What was done
In studying the skylark (Alauda arvensis) -- which is migratory in Eastern and Northern Europe, but resident in areas with milder winters (such as the UK) -- the authors analyzed the dates of their spring arrival in the Volga-Kama region of the Tatarstan Republic of Russia, using "one of the longest extant records of bird migration in the world," which runs from 1811 to 2008, but with sporadic missing years in the first half of the record.

What was learned
The three researchers report that March temperatures rose about 5.2°C over the duration of their study period, but that 3.7°C (~70%) of that warming occurred over the final three decades of the nearly two-century-long interval. In addition, they found there was a corresponding inflection point in the rate-of-advancement of skylark spring arrival-date, after which the rate-of-advancement of the migrant birds' arrival-date was approximately 15 times greater (0.368 day per year) than it was over the study period's first 17 decades (0.024 day per year). Then, plotting arrival-date as a function of temperature, they found the inflection point occurred at a mean March temperature of -3.5°C, suggesting a mean rate-of-advancement of spring arrival-date of 0.5 day per °C for mean March temperature increases below a negative 3.5°C, and a mean rate-of-advancement of arrival-date of 3.4 days per °C for mean March temperature increases above -3.5°C, which represents close to a seven-fold increase in the rate-of-advancement of spring arrival-date upon exceeding this biological "tipping point" temperature.

What it means
Askeyev et al. conclude that "the nonlinear nature of the relationship between first arrival and temperature suggests that above a critical temperature threshold [our italics] a much more rapid change in first arrival date will occur," which is indeed what has happened with the skylarks that annually return to the Volga-Kama region of Russian Tatarstan. This observation thus suggests that whether or not "tipping points" occur in the realm of climatology -- as claimed by Al Gore and NASA's James Hansen -- they do in fact occur, and in some cases have already occurred, to the benefit of certain of earth's birds: the hotter it gets, the better they do what they need to do to deal with the evolving situation.