|American Robin with two fledglings|
Now consider: if every egg hatches and every hatchling fledges and every fledgling survives, this would mean that a single pair of robins would add eight to twelve new robins to the robin population in a single year. If all robins were similarly successful, the robin population would increase on average five times in one breeding season. Clearly, we would soon be overrun with robins.
|Rose-breasted Grosbeak fledgling flutters and calls for food|
Food supplies effect the ability of birds to successfully raise young. A few years ago, cuckoos were common in our area, in conjunction with a variety of caterpillars chewing up the leaves of the trees. Several years ago, our yards were overrun with chipmunks. That same summer, Barred Owls had remarkable success in raising their young. But then came the winter when the food disappeared beneath the snow pack and the young owls were unable to feed themselves. A few were found and taken to wildlife rehabilitation centers; the others perished. This summer has been another banner year for chipmunks, and probably for owls and hawks as well. What will happen to those young if the coming winter is typically harsh?
The likelihood that a nesting pair of birds will successfully hatch and raise a full clutch is quite low: an egg may not hatch, a nestling may be out-competed for food by its nest mates, a fledgling may be too quiet or too weak to attract a parent with food or simply get too far away from its parents. Cornell Lab or Ornithology has citizen scientists doing nest box studies. For the Eastern Bluebird, for example, nesting success is about 72%; this is the percentage of nests that produce at least one fledgling.
|Tufted Titmouse juvenile|
The next day, the young grosbeak was at the feeder by himself and feeding himself, endlessly scarfing down seed and even chasing off other, sometimes larger, birds. But he had no sense of fear. I approached to within three feet and felt as though I could have picked him up, he was so tame - or perhaps, clueless. Instead, I clapped my hands and shouted. This finally induced him to fly off. Young Downy Woodpeckers on the suet feeder also show a cluelessness to danger, so I’ve taken to chasing them off as well in an attempt to teach a bit of wariness.
They need wariness, as the pile of dove feathers on the back lawn one morning testified. Probably a young Mourning Dove roosted on the tree branch too long, and became a meal for a hawk or owl. But in doing so, that dove fulfilled its life purpose; it became a part of the food chain.
Therein is a truth that is an unpleasant one for many people. We look on the birds at our feeders as entertainment, cute bundles of energy and activity. We give them human attributes. The titmice are sweet, the chickadees acrobatic, and the nuthatches comic. The crows are noisy and the jays are greedy bullies. But they are all “our” birds.
|Blue Jay juvenile|
The life expectancy of small birds is not long. Occasionally a song bird may live 5 to 10 years. The host of dangers and challenges facing small birds probably means that few ever “die of old age,” though old age may eventually weaken them or slow them so that they succumb to one of the dangers or challenges. Some studies suggest that only 20% of the birds which are successfully raised during a breeding season survive to the second year, while 80% of the adults survive to the following year. Clearly, mortality among small birds is very high.
Early this month, I was entertained by a young Chipping Sparrow which begged from a Song Sparrow, then begged from another young Chipping Sparrow, and eventually was fed by a parent. I am amused by the demanding calls of the young Blue Jays; they seem like the noisiest, most obnoxious juveniles imaginable. I am taken by the simple naivete of the young birds and wish them well even as I try to teach them a few lessons about getting on in the world.
|Male Downy Woodpecker feeds his fledgling son|
Posted by Chris Petrak, Tails of Birding