Removing Brown-Headed Cowbirds Increases Seasonal Fecundity and Population Growth in Song Sparrows
Bibliographic record
Abstract
Parasitic Brown-headed Cowbirds may reduce the reproductive success of their songbird hosts and thus threaten rare and frequently parasitized host populations with extinction. The demographic impacts of cowbirds on hosts, however, have seldom been studied experimentally. We removed cowbirds to estimate how much they reduce the seasonal fecundity and population growth rates of a non-endangered cowbird host, the Song Sparrow. Over five control years, 61% of 515 sparrow nests were parasitized at three riparian study sites. When we removed female cowbirds from two of these sites, parasitism fell to 43% of 65 nests at Westham in 1996, to 30% of 117 nests at Westham in 1997, and 18% of 78 nests at Delta in 1998. The mean seasonal fecundity of sparrows increased from 1.67 fledglings per territory in controls to 3.40 fledglings per territory at Westham and from 1.23 to 3.16 fledglings per territory at Delta. At the third control site, Deas, seasonal fecundity averaged 2.07 fledglings per territory. The mean sizes of sparrow broods at fledging were increased in removal years from 0.53 to 0.95 young at Westham and from 0.47 to 1.15 young at Delta. Daily nest failure rates were reduced from 0.046 in control years to 0.030 during cowbird removals at Westham and from 0.049 to 0.035 at Delta. Survival over a 25-d nest cycle rose from an average of 0.32 in eight controls to 0.45 in three removals. Cowbird removals increased local population growth rates of song sparrows from well below replacement levels (λ = 0.70–0.87) to a stable condition (λ = 0.93–1.06). If cowbirds have similar or stronger effects on rarer and more vulnerable hosts, cowbird removal could improve the demographic performance of such hosts substantially.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.001 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".