Several people in the first half of May told me about male Scarlet Tanagers in their breeding finery visiting their sunflower seed feeders. These birds usually glean caterpillars and other insects from leaves in the upper part of the forest canopy. They rarely visit feeders in June and July.
What’s going on? The explanation is that caterpillars are hard to find in early May so the tanagers need to find alternative food until the trees fully leaf out with the insect herbivores following thereafter.
These observations point to a more general point about spring migration. Males are on the horns of a dilemma. A male needs to arrive as soon as possible to stake out a good territory. We know there are satellite males around in the summer, looking to sneakily mate with some other male’s mate. Arriving early is a way to guarantee one of the limited nesting territories.
But the downside of arriving early is that there may be insufficient food to allow an eager male to survive long enough to mate. So, males need to find an arrival time that is just right, balancing the competing pressure of having enough food and securing a territory.
How do migrants know when to arrive? The answer is complicated and requires that we distinguish between short-distance migrants and long-distance of neotropical migrants.
Our short-distance migrants overwinter south of us in North America. Yellow-rumped Warblers are abundant in the winter along the coast of the mid-Atlantic and southern states. Red-winged Blackbirds winter broadly across the southern half of the United States. Some of our American Robins may migrate only as far south as Massachusetts to spend the winter.
Using daily temperature records for April and May from the National Oceanographic and Atmospheric Administration, I looked for broad-scale patterns of temperature variation. Does a warm April in New Jersey predict a warm April in Maine?
I found that the states between Maine and Delaware behaved as a unit. All points within this broad area showed correlated warm or cool springs in a given year.
These patterns provide a useful cue for the spring migration of birds. A bird in Pennsylvania may arrive when insects are hard to find so delaying a few days is a good strategy. By following spring north, birds will arrive on their breeding grounds when food is starting to become adequate.
Long-distance migrants, on the other hand, have no easy way to know what the weather is like in North America. Ruby-throated Hummingbirds in Costa Rica, Bobolinks in Argentina or Blackburnian Warblers in Ecuador rely on changes in daylength.
Different species of neotropical migrants arrive on different schedules so the critical daylength varies. Since day lengths at this time of year are growing shorter south of the equator, Bobolinks respond to decreasing daylength.
Once the critical photoperiod is reached, migratory birds manifest a behavior called migratory restlessness. This behavior was first documented in captive birds who repeatedly tried to move in a northward direction.
The reliance of short-term migrants on current weather conditions versus the reliance of neotropical migrants on photoperiod has some important implications for climate change. One study has shown that short-term migrants are arriving now about 13 days earlier on average than they did from 1950-1999. Long-distance migrants are arriving only about four days early.
The threat that looms for long-distance migrants is that a variety of biological events like leaf-out, caterpillar emergence and flowering are occurring earlier. So, the photoperiod cue for migration causes neotropical migrants to arrive after the peaks of these biological events. These birds are arriving late to the party when most of the food is gone. The most striking example of this asynchrony of events involves the Pied Flycatcher in Europe. These birds winter in Africa and breed broadly across Eurasia. The adults feed their nestlings caterpillars. Earlier leaf-out and caterpillar emergence means that the flycatchers arrive to find caterpillars hard to find. The pied flycatcher population in Europe has declined by 44% in the past 22 years.