Eggs in a Basket

Susan Thurn,
CNHM Director of Education

The Eastern phoebe is nesting in the same nest again in my house eaves. Not just for the second year, but for the second time this year. Many bird species have more than one brood in a season. Birds may lay more than one brood per year as a survival strategy. A bird is driven to pass on its genetic material through reproduction, and more young can mean more of them reach adulthood. Many songbirds have enough time to raise more than one family with a short period of time between egg-laying and fledge time. Their young can still have time enough to fatten up for their southern migration, or to prepare for our northern winters. It used to be thought that most birds nested for life. However, some birds do not nest for life, and some birds do not even keep the same mate for a second brood. How many eggs does a bird “put in a basket,” anyway?

Eastern phoebes are very loyal to their nesting locations, using them not only for their typical two broods per season, but also using them for many years. The phoebe makes repairs to the nest, apparently following the 3-R’s – reduce, reuse, recycle, by using the nest again. Most research suggests that phoebes keep the same mate through the season. Other birds in our region that can have two broods per season with the same mate include chipping, field, and song sparrows, juncos, towhees, catbirds, and robins.

Birds that often have two broods per season and sometimes choose different mates for the second brood include brown thrashers, bluebirds, and house wrens. The story gets even more interesting, however. Bluebird studies have shown that generally, northern birds have fewer broods than southern birds, but still have about the same number of young each season. In our neck of the woods (or fields, as that is the habitat bluebirds prefer,) bluebirds have two broods. Further south, where the food supply of insects is greater because of a longer warm season, bluebirds have three broods. Broods in the north lay more eggs than bluebirds in the south, and so end up with about the same number of young.

Would it be better to have more babies at once, or spread them out over one season? Would there be an advantage to having more nests, but fewer mouths to feed? Do northern birds sacrifice time to raise and train fledglings? Perhaps only a parent could know. Actually, it appears that there is a trade-off because although southern birds might have a “basket” full of more eggs, they have higher hatching failures.

It is fascinating to continue discovering about the natural history and science of birds. It was a Museum member who brought this question about birds and their second broods to my attention. It is often this spark of curiosity that will lead us to our own discoveries, perhaps to a bird in our own neighborhood that is in the midst of its second brood.

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility and new exhibit, On Lake Owen: The Art of Walter Bohl, in Cable at 13470 County Highway M or on the web at www.cablemuseum.org to learn more about exhibits and programs.