A few days ago Jerry Farrell told me about a personal experience that shocked him. It startled me as well.
For many years Jerry has been raising pheasants for release. In the past, he told me, without exception his hens began egg laying in late March or early April. But this year he found egg clutches at the beginning of February, a temporal difference of about two months.
All my life, he went on, I have believed in photoperiodism as the driving force behind this physiological change; now I have to rethink my assumptions. That technical term, photoperiodism, refers to day length. We know that spring is coming when the days get longer. So too do those hen pheasants.
I know what you are thinking: the early laying was simply due to our extremely mild winter. That response seems quite reasonable until you think about it. If mildness establishes those laying dates, why didn't they change earlier or later during other less extreme years? The driving forces behind physical changes like egg laying are surely nurtured over millennia and are deeply buried in those hen's evolution-established drives.
Less striking but equally important evidence of change has been presented in a technical paper published in the journal, The Condor, by University at Buffalo professor Bob DeLeon and his daughter Emma, a faculty member at Louisiana State University.
The DeLeons examined the spring migration records of 93 bird species gathered over a 42-year period by the Buffalo Ornithological Society. The birds included not just species like robins and red-winged blackbirds but also shorebirds like killdeer and woodcock, raptors like broad-winged hawk and osprey, waterfowl like blue-winged teal, marsh birds like Virginia rail and green heron as well as common terns and ruby-throated hummingbirds.
Those BOS migration records are carefully monitored by a statistics committee. Verification reports are required for birds that appear outside the normal range of migration dates. Members are provided a Verification Date Guide that is updated about every ten years, the most recent edition by a committee led by Bob Suggs.
The DeLeons' paper is highly technical, full of statistical terms like regression lines, standard errors and p-scores, and there is variation among the arrival dates. Many birds appeared earlier but a quarter of them appeared later.
The overall finding, however, is very clear. Through the 42 years the average arrival date for those bird migrants is one day earlier per decade. In other words, over that period those migrants are appearing here more than four days earlier.
The DeLeons went further, separating the migrants into short-distance and long-distance groups. Both groups fled south each winter, but the short-distance migrants stayed within North America while the long-distance migrants went on to Central or South America or the West Indies. Among their short-distance migrants are turkey vultures and phoebes while their long-distance migrants include wood thrushes and kingbirds.
Their findings: the short-distance migrants averaged over six days earlier, the long-distance migrants only 2 1/2 days earlier. The average arrival dates for those two groups also differed strikingly. The average date for short-distance migrants to appear was April 14 and, as you might expect since they had farther to travel, the long-distance fliers' average arrival was May 3.
The DeLeons carried out another analysis as well. They compared their data with spring temperature records from Houston, Texas. This confirmed what we should also expect: during warmer years the birds migrated earlier, colder years later than average.
Are the findings of the DeLeons consistent with those of others? Indeed they are. They sought out other stations where long-term migration records were amassed, finding them in Manitoba, Massachusetts, Minnesota, South Dakota and Pennsylvania. Those records showed general agreement with the DeLeons' findings.
Is there a cause for these results? Their paper offers among its conclusions, "These results are consistent with the hypothesis that climate change has a strong influence on the phenology of bird migration."
My sense is that indeed the evidence for global warming is piling up. And our non-winter offered further evidence, not as what will happen next year and the years following but as fulfillment of another prediction associated with climate change: wide swings in weather as the temperature average more slowly creeps up.-- Gerry Rising