How do frugivores track resources? Insights from spatial analyses of bird foraging in a tropical forest

Abstract

Frugivores often track ripe fruit abundance closely across local areas despite the ephemeral and typically patchy distributions of this resource. We use spatial auto- and cross-correlation analyses to quantify spatial patterns of fruit abundance and avian frugivory across a 4-month period within a forested 4.05-ha study grid in Puerto Rico. Analyses focused on two tanager species, Spindalis portoricensis and Nesospingus speculiferus, and their principal food plants. Three broad questions are addressed: (1). at what spatial scales is fruit abundance and frugivory patchy; (2). at what spatial scales do frugivores respond to fruit abundance; and (3). to what extent do spatial patterns of frugivory overlap between bird species? Fruit patch size, species composition, and heterogeneity was variable among months, despite fruit patch locations remaining relatively consistent between months. Positive correlations between frugivory and fruit abundance suggested tanagers successfully tracked fruit abundance. Frugivory was, however, more localized than fruit abundance. Scales of spatial overlap in frugivory and monthly variation in the foraging locations of the two tanager species suggested that interspecific facilitation may have been important in determining bird foraging locations. In particular, S. portoricensis, a specialist frugivore, may have relied on the loud calls of the gregarious generalist, N. speculiferus, to find new foraging areas. Such a mechanism could help explain the formation of mixed species feeding flocks and highlights the potential importance of facilitation between species that share resources.

Species richness matters for the quality of ecosystem services: a test using seed dispersal by frugivorous birds

“…Such behavioural differences would increase the correlation between frugivore functional diversity and species richness, making seed arrival and colonization rates especially sensitive to richness changes. Finally, some facilitation effects may also come into play, such as higher richness being derived from some species of thrushes tracking the presence of others across the foraging landscape [42], and thereby providing an additive local dispersal service. These sort of facilitative interactions do probably occur in our system, given that observations of different species feeding simultaneously at the same group of neighbour fruiting trees, or flying together in multi Further to complementarity and facilitation, our study also provides some clues about sampling effects, whereby a richer frugivore assemblage should have an increased probability of containing dominant species, monopolizing seed dispersal function and hence increasing greatly the magnitude of service [1].…”

Birds foraging for fruits and insects in shrubby restinga vegetation, southeastern Brazil

“…This last species is known to forage in mixed flocks (Machado 1999). The fact that fruits of a given species are ephemeral and patchy may explain flocking in birds, which facilitates encountering such resources Saracco et al (2004). Besides that, mixed flocks also take advantage of a greater vigilance against predators (Powell 1985), which in an open habitat as the shrubby restinga may be critical.…”

Disentangling habitat use by frugivorous birds: Constant interactive effects of forest cover and fruit availability

“…between birds foraging in the same tree). In fact, the relative abundances of the different thrush species are frequently correlated in our study system (García & Martínez 2012), suggesting interspecific facilitation rather than competition at the population level (see also Saracco et al 2004). …”

“…Further studies should address whether the temporally consistent interactive effects of habitat cover and resource availability evidenced here are generalizable to more complex plantfrugivore systems (e.g. tropical forest; Luck & Daily 2003;Lehouck et al 2009;Saracco et al 2004), but also to other ecosystem services derived from the trophic activity of animals (e.g. pollination, pest control; Kremen et al 2007) in real-world landscapes.…”

“…In the case of frugivorous birds, fruit abundance and woody plant cover have been frequently described as major drivers of habitat use (Herrera 1985;García, Zamora, & Amico 2011;Blendinger et al 2012). Birds are able to optimize their foraging by tracking fruit abundance through space and time, resulting in more intense use of fruit-rich patches (Herrera 1985;Rey 1995;Saracco, Collazo, & Goom 2004;Blendinger et al 2012; but see Tellería, Ramírez, & Pérez-Tris 2008 for examples of non-optimal foraging). Protection against predators, often provided by dense woody plant cover, is also a fundamental habitat condition for frugivorous birds (but see Luck & Daily 2003), who frequently avoid open areas where they are more vulnerable (Cardoso da Silva, Uhl, & Murray 1996;Cordeiro & Howe 2003;García et al 2011).…”