A summer tanager seen this week at Fort Zachary Taylor State Park. MARK HEDDEN/Keys Weekly

The Gregorian calendar, the one most of us use in our everyday life, is a pretty accurate gauge for the earth’s rotation around the sun. It’s a way for us to orient ourselves in the long term amid the swirling vortex of passing time, a way for us to figure out what day we are supposed to show up where, a way for us to remember deadlines and mandatory court appearances and greeting card-requiring holidays imposed on us by the greeting card industrial complex.

Every year the Gregorian calendar tends to slide away from the vernal and autumnal equinoxes, and the summer and winter solstices, by about a quarter of a day, but that is mostly corrected by adding a day to February every four years, when we all experience what is known as a leap year. Such things have serious cultural significance. For instance, people born on Feb. 29 during a leap year – approximately .2% of the human population – get screwed out of their birthday three out of four years. 

On the flip side, Leap Year was the inspiration for the “Leap Day” episode in the 6th season of “30 Rock,” which tells the story of the mythical Leap Day William, who has white gloves and a blue top hat, a handlebar mustache and gills, and emerges every four years from the Mariana Trench to exchange children’s tears for candy. 

The Gregorian calendar was an update to the Julian calendar, which was largely used in Europe from 45 B.C. until 1582 A.D. It also had a leap day every four years, but did not account for the fact that a trip around the sun doesn’t actually take 365.25 days, but more accurately, 365.2425 days. Turns out those .0075 days every year, over the course of 1,625 years, mattered. The equinoxes and solstices had drifted away from their original dates on the calendar and were happening about 10 days too early, which was screwing up the scheduling of all sorts of Christian and Pagan holidays. 

So they updated and rebooted with the Gregorian calendar, which also added a day to February every four years, with the exception of years that could be divided by 100, which requires skipping an added day, except on years that could be divided by 400, when everyone is just expected to carry on and add a day like it was a normal leap year. Easy peasy.

They also just deleted 10 days from the month of October in 1582 in order to properly line things up with the equinoxes and solstices, and they moved the official start of the New Year from March 25 to Jan. 1. Which is why the months of September, October and November, whose names mean the 7th, 8th and 9th months, are now the 9th, 10th and 11th months. 

Birds, of course, care about none of this. Like most lifeforms on this planet, they live their lives on an annual cycle, but don’t decide what to do when because a particular date is circled on a piece of paper hanging on a wall. 

Birds are not planners. Their schedule is more instinctive, more, uh, loosey goosey. It’s dictated by the length of the day and hormones and food availability and weather patterns and flowering trees and other factors we haven’t quite figured out yet. 

For birds that migrate, a lot of these factors inspire a phenomenon known as zugunruhe, which sounds like an outdated Volkswagen sales slogan, but actually means migratory restlessness. They start building up fat stores and seem to generally get anxious. They start orienting themselves. They wait for conditions to align and then one day, or for diurnal migrants, one night, they just step off, often moving themselves across large swaths of the globe in amazingly short periods. 

Living in the Keys, we are in one of the best places in the world to see migrating birds. We are the last safe stop to fuel up before a large water crossing on the way south, and the first safe stop after a large water crossing on the way north. 

In the fall, you tend to see migrants between late August and mid-October. In the spring, when birds are a little more focused, trying to return to their breeding grounds to stake out the best territories, things are most intense between mid-April and early May. 

To my mind, April 15 is always the first official day of spring migration. I know full well the birds aren’t paying attention to the calendar, but for me, it’s always seemed like opening day of baseball season. So I try to make a point of going birding on that date. 

The hammock at Fort Zach was pretty quiet when I got there. Nothing at first, then about 50 yards in I spotted something low and intensely yellow – a hooded warbler. I hadn’t seen one at least since last fall. So it was pretty clear that migration was afoot. 

Over by the blacksmith shop I caught sight of another yellow bird on the ground, which I thought would be a second hooded warbler, but turned out to be a Kentucky warbler. I tend to only catch sight of them every two or three years, and they are cool, skulky little things, brighter than the brightest crayon in the box, but not inclined to display it. Also, whenever I see one, I tend to utter under my breath, pronouncing the word “Kentucky” as KAN-tuck-EE, because that’s the way Daniel Day Lewis does it in “Last of the Mohicans.” 

Walking around the park I caught sight of a few other migrants. There was a northern rough-winged swallow flying over the pines. There was an eastern kingbird on the edge of the moat. And there were a lot of indigo buntings that I heard but never saw. And just as I was nearly back to my car, a fire-engine red summer tanager, giving its two-note chuck up in a sea grape.

It was a sparse showing, but none of those birds was in the park last week, and none of them would be in the park in a few days, if they stayed that long. 

There will be more birds in the coming weeks. How many more depends on the wind and the weather. I hope to get out and see more of them. But it was nice to feel like I was at the beginning of something for once.

Also, when I checked the time on my phone, I noticed it was April 16, not April 15. So maybe I’m more birdlike in my relationship to the calendar than I thought.

Mark Hedden is a photographer, writer, and semi-professional birdwatcher. He has lived in Key West for more than 25 years and may no longer be employable in the real world. He is also executive director of the Florida Keys Audubon Society.