Does the proportion of arthropods versus fruit in the diet influence overwintering condition of an omnivorous songbird?
Bibliographic record
Abstract
Many migratory songbirds switch from a primarily insectivorous diet during the breeding season to either a mixed diet or fruit diet during the non-breeding season. However, for species with mixed diets, arthropods may be superior food items because of their higher protein content and easier digestibility. We tested this hypothesis by analyzing the diet and body condition of omnivorous Wood Thrushes (Hylocichla mustelina) at a non-breeding site in tropical forest in Belize, Central America. We used analysis of stable isotopes δ15N and δ13C in the blood to measure diet. Our objective was to determine if a higher dietary proportion of arthropods relative to fruit (i.e., higher δ15N and δ13C) was associated with better body condition. We also examined the possible effect of age, sex, and habitat type on Wood Thrush diets, as well as any changes in diet through the overwintering period. We used a hierarchical Bayesian mixing model (MixSIAR) to estimate the proportion of different prey items in the diet of overwintering Wood Thrushes overall, in each habitat type, and over time during the non-breeding period. From January to April, we found a significant decline in δ15N in forest habitats, whereas δ15N increased in scrub habitat. There was no significant seasonal change in δ13C. Birds with higher δ15N or δ13C values were not in better body condition. Females in dry-scrub habitat consumed more fruit than males, but this did not affect body condition. Mixing model results indicated that most Wood Thrushes at our study sites consumed primarily arthropods, even during the driest times of the non-breeding season and in the driest habitat. Overall, our results suggest that the diet of Wood Thrushes varies with habitat and during the overwintering period, but diet alone was not a predictor of body condition. Wood Thrushes, and possibly other omnivorous migratory songbirds, are apparently flexibly able to meet their wintering and pre-migration nutritional demands with a variety of diets. Muchas aves canoras migratorias cambian de una dieta principalmente insectívora durante la temporada reproductiva a una dieta o bien mixta o bien frugívora durante la temporada no reproductiva. Aún así, para especies con dietas mixtas, los artrópodos pueden ser items alimentarios superiores debidos a su mayor contenido protéico y fácil digestión. Pusimos a prueba esta hipótesis analizando la dieta y la condición física del omnívoro Zorzal del Bosque (Hylocichla mustelina) en un sitio no reproductivo en el bosque tropical de Belize, América Central. Utilizamos análisis de isótopos estables δ15N y δ13C en la sangre a fin de medir dieta. Nuestro objetivo fue determinar si una proporción más alta de artropodos en relación a fruta en la dieta (i.e., δ15N y δ13C más alto) está asociada con una mejor condición física. Examinamos también el posible efecto de la edad, el sexo y el tipo de hábitat en la dieta del Zorzal del Bosque, al igual que cualquier cambio en la dieta durante el período invernal. Usamos un modelo Bayesiano mixto de jerarquías (MixSIAR) para estimar la porporción de presas en la dieta de los Zorzales del Bosque invernales en total, en cada tipo de hábitat y a través del tiempo durante el período no reproductivo. De Enero a Abril, encontramos una disminución significativa de δ15N en los hábitats de bosque, mientras que un incremento de δ15N en los habitats de arbustos. No hay diferencias estacionales significativas en δ13C. Las aves con mayorer valores δ15N o δ13C no estaban en mejor condición física. Las hembras en hábitats de arbustos secos consumieron más fruta que los machos, pero esto no afectó la condición fisica. Los resultados de los modelos mixtos indicaron que la mayoría de los Zorzales del Bosque en nuestros sitios de estudio consumieron principalmente artrópodos, aún durante los momentos más secos de la temporada no reproductiva y en los hábitats más secos. En general, nuestos resultados sugieren que la dieta de los Zorzales del Bosque varía durante el período invernal, pero que la dieta sola no es un predictor de la condición física. Los Zorzales del Bosque, y probablemente otras aves canoras migratorias, parecerían ser flexiblemente capaces de llegar a sus requerimientos nutricionales invernales y pre-migratorios con una variaded de dietas. Appendix S1. Methods Table S1. Mean (± SE) stable isotope values for nitrogen and carbon in Wood Thrush cellular blood and in potential prey items. Fig. S1. Wood Thrush isospace plot with three source items, fruit, Formicidae-Coleoptera combined (“AntsBeetles”), and Araneae. Values shown are sources corrected with trophic discrimination factors. Vertical and horizontal bars are standard deviation for each source, and points show the values of the consumers (Wood Thrushes), color-coded by habitat. Table S2. Model list with AICc values, model weights. Only one model for each isotope had ΔAICc < 2. Table S3. Results of Bayesian mixing models run by using MixSIAR to estimate posterior density distributions of Wood Thrush diet in Belize. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.
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How this classification was reachedexpand
Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 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 | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 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 teacher head, 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".