In our last dramatic episode, you may remember that the eastern phoebes nesting above my garage door were forced to raise a cowbird chick. In the end, the cowbird and two phoebe chicks disappeared mysteriously, and the phoebes managed to raise the two remaining offspring successfully, despite the odds.
Also, they hated me.
The phoebe parents found my curiosity to be a nuisance, and they sounded the alarm whenever I was near. I thought that was the end of it.
I was wrong.
Two weeks after the curtain came down on Act One, the phoebes returned and started another brood in the same nest. As of this Monday, all four eggs had hatched, this time without a cowbird. Also, I was apparently forgiven, because they’ve stopped chirping at me whenever I’m in the driveway, at least for the present.
They now deem me unworthy of their attention.
Raising two broods is not rare. In general, smaller birds need more babies to replenish the population. The life expectancy of warbler-sized birds is only about two to five years. Mortality is high among first-year birds who may be less savvy about predators. Furthermore, all songbirds face new threats in the modern era. Outdoor cats kill over two billion birds a year in the U.S. and Canada — about four times the estimated rate of those killed by collisions with tall objects, such as towers, skyscrapers and wind turbines.
Small, non-migratory birds such as titmice, chickadees and nuthatches can get by with one brood per year. But a severe winter takes a toll on the tinier golden-crowned kinglets, and they typically raise two broods. The tiny winter wren, which does not migrate very far, does likewise.
Climate matters. Mourning doves in Maine may try to raise up to three broods a year. In the southern U.S., with longer summers, they may attempt nesting up to six times annually. Eastern bluebirds in the south are apt to raise three broods.
There are many nesting strategies to replenish the population of a species. Game birds frequently have bigger broods to make up for the losses to predators and hunters. Maine’s ruffed grouse can lay up to 14 eggs. Mallards lay almost as many. Wood ducks have been known to outdo both of them.
Re-nesting is another strategy for species that can ill afford nest failure. Adverse weather, predation and food shortages — even that parasitizing cowbird egg — can all lead to failure. Many species will try again. It’s most amusing among grouse and woodcocks. The males play no role in raising the chicks. Generally, the females are on their nests in May, yet the males continue to strut their stuff well into June. They’re ready to service a willing female if her first brood fails and she decides to start again.
I’ve been doing bird surveys in the north woods a lot lately, volunteering on the Maine Bird Atlas for the Maine Department of Inland Fisheries and Wildlife. Ruffed grouse hens are frequently seen at the edge of logging roads this time of year, drawn there by the ripe strawberries, and a good supply of insect protein for their chicks. I’ve startled countless families at this point.
On one particular mile-long walk, I surprised four different families. There’s a wide range in the size of the chicks between broods. Some chicks are nearly as large as their mother. Some are still just tiny puffballs – a clear sign that the hen had a late start, or a re-start.
Typically, large birds live longer than small ones. Common loons live 20-plus years, and lay only one or two eggs annually. Bald eagle life expectancy is similar, and rarely do they lay up to three. When you have many years to replace yourself, nest failure is less catastrophic. Even the much smaller Atlantic puffin can live over two decades, and they lay only a single egg per year. The downside is that when the population suffers a serious crash, due to natural or man-made causes, it can take a long time to rebuild the population. The rampant use of DDT nearly wiped out peregrine falcons, and decimated eagles and ospreys. All were very slow to recover.
If all goes well, the eastern phoebes nesting on my garage will fledge six chicks this year. The odds are that cats will get two of them, a predator will get another, and a collision with a structure will take a fourth.
But if two return next spring, the phoebes win.