch will reduce the productivity of many bird species.
Migratory birds synchronize their egg-laying to coincide with the emergence of caterpillars and other insects that nibble nutritious, newly emerged leaves. Timing is everything. Birds with the fewest miles to migrate are better able to adjust egg-laying times to earlier springs caused by warmer temperatures.
Northern Hemisphere birds that winter the farthest south are less able to advance their migratory schedules to match local conditions.
In a study of 100 European migratory birds, the species that declined over a 10-year period ending in 2000 did not advance their spring migration to match the earlier emergence of leaves in spring. “Whereas those with stable or increasing populations advanced their migration considerably,” wrote French, Italian and Finnish researchers in a 2008 paper in the journal Proceedings of the National Academy of Sciences of the U.S. “We predict that, under current climate change scenarios, species with a threatened population status and declining breeding populations will suffer further losses.”
The Black-throated Blue Warbler, a beautiful songbird of the American Northeast, seems to be a winner in the early-spring scenario unfolding in Massachusetts and surrounding states. A recent study found that female Black-throated Blue Warblers advanced their average date of egg-laying by about six days in the Hubbard Brook Experimental Forest in New Hampshire while leaf-emergence there had advanced 10 days.
“We think the benefit of breeding earlier in early springs is that there is more time for re-nesting if the first nest fails for some reason,” said Nina Lany, a postdoctoral research fellow at Michigan State University and the lead author of the warbler study published in 2013 in the journal PLoS ONE. “However, if leaf-out continues to advance, the downside for migratory birds might be that they won’t arrive on the breeding grounds in time to adjust breeding as they have in the past.”