Reproductive success measures how well birds produce viable offspring. It can be calculated and interpreted in various ways depending on the stage of the nesting cycle. Success rates can offer insight about the most critical stage of the nesting cycle and provide useful information for conservation efforts.

In Table 1, we summarize nest-box success rates for six of The Birdhouse Network's most common species and six of our "Most Wanted" birds. We used three different measures to help us determine the stage when a species is most vulnerable to nest failure. The nestling rate is the percentage of eggs that actually hatched, and fledgling rate measures the percentage of eggs in completed clutches that ultimately produce fledglings. For both the nestling and fledgling rates, we considered only nests with complete clutches and at least one nestling present. This enabled us to measure parental ability in nests that made it to the nestling stage. Overall nesting success is the proportion of all nest attempts that produced at least one fledgling. This measure includes nests with at least one egg recorded, even if the eggs never hatched.

SPECIES # of Nest Attempts # of Complete Clutches Nestling Rate (%) Fledgling Rate (%) Overall Nesting Success (%) Average Clutch Size Average # Nestings Average # Fledglings Eastern Bluebird 3,313 2,690 90 88 72 4.4 4.0 3.9 Tree Swallow 2,283 1,803 89 87 70 5.3 4.8 4.6 House Wren 737 555 90 90 65 5.6 5.2 5.1 Mountain Bluebird 597 518 88 88 78 5.0 4.5 4.4 House Sparrow 354 112 83 81 17 4.5 3.6 3.9 Western Bluebird 311 275 87 83 77 5.1 4.5 4.3 Purple Martin 88 71 90 88 76 5.4 5.0 4.7 American Kestrel 44 34 96 93 80 4.6 4.4 4.2 Mountain Chickadee 36 32 91 88 81 6.3 5.8 5.7 Bewick's Wren 32 23 89 84 72 5.8 5.5 5.3 Prothonotary Warbler 30 22 73 73 67 4.9 3.6 3.7 Great Creasted Flycatcher 28 26 91 90 82 4.3 4.3 4.4 Table 1. Summary of 2000-01 nest data. The Birdhouse Network's "Most Wanted" shaded in pink. # of Nest Attempts = nests with at least one egg. # of Complete Clutches = nests with at least one nestling. Nestling Rate = % eggs hatched in nests with at least one nestling. Fledgling Rate = % nests with at least one nestling that produced at least one fledgling. Overall Nesting Success = % nest attempts producing at least one fledgling in nests that had at least one egg. Average Clutch Size and Average # Nestlings are based on # Complete Clutches. Average # Fledglings is based on nests with at least one fledgling, accounting for higher fledgling averages than nestling averages. For other species, click here.

To discern what happens during the various stages of the nesting cycle, choose a species from Table 1 and follow the rows from nestling rate to overall nesting success. For example, the Tree Swallow shows a nestling rate of 89 percent and a fledgling rate of 87 percent. This suggests that once Tree Swallows make it to the nestling stage, they have a high probability of fledging. Now look at the overall nesting success rate, which includes nests that never produced nestlings. The rate of 70 percent is lower than the rates for more than half of the other birds listed and suggests that Tree Swallows have a relatively high rate of failure during egg laying and incubation. Note that Table 1 is sorted from highest to lowest sample size and thus, highest to lowest reliability.

The overall nesting success rate ranged from 65 percent (House Wrens) to 82 percent (Great Crested Flycatchers), except for House Sparrows, with an anomalously low 17 percent. Figure 1 indicates why nests failed. Ninety-two percent of House Sparrow nest failures were caused by human intervention, a practice both legal and encouraged for nonnative species that usurp cavities from native birds. Among the other species, the most common reason for nest failure was predation, accounting for more than 40 percent of all failed nests for Eastern Bluebirds, Tree Swallows, House Wrens, and Mountain Chickadees. Nest abandonment was another major cause of failure, accounting for at least 20 percent of failed nests for seven species.

Figure 1. Causes of nest failures, 2000-01, for nests with at least one egg but no fledglings reported. Numbers in parentheses indicate number of failed nests.

Providing a nest box is a great way to increase nesting sites for cavity-nesting birds, but if it also increases the likelihood of predation, guards should be erected. The Birdhouse Network strongly encourages the use of predator guards, particularly in areas where predators are abundant. In addition, regular, weekly monitoring and recording of nest-box activity will help increase our overall knowledge of nesting success and failure.

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