Column: Nesting season is fraught with potential danger

Steve Lekwa, Naturally Speaking
Special to the Ames Tribune

Trees have reached full leaf and the last of the woodland spring flowers are fading. Baby geese (goslings) are already growing rapidly. The dry weather should allow a good early hatch of pheasants and turkeys.

The first baby robins, bluebirds and house finches have left their nests, and their parents are busy with their second nest attempts. “Attempt” is the key word, for the challenge of raising a nest full of baby birds to the point where they can survive on their own is difficult. The odds of success are not all that good.

Robins in my yard lost their first clutch of eggs to a blue jay (I assume). I found predator-destroyed robin eggshells while I was mowing the yard, and I had seen the male robin chasing a blue jay. Bird eggs are a great source of protein for many kinds of wildlife other than jays. Squirrels, chipmunks, snakes, crows and even grackles enjoy bird eggs when they can get them, as do a host of other animals. Predators often leave roughly broken shells with yolk and blood stains. Hatched shells are relatively clean and show a straight break line around the middle.

On the plus side, I also found recently hatched mourning dove eggshells. I hope the little chipping sparrows, cardinals and catbirds that visit our feeders can avoid cowbird parasitism this year, but cowbirds are pretty common, and chances for escaping their notice are pretty slim. A single cowbird egg in a nest means there’s little chance that a baby of the host bird’s family will survive. And a female cowbird may lay as many as 20 eggs in 20 different nests.

The 15 bluebird nest boxes I monitor at Hickory Grove Park are getting lots of use, but with only limited success. Bluebirds have initiated six nests but only one successfully fledged five young so far. One had its eggs destroyed most likely by a wren. The nest showed no disturbance, but four eggs disappeared.

Another two nests had all their eggs taken by a raccoon climbing up to the box. Muddy paw prints showed that the raccoon accomplished that in spite of plastic pipes placed over the steel fence posts the boxes are mounted on. The plastic was intended to make the post too slippery and difficult for predators to climb. The nests showed extensive disturbance where the raccoons had reached in and pawed around to pull out the eggs. The adult bluebirds apparently survived the raccoon attacks because three pairs have started nests and laid eggs in other boxes.

I recently sprayed the plastic pipes with a Teflon spray to make them slipperier, but those raccoons know there are tasty treats up there. They’re persistent and smart. I may have to go with a more expensive cone type predator guard to deter them.

Tree swallows started a bit late this year due to the unusually chilly weather we had for a while. The female of the first pair of swallows to select a nest box was pecked to death by a house sparrow before she could lay any eggs. The sparrows wanted the box for their own nest but gave up on that box after I threw their nest out several times. They just moved to another box, built a new nest and had five eggs a week later.

Swallows recently lost two nests to raccoons and lost another nest to a second pair of house sparrows. Four boxes have baby swallows that began hatching at the end of May, and two more swallow pairs are still incubating eggs. The babies are still very vulnerable now that a raccoon or two have learned that bird boxes are actually lunch boxes.

Wrens moved into a couple of boxes after other nests were destroyed. One is sitting on eight eggs. Last, but not least, is kind of a surprise. Chickadees often fledge their young by mid-May, but one of the bluebird boxes has a little moss chickadee nest that was started in late May. She wouldn’t leave the nest when I opened the box to check how many eggs she has. Chickadees often have big families but maybe not eight like her wren neighbor.

The boxes will continue to be actively used by bluebirds and wrens well into July and even August. Tree swallows will fledge most of their young by mid-June. There is still time to raise a lot of baby birds – if I can outsmart the nest predators.