Timing of Migration

Are you a Fantasy Football enthusiast or a fanatic supporter of an NFL team? Here’s something to consider: scientists studied 40 years of football scores and determined that west coast teams have an advantage over east coast teams when the game is played after 8pm on the east coast of the U.S. Similar results were found with baseball scores. What’s happening? It’s the players’ internal clock. West coast teams are more alert in the evening on the east coast than are east coast teams.

Did you get up at about the same time this morning as you did yesterday? Do you feel tired in the mid-afternoon? Do you go to bed at about the same time every evening? Probably. You are showing ‘circadian’ (Latin for “around the day”) rhythms that are reflected in your brain activity, body temperature, hormone levels, blood pressure, and other metabolic functions. This is your body’s internal clock. These patterns of daily activity are tied to the photoperiod (length of daylight) and are critical to organisms’ survival. You interrupt your circadian cycle every time you experience “jet lag” by flying several time zones away from home, but in a few days you adjust to the new daylight regime. The quickest way to adjust is to expose yourself to sunlight as the sun will reset your cycle.
               Now think about why you awoke and got out of bed this morning. The immediate reasons are your daily rhythm – you usually get up at that time, perhaps assisted by an alarm clock or your significant other bringing you breakfast in bed. As unlikely as the latter might be, all these are proximate factors – the immediate reasons you rousted yourself out of the sheets. But why get up at all? Because you have things to do – your boss expects you at work, you have a fishing trip planned, you have to get the kids to school, or you are hungry. These are ultimate factors - the actual reason(s) you get up and out of bed.

The proximate factors of migration are those environmental cues that set birds off on their journey, primarily day length; and the ultimate factors – the reasons they leave on this annual journey- are such things as food and a milder climate. The timing of migration is pretty predictable. Birds arrive on their breeding grounds at about the same time every year. Tradition has it that swallows return to Capistrano on March 19th, but the actual times vary, often by several days. (Unfortunately, due to the remodeling of California’s Mission San Juan Capistrano and increasing development in the area, the Cliff Swallow population has declined to virtually zero, but efforts are being made to entice the birds back.) Since 1957 the town of Hinckley, Ohio has been holding a “Return of the Buzzards” festival to celebrate the arrival of Turkey Vultures, predicted to be March 15th each year. Vagaries of the weather affect the travel of migrants so the dates of arrival are not exact, but they are pretty close. How do birds time it? The answer is photoperiod, the main proximate factor.