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Another Record Year: 28,244 Bluebirds Fledged
by Peter Dunn and Kent D. Hall, Data Collection & Analysis Committee
The number of fledgling bluebirds reported increased by over 34% in 2007 to another all-time record of 28,244 birds (Table 1). Last year was the first time BRAW members went over the 20,000 mark; will we be able to go over 30,000 next year?
This increase is largely due to an increase in total reports (N= 291 in 2007, 171 in 2006) with the addition of 1,840 more nest boxes in 2007. A record number of these 291 reports (98) were made by first-time monitors. Since many of these reports were only 1-5 nest boxes, the average number of boxes reported/ trail dropped from 35 in 2006 to 30 in 2007.
Figure 1. Number of fledging bluebirds and tree swallows, and the number of nest-boxes presented, from 1994 to 2007. Note that most BRAW monitors dropped paired boxes in 1997, and most also dropped crowded single boxes in 2002.
The number of tree swallow fledglings increased by 53% from 6,574 in 2006 to 10,051 in 2007 (Table 1). This may have been due to increased productivity of swallows (fledglings per box), which increased 18% (from 1.1 to 1.3; Table 1). In comparison, bluebird productivity only increased 3% (from 3.5 to 3.6). As a consequence of the greater increase in swallow fledglings, there was a drop in the ratio of bluebird to swallow fledglings from 2006 to 2007 (Table 1).
Table 1. Comparison of Eastern Bluebird and Tree Swallow fledging success in 2005-2007.
| Year | Bluebirds |
Bluebirds Fledged per Box |
Swallows Fledged |
Swallows Fledged per Box |
Total Boxes | Bluebirds: Swalllows |
|---|---|---|---|---|---|---|
| 2007 | 28,244 | 3.6 | 10,051 | 1.3 | 7,861 | 2.81:1 |
| 2006 | 21,047 | 3.5 | 6,574 | 1.1 | 6,021 | 3.20:1 |
| 2005 | 17,670 | 2.9 | 8,440 | 1.4 | 6,016 | 2.09:1 |
An important factor influencing swallow production could have been the weather. On April 11, a major snow storm hit the eastern half of Wisconsin. For the few days prior to and after, the weather was extremely cold. Data collected on the Audubon Trail in central Wisconsin indicates nest building and egg laying in bluebirds halted in this prime nesting period. It is hypothesized that sharply reduced numbers of boxes were initially occupied by bluebirds in 2007 because of this weather. When the weather did begin to warm, swallows were more competitive than normal, due to the large number of unoccupied boxes. Data reported from the eastern half of the state as a whole, support this hypothesis.
Fledging success (% of eggs that resulted in fledglings) of both bluebirds and tree swallows was about 5% higher in 2007 than in 2006. Bluebirds fledged 82% of eggs in 2007 and 78% of eggs in 2006 (Table 2). Swallows also fledged 82% of their eggs in 2007 and 79% of eggs in 2006 (Table 2).
Table 2. Productivity of birds during 2007. Estimates are based on total numbers of eggs, nestlings and fledglings for each species (in parentheses).
| BLUEBIRD | SWALLOW | CHICKADEE | WREN | |
|---|---|---|---|---|
| % of eggs hatching | 86 (25280) | 87 (9781) | 64 (647) | 84 (2176) |
| % of nestlings fledging | 96 (21624) | 94 (8493) | 85 (412) | 94 (1821) |
| % of eggs fledging | 82 (20745) | 82 (8012) | 54 (348) | 79 (1717) |
For the first time in BRAW history, members reported the number of chickadees and wrens fledged. There were 127 chickadee nests, fledging 386 young, and 538 house wren nests fledging 2,090 young. In 2006, there were 97 chickadee and 586 house wren nests reported. Combining all of the fledglings from these species means that in 2007 BRAW members reported fledging of 40,771 songbirds!
Chickadees had the lowest productivity of the four most common species - just 54% of eggs led to fledglings (Table 2). The Audubon Trail produced nearly 1 / 3 of the chickadee fledglings reported to BRAW. On their trail, they put out 327 new nest boxes, many of which were nested by chickadees before bluebirds found them. Later, however, when bluebirds located and chose to build in them, the chickadees were displaced by the larger bluebirds. In fact, in 7 cases for the Audubon trail, bluebirds built their nests on top of chickadee chicks (all of which died). Such competition led to poor production of chickadees.
Multiple broods in Bluebirds
In 2006, it appeared that much of the increase in production was due to second broods as there was a 14 percentage point increase from 2005 (23 and 37% of all successful nesting attempts were second broods in 2005 and 2006, respectively). In previous years, we only knew the percentage of successful nesting attempts that were from second broods, and not the number of fledglings that came from second (or third) broods, so we could not determine exactly how second broods influenced overall productivity (number of young fledged per box). This year (2007) we asked bluebird monitors to collect the data that would allow us to look at the influence of second and third broods in future years. Note that the results in Table 2 only include records that are complete for the number of eggs, nestlings and fledglings at all nests.
Table 3. Productivity of Bluebirds from first, second and third broods during 2007 (nests with complete data only). Numbers of nests, eggs, nestlings and fledglings are in parentheses.
| all broods | first | second | third | |
|---|---|---|---|---|
| % of eggs attempts | 100 (5600) | 61 (3432) | 36 (2018) | 3 (150) |
| % of successful nests | 91 (5112) | 90 (3073) | 95 (1910) | 86 (129) |
| % of eggs hatching | 86(21624/25280) | 87(13700/16022) | 86(7403/8637) | 84 (521/621) |
| % of nestlings fledging | 96 (20745/21624) | 94 (13123/13700) | 96(7124/7403) | 94 (498/521) |
In 2007, 61% of nesting attempts (nests with at least one egg laid) were first broods. Second and third broods were 36% and 3% of all records. First, second and third broods had relatively similar proportions of nests producing at least one fledgling (90, 95 and 86%, respectively). Moreover, the percent of eggs hatching and percent of nestlings fledged were almost identical.
Productivity varied across the state, from less than one bluebird fledged per box in Florence, Iron and Outgamie counties to over five bluebirds fledged per box in La Crosse, Pepin, Trempealeau and Vernon counties (Fig. 2). As in 2006, there was a strong negative relationship between bluebird and swallow production (Fig. 3) using these data from each county. This means that bluebird production was highest in counties with low swallow production. As in previous years, there was also higher production of bluebirds in western than in eastern counties (Fig. 2). If you look at figure 2, the darker counties for bluebirds, with higher productivity, tend to be in the west (Portage and Milwaukee counties are notable exceptions).
Figure 2. Bluebirds fledged per box
Figure 3 Tree swallows fledged per box
We thank our monitors for contributing the data and your outstanding efforts to help bluebirds in Wisconsin. We look forward to breaking the 30,000 mark in 2008!