Photo credit: C. Griffis

We found that in many regions it was indeed an unusual year for Tree Swallows. Six of ten regions reported fewer fledglings on average compared with the previous five years. In contrast, Tree Swallows in two regions fledged significantly more young (Figure 1).

What kinds of factors might influence the nesting success of Tree Swallows across such large regional scales? Weather is one obvious suspect. Tree Swallows are aerial insectivores—they rely on flying insects for their entire diet during the breeding season. Because flying insects are harder to come by during heavy rains and severe droughts, Tree Swallows can be especially susceptible to the vagaries of the weather. During the nesting season, food scarcity can cause parents to delay nesting, abandon eggs or chicks, or spend less time on parental duties and more time trying to keep themselves alive: All of these can adversely affect nesting success.

To understand the pattern of geographical variation in Tree Swallow breeding failures and successes, we categorized the data into 10 regions spanning most of the Tree Swallow breeding range. To assess whether 2002 was an unusual year, we compared lay dates and average fledging rates in 2002 with the averages from the previous five years combined. We analyzed 6,813 records of first nesting attempts for which at least one egg was laid from 1997 through 2002. We began by comparing dates when the eggs were laid, the number of eggs in the clutch, and the number of young fledged between the two time periods.

Knowing the egg-laying date is important because many single brooded species like the Tree Swallow lay smaller clutches as the nesting season progresses. Tree Swallows usually lay from four to seven eggs. Optimally, they would lay the maximum number that they would be able to care for during the incubation and nestling periods. A cold spring with few insects may cause nesting delays and smaller clutch sizes, but smaller numbers of eggs do not necessarily mean that fewer young will fledge overall. In some cases, a smaller family might be easier to rear, particularly during severe weather. To gauge how well Tree Swallows were able to care for the eggs they did lay, we calculated fledging rate—the proportion of eggs that survived to fledging in different regions and time periods.

Tree Swallows in several regions had later egg-laying dates, smaller clutch sizes, and fewer fledglings in 2002 than the previous five-year average (Figure 1). In 8 out of 10 regions, females laid their first egg significantly later in 2002. In seven of these regions, they laid significantly fewer eggs as well, suggesting that they may have reduced their clutch sizes along with delaying lay dates in response to weather. Smaller numbers of fledglings were reported in five of these regions—New England, Allegheny, Mid-Atlantic, East Central, and Northern Rockies.

Fledging rates in 2002 were much lower in the Mid-Atlantic and East Central regions and slightly lower in New England (Figure 2). These data indicate that the chances for each egg's success would have been higher if Tree Swallows in those regions had laid even smaller clutches.

In California, Tree Swallows laid eggs earlier than usual for that region and produced more eggs per nest than in the previous five years. But greater numbers of eggs did not mean it was a boom year for fledglings; nestbox monitors reported lower than average fledgling numbers. From an economic perspective, Tree Swallows in California invested in larger clutches but got back less on their return with lower fledgling numbers. Thus, the fledging rate was much below average.

In contrast, Tree Swallows in the Great Lakes and North Central regions had much higher than average fledging rates. Although they laid their eggs later and invested in smaller clutches, they got more on their return with higher fledgling numbers.

Overall, egg-laying dates and fledging rates varied significantly among regions and between time periods. What is the evidence that this variation could be attributable to weather? According to 108 years of climatic records from the National Climatic Data Center, the spring of 2002 chronicled some of the wettest, hottest, or driest months in states across the country. Particularly wet weather occurred March through May in the New England, Allegheny, Mid-Atlantic and East Central regions—the same regions with lower or much lower than average fledging rates. California experienced its eighth driest season on record, suggesting that severe drought may have been to blame for the breeding failures in 2002. Much of the rest of the country received relatively normal precipitation. These states were fairly representative of regions with fledging rates comparable to or much higher than in the previous five years.

To test whether other species experienced the same phenomenon, we ran the same analysis on Eastern Bluebirds across their range. We found that in the Allegheny, East Central, Great Lakes, and New England regions, the Eastern Bluebird fledging rate was significantly lower in 2002 than in the previous five years. In the Southeast, where weather conditions were less severe, the fledging rate was significantly greater.

The data from participants of The Birdhouse Network and the data on climate suggest that prolonged severe weather, such as drought or much greater than normal precipitation, was to blame for the striking differences in nesting success of Tree Swallows and Eastern Bluebirds in some regions during 2002. The insights we are gaining are some of the very first to give us a “bird-weather map” during the breeding season, helping to create a broad-scale picture of the effects of climate on bird reproduction. Such a perspective will be very valuable as we continue to grapple with the consequences of global warming for birds.

We thank our dedicated nest-box monitors, who made it possible to study Tree Swallow nesting success across nearly the entire Tree Swallow breeding range.