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

Abstract

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 Electronic supplementary material The online version of this article (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.

Altitudinal migration by birds: a review of the literature and a comprehensive list of species

“…Boyle et al. () tested this hypothesis in Costa Rica, comparing phylogenetically close migrating and non‐migrating pairs of species. Migrant species were found to have a more frugivorous diet and were competitively superior (rather than inferior) foragers for fruit than their resident counterparts.…”

“…Altitudinal migration has been reported at both temperate (e.g., Rabenold and Rabenold , Laymon , Boyle and Martin ) and tropical latitudes (e.g., Loiselle and Blake , Powell and Bjork , Boyle et al. ,b, Norbu et al. ).…”

Phenology of tropical understory trees: patterns and correlates

“…We chose species due to their importance in the diets of understory frugivorous birds based on fecal samples (Rosselli 1989, Loiselle & Blake 1990, Stiles & Rosselli 1993, Loiselle & Blake 1999, Boyle et al 2011 and traits indicative of avian seed dispersal (fruit size, color and fleshiness). Voucher specimens of all species in this study were deposited at the Museo Nacional de Costa Rica, INBio and the Missouri Botanical Garden.…”

“…We classified birds as frugivores and potential seed dispersers based on Stiles & Skutch (1989). We banded birds, collected data on age, sex, diet, and morphology for other studies (Boyle 2008, Boyle et al 2011, then released birds at the capture site.…”

Partial Altitudinal Migration of a Himalayan Forest Pheasant

“…In addition to long distance, cross continental latitudinal movements, many birds also undertake annual migrations along elevational gradients in montane environments [10]. So far, only a few studies have focused on altitudinal migration systems [11]–[25] and have mostly been viewed as individuals moving from higher elevations to more favorable lower elevations and vice-versa in response to fluctuating environmental conditions such as availability of food [14], [18]–[21], changes in weather [22], or trade-offs between survival and predation risks [23]. However, very few studies have examined these in detail [11], [12], [14], [15], [17]; and very little is known about altitudinal migration patterns (but see [24] and [13], [25]).…”