Evolutionary Ecology

, Volume 25, Issue 1, pp 219–236 | Cite as

Why do some, but not all, tropical birds migrate? A comparative study of diet breadth and fruit preference

  • W. Alice Boyle
  • Courtney J. Conway
  • Judith L. Bronstein
Research Article


Annual migrations of birds profoundly influence terrestrial communities. However, few empirical studies examine why birds migrate, in part due to the difficulty of testing causal hypotheses in long-distance migration systems. Short-distance altitudinal migrations provide relatively tractable systems in which to test explanations for migration. Many past studies explain tropical altitudinal migration as a response to spatial and temporal variation in fruit availability. Yet this hypothesis fails to explain why some coexisting, closely-related frugivorous birds remain resident year-round. We take a mechanistic approach by proposing and evaluating two hypotheses (one based on competitive exclusion and the other based on differences in dietary specialization) to explain why some, but not all, tropical frugivores migrate. We tested predictions of these hypotheses by comparing diets, fruit preferences, and the relationships between diet and preference in closely-related pairs of migrant and resident species. Fecal samples and experimental choice trials revealed that sympatric migrants and residents differed in both their diets and fruit preferences. Migrants consumed a greater diversity of fruits and fewer arthropods than did their resident counterparts. Migrants also tended to have slightly stronger fruit preferences than residents. Most critically, diets of migrants more closely matched their preferences than did the diets of residents. These results suggest that migrants may be competitively superior foragers for fruit compared to residents (rather than vice versa), implying that current competitive interactions are unlikely to explain variation in migratory behavior among coexisting frugivores. We found some support for the dietary specialization hypothesis, propose refinements to the mechanism underlying this hypothesis, and discuss how dietary specialization might ultimately reflect past interspecific competition. We recommend that future studies quantify variation in nutritional content of tropical fruits, and determine whether frugivory is a consequence or a cause of migratory behaviour.


Diet Elevational gradient Fruit preference Interspecific competition Nutrient limitation Resource variability 



H. Reider, R. Repasky L., Cholodenko, A. Zambrano, and J. Montoya-Morera, and provided exceptional field assistance. B. Boyle, B. Hammel, F. Morales, N. Zamora, C. Taylor, R. Kriebel, J. Gonzalez, and O. Vargas helped identify plants. Financial support was provided by the National Science Foundation (Grant No. 0410531), an NSERC (PGS-B) fellowship, the International Arid Lands Consortium, the American Ornithologists’ Union, and the University of Arizona. A. Bien, R. Tenorio, J. Guevara (MINAE permit #154-2002), and the University of Arizona IACUC (permit 02-068) provided permits and logistical support. R. Steidl, D. Levey, T. Fontaine, R. Greenberg and anonymous reviewers provided helpful comments on earlier versions of this manuscript.

Supplementary material

10682_2010_9403_MOESM1_ESM.pdf (86 kb)
Supplementary material 1 (PDF 86 kb)


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Copyright information

© Springer Science+Business Media B.V. 2010

Authors and Affiliations

  • W. Alice Boyle
    • 1
    • 2
  • Courtney J. Conway
    • 3
  • Judith L. Bronstein
    • 4
  1. 1.Department of Ecology and Evolutionary BiologyUniversity of ArizonaTucsonUSA
  2. 2.Department of BiologyUniversity of Western OntarioLondonCanada
  3. 3.U.S. Geological Survey, Arizona Cooperative Fish and Wildlife Research Unit, School of Natural Resources and the EnvironmentUniversity of ArizonaTucsonUSA
  4. 4.Department of Ecology and Evolutionary BiologyUniversity of ArizonaTucsonUSA

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