Full of Sap

Wintering birds are starting to feel the urge to migrate

As I type, my yard is full of robins, up in the trees, down on the ground, flying overhead. Looking over the few acres I can see from my porch, I count at least a thousand birds. Obviously, "first robin of spring" is a Yankee expression. Indeed, my yard is full of Yankee robins. Robins here now will fly north to breeding grounds in the Northern Appalachians, Canada, and New England suburbs. Each robin in my yard was once a pale blue egg in a nest in a Yankee tree, and any of them could be some Yankee's first robin of spring.

Pale blue eggs in Tennessee trees are now part of a flock further south, along the Gulf coast. All robins will start flying north soon, and our summer pairs will arrive after the winter flocks start dispersing. Watch robin flocks closely over the next month to see migration in progress. First they molt into fresh feathers and start singing more often, then they will head north, flying at night, feeding and resting during the day. When their breeding hormones are at full flow, robins need little sleep.

Hormones started rising in local birds in recent weeks. Chickadees started singing in January, and woodpeckers are drumming. Robins started growing black and orange feathers to replace worn grays and browns. In the coming weeks, the heads of male robins will turn black, slightly darker than the females, but as the southerly flocks come through, the gender difference fades. Robins from different regions vary in size and coloration, and mixed winter flocks offer a chance to study the subtle variations.

Sapsuckers are getting their breeding colors, and males acquire brilliant reds and yellows just days before they head to the highlands to breed. Try not to miss it.

Some of our winter birds live here all year. In fact, they live in the same place all their life. Wrens, chickadees, woodpeckers, cardinals and others prefer to stay on familiar territory. Even if they stray into new areas for food, they return home to roost. Robins, sapsuckers, goldfinches and others are more nomadic, and when breeding season arrives, they migrate northward and up elevation.

Migratory birds travel in flocks. This allows first-year birds to follow older birds and learn the route, and it prevents errors in older birds. If a bird makes a turn and the flock does not follow, it just rejoins.

Birds can perceive direction through both the polarity of sunlight and a sensitivity to Earth's magnetic field. This lets them know which way they are going but not necessarily which way to go. In fact, they follow mountain ranges, rivers and storm systems rather than simply flying in a direction. An ovenbird who nested in a patch of hay-scented ferns in the Smokies last year will return to the same ravine this year not because she has a genetic map, but because she traveled there and back before with relatives, learned the route and considers the ravine home.

Ovenbirds winter in South America and cross the Gulf of Mexico twice a year, but migration journeys are not always so long and difficult. Phoebes winter in our valleys then ascend rivers to breeding grounds in the foothills and mountains as spring progresses. Like most flycatchers, phoebes stay near water and the blooms of craneflies and mayflies our waters produce. Most aquatic insects are lumbering in flight, and some get fairly large, perfect for keeping flycatchers fat and happy. Phoebes follow the bonanza up rivers and streams.

Wintering birds are not territorial. In fact, they become quite gregarious. There is safety in numbers and wisdom in elders. If you can get close enough to robins to hear their quieter vocalizations, you can't help but be impressed with all the sounds they can make. There is plenty of room for a vocabulary, surely not one broad enough to support philosophical ruminations, but perhaps just big enough for making fun of squirrels.