FSD 2010 - Frugivore-Insectivore Birds

5th International Symposium-Workshop on Frugivores and Seed Dispersal (1985-2010)

Frugivore-Insectivore Birds*

*This page was prepared by Tomás A. Carlo for fsd2010.org and edited by ATBC2008 Association

Definition

Frugivore-insectivores are remarkable birds that provide important services to plants and ecosystems. But let’s first define what they are. As the name suggests, a frugivore-insectivore (or vice versa) it’s a bird that feeds both on fruit and insects, invertebrates, or other animal matter as a regular part of its diet. Of course, to be considered legitimate frugivores, these birds have to effectively disperse the seeds of plants in viable state and contribute to their population persistence and structure. Traditionally, frugivore-insectivores were considered “opportunist” frugivores and received little attention (McKey 1975), because fruit comprise, overall, less than 50% of their diets, sometimes as small a fraction as 15% (Carlo et al. 2003). But it is this precise characteristic that makes them so ecologically cool and important - let’s see why.

Specialist versus Opportunist Frugivores

First we have to consider the relative abundance of frugivore-insectivores in ecosystems and habitats. Some frugivore-insectivores can be amongst the most common, abundant, and ubiquitous fruit-eating birds, often much more than more heavily frugivorous species. Exemplary Neotropical and Nearctic frugivore-insectivores are found in the genera Turdus, Tyrannus, Pitangus, Mimus, and Vireo, just to mention a few (Skutch 1983). Frugivore-insectivores can be very abundant precisely because of their lack of specialization, allowing them to exploit both animal and fruit resources in different habitats, sometimes as sharply contrasting as pastures and forests. Also, their generalist nature makes their populations more resilient in the face of environmental fluctuations and perturbations, providing a backbone for plant-frugivore mutualistic networks (Bascompe & Jordano 2007). Let’s consider an example from Puerto Rico, an island in the Caribbean archipelago.

Frugivore-insectivores like this Black-Whiskered Vireo (photo left) can be very important providers of seed dispersal services, often being more important than species that rely more heavily on fruits. This is because frugivore-insectivores can be very abundant in many different habitat types and because they do not rely on fruit for survival. Fruit of Clusia rosea (Clusiaceae) (© Tomás A. Carlo).

A key example from Puerto Rico

In Puerto Rican forests, most of the avian community consume and prefer the fruits of Guarea Guidonia (Meliaceae) and Clusia rosea (Clusiaceae), which contain a lipid-rich red aril coating their seeds (Carlo et al. 2003). The fruits of Guarea and Clusia are particularly important for the endemic and largely frugivorous (85% fruit diet) Bullfinch (Loxigilla portoricensis, Emberizidae), both fruits composing an average of 50.2 % of their diet across the forested landscape (Carlo et al. 2003). The fruits are also important for large-bodied frugivores such as the Red-Legged Trush (Turdus plumbeus) and the Perly-eyed Thrasher (Margarops fuscatus). In contrast, the Black-Whiskered Vireo (Vireo altiloquous, Vireonidae) is a small frugivore-insectivore (35% fruit diet) that uses Guarea and Clusia only for up to 15.2 % of its diet.

Many fruit-eating animals are just consumers, and do not provide plants, in this case a Pithecellobium dulce tree with seed dispersal services, such as the White-winged parakeet (Brotogeris versicolorus, right) and the Bullfinch (Loxigilla portoricensis, see below) (Photos © Alfredo D. Colón Archilla). In turn, insectivore-frugivores, such as the Grey Kingbird (Tyrannus dominicensis, right) provide plants with effective seed dispersal despite that fruit constitutes a small fraction of their diets and are sometimes viewed as “opportunistic”. (Photo © Tomás A. Carlo).

Frugivores visitation and seed dispersal

By combining quantitative data on the abundance of each bird species in different sites, the frequency of use of each fruit species at each site, the average time spent on fruiting plants, and the number of seed-removal events per visit to each fruiting plant, Carlo et al. (2003) was able to estimate the relative contribution of each bird species to the dispersal Clusia and Guarea seeds across sites (Figure 1). It is outstanding that the Vireo, the bird that relies less on these fruits, appears as the quantitatively more important disperser of these important plants for the bird community! Thus, collectively, frugivore-insectivores like the Black-whiskered Vireo can be very important providers of seed dispersal services because of their great abundance and high fruit-removal services despite that fruits are not a main constituent of their diets. Moreover, frugivore-insectivores are probably keystone species in the architecture of frugivore-plant mutualistic networks (Bascompte & Jordano 2007).

Figure 1. Quantitative estimation of the contribution of different bird frugivore consumers to the dispersal of Guarea guidonea and Clusia rosea, two keystone fruiting trees in Puerto Rican forests. The most predictable consumer is the Bullfinch (Loxigilla portoricensis, below) (Photo © Alfredo D. Colón Archilla), but this species do not disperse seeds and just rob the aril. Other large frugivores are potentially good dispersers but are more scarse or rare in the environment, and thus, Vireo, the frugivore-insectivore that relies very little on these two species is the one providing the bulk of the dispersal services. The Y-axis is a normalized fruit-removal index considering the abundance of each bird species, the frequency of use of the fruits, and the fruit-handling behavior. The x-axis are different study sites and time periods. (from Carlo et al. 2003)

References

Bascompte, J., Jordano, P. 2007. Plant-Animal Mutualistic Networks: The Architecture of Biodiversity. Annu. Rev. Ecol. Evol. Syst. 38:567–93. Abstract.

Carlo , T. A., Collazo, J. A., Groom, M. J. 2003. Avian fruit preferences across a Puerto Rican forested landscape: pattern consistency and implications for seed removal. Oecologia 134:119-131. pdf

McKey, D. 1975. The ecology of coevolved seed dispersal systems. In Gilbert, L. E., and P. H. Raven, editors, Coevolution of animals and plants. University of Texas Press, Austin, Texas, USA. Pages 159-191.

Skutch, A. 1983. Birds of Tropical America. University of Texas Press, Austin, TX. Wikipedia

Link to experts

Jordi Bascompte

Tomás A. Carlo (photo)

Pedro Jordano

Doyle McKey

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