A classic question revisited in red-winged blackbirds: disentangling confounding hypotheses surrounding parental investment theory and nest defense intensity
- 375 Downloads
The pattern of increased nest defense effort over the course of a nesting season could result from three distinct (albeit non-exclusive) mechanisms: increased value of offspring to parents with progression toward independence (parental investment theory), decreased opportunity for renesting (renesting potential hypothesis), or decreased perceived costs of defense after repeated encounters with human observers (positive reinforcement hypothesis). To gauge relative empirical support for each of these mechanisms, we disentangle these three often-confounded hypotheses using multimodel inference with mixed-model ordinal regression applied to an extensive red-winged blackbird (Agelaius phoeniceus) nesting data set (4518 monitoring visits to 1330 nests). Parent aggression was rated on an ordinal scale (0–4) during repeated monitoring visits. Additionally, we assessed clutch/brood size, nest density, time of day, and nest concealment effects on aggression. In a preliminary analysis, including all three major hypotheses, male and female nest defense was most strongly explained by parental investment (nest age). Positive reinforcement (visit number) and renesting potential (Julian date) were also well-supported predictors in males. The interactions of decomposed nest age (within-individual and between-individual centered) with Julian date were particularly important in the top male model. Additional factors, such as clutch/brood size, nest density, and nest concealment, appeared to have larger predictive roles in explaining female aggression relative to males. These patterns are likely explained by different sexual reproductive roles within a polygynous mating system. Our study highlights the importance of interacting mechanisms involving parental investment theory and the use of within-individual standardization to help disentangle competing and empirically confounded hypotheses.
Avian nest defense generally increases over the course of a nesting season, potentially from the result of three different mechanisms: parental investment theory, renesting potential hypothesis, or positive reinforcement hypothesis from repeated nest visitation. We revisit this classic question through a comprehensive analytical approach with an extensive observational data set with red-winged blackbirds, employing multimodel selection and within-individual and between-individual centering techniques. We found that parental investment (nest age) was the strongest predictor of nest defense for both sexes; however, positive reinforcement and renesting potential also appeared to help explain additional variation in nest defense for males. Competitiveness of models with interactive effects indicated that these mechanisms do not operate independently for either sex, and additional covariates (e.g., clutch/brood size) especially aided female model competiveness. Our study highlights the importance of multiple and often interacting factors that influence avian nest defense.
KeywordsNest defense intensity Red-winged blackbird Parental investment theory Renesting potential hypothesis Positive reinforcement hypothesis Ordinal regression
We thank our main funding source, the Illinois DNR (funding source, Federal Aid Project W-106-R), and further support was provided by the Cooperative Wildlife Research Laboratory, Department of Zoology at Southern Illinois University Carbondale and Illinois Ornithological Society. We especially thank private landowners, Natural Resources Conservation Service/Farm Service Agency county offices, and research technicians (A. Annis, A. Fleming, N. Peters, L. Schroeder, W. Rodriguez, J. Daub, E. Comeau, A. Potash, S. Michalet, K. Pangan, J. Nawrocki, S. Dalinsky, M. Kneitel, and Z. Channic) who made our research possible. We also thank data proofers/entry (J. Whitaker, A. Blackert, S. Ramakrishnan). We are grateful to anonymous reviewers and to D. Westneat and P. Porneluzi for their reviews that greatly improved our manuscript.
Compliance with ethical standards
For this type of study, formal consent is not required. Birds were not handled during this study, and nest monitoring was conducted in a manner to minimize disturbance to the area and blackbird parents. All applicable international, national, and/or institutional guidelines for the care and use of animals were followed. This article does not contain any studies with human participants performed by any of the authors.
Our research was funded through Illinois DNR (funding source, Federal Aid Project W-106-R), and further support was provided by the Cooperative Wildlife Research Laboratory, Department of Zoology at Southern Illinois University Carbondale and Illinois Ornithological Society.
Conflict of interest
The authors declare that they have no competing interests
- Barash DP (1975) Evolutionary aspects of parental behavior: distraction behavior of the alpine accentor. Wilson Bull 87:867–373Google Scholar
- Burnham KP, Anderson DR (2002) Model selection and multimodel inference: a practical information-theoretic approach, 2nd edn. Springer Science and Business Media, New YorkGoogle Scholar
- Caccamise DF (1977) Breeding success and nest site characteristics of the red-winged blackbird. Wilson Bull 3:396–403Google Scholar
- Christensen RHB (2015) Ordinal–regression models for ordinal data. R package version 2015.1–21, https://cran.r-project.org/web/packages/ordinal/index.html
- Cohen J, Cohen P, West SG, Aiken LS (2003) Applied multiple regression/correlation analysis for the behavioral sciences, 3rd edn. Lawrence Erlbaum Associates Inc., New JerseyGoogle Scholar
- Ehrlich PR, Dobkin DS, Wheye D (1988) The birder’s handbook: a field guide to the natural history of north American birds; the essential companion to your identification guide. Simon & Schuster Inc., New YorkGoogle Scholar
- Geupel GR, Thompson CF (2013) The Palomarin handbook. Point Reyes Bird Observatory, Stinson beach, CAGoogle Scholar
- Knight RL, Temple SA (1986a) Why does intensity of avian nest defense increase during the nesting cycle? Auk 103:318–327Google Scholar
- R Development Core Team version 3.0.3 (2014) R: A Language and Environment for Statistical Computing. R Foundation for Statistical Computing, Vienna, http://www.R-project.org/
- Searcy WA, Yasukawa K (1981) Sexual size dimorphism and survival of male and female blackbirds (Icteridae). Auk 98:457–465Google Scholar
- Trivers RL (1972) Parental investment and sexual selection. In: Campbell BG (ed) Sexual selection and the descent of man 1871–1971. Aldine, Chicago, pp. 136–179Google Scholar
- Walk JW, Ward MP, Benson TJ, Deppe JL, Lischka SA, Bailey SD, Brawn JD (2011) Illinois birds: a century of change. Illinois Natural History Survey Special Publication 31, IllinoisGoogle Scholar
- Westneat DF (1989) Intensity of nest defense in indigo buntings increases with stage and not number of visits. Auk 106:747–749Google Scholar
- Yasukawa K, Searcy WA (1995) Red-winged blackbird (Agelaius phoeniceus). In: Poole A (ed) The Birds of North America Online. Cornell Lab of Ornithology, Ithaca, New York, http://bna.birds.cornell.edu/bna/species/184/articles/introduction
- Zuur A, Ieno EN, Walker N, Saveliev AA, Smith GM (2007) Mixed effects models and extensions in ecology with R. Springer, New YorkGoogle Scholar