What was done
The authors studied a natural population of individually recognizable, color-banded Mexican jays (Aphelocoma ultramarina) in the Chiricahua Mountains of Arizona over the period 1971-1998 for trends in egg-laying dates and monthly minimum air temperatures.

What was learned
Over the 29-year period of the study, the date of first nest construction in the population occurred 10.8 days earlier, while the date of first clutch in the population came 10.1 days earlier.  These changes were associated with significant trends toward increased monthly minimum temperatures in the study area.

What it means
The authors note that in many bird species, "breeding is timed so as to have young in the nest when the principal food of the nestlings is at its peak."  And with warmer minimum temperatures occurring earlier and earlier over the study period, they suggest that this climatic trend could be producing an earlier abundance of such food, which would help to explain the increasingly-earlier-occurring egg-laying date.  We note that this phenomenon (the earlier occurrence of food availability) could also be augmented by the increase in atmospheric CO2 concentration that has occurred over the study period.  The authors suggest that a second way in which earlier-occurring warmer night temperatures may lead to earlier breeding dates in birds is by alleviating thermal stresses on females on cold nights.

The authors cite several studies that reveal similar breeding trends in European birds, stating that "recognition of similar trends on both continents in very different environments is consistent with the interpretation that some avian populations are already responding to climate changes in the last 29 years or so."  Once again, we consider these widespread changes to be positive in nature.  Indeed, in view of the findings of our Journal Reviews dealing with Global Warming and Shifts in British Bird Ranges and Global Warming and Shifts in European Butterfly Ranges, it would appear that not only are animal environments increasing in size as the atmosphere's temperature and CO2 content rise in tandem, they are also increasing in temporal availability for important life processes.