Birds Project

Use of space by granivorous birds (for feeding)

Seeds are an important resource in arid and semiarid ecosystems due to their abundance and diversity. In the open woodland (algarrobal) of Ñacuñán, as well as in many deserts, seeds have a very heterogeneous distribution at small scales (from one to a few meters: microsites) associated with the structure and composition of the vegetation (see Seeds). Fluctuations in time are also typical, so the relative abundance of different types of seeds (e.g., grasses vs. forbs) are quite predictable. On the other hand, granivorous birds, the main seed consumers in autumn-winter, have marked preferences for grass seeds (see Feeding preferences).

Granivorous birds could be constrained in their use of feeding sites due to those variations in their resource, but they could also be adapted for searching in those sites in which the abundance of the preferred seeds is higher. This could have consequences not only on birds (bottom-up effects), but also on plant populations (top-down effects), which particularly depend on their seed stage in arid ecosystems. The consequences of a non-random use of space are even deeper because, even if the distribution of birds in space is a consequence of individual decisions, these individual selection patterns can affect the species coexistence and therefore modify the structure and organization of communities. Furthermore, by knowing the non-random use of space we can assess more accurately the actual seed availability for birds and determine whether food is limiting the populations.

However, there are some factors besides seed abundance involved in the decision of an individual bird's of where to feed. Examples of this are accessibility and efficiencies in searching and consuming seeds, microclimatic conditions and the degree of exposure to predators. Moreover, the observed patterns of use at the microsite scale could in fact be a consequence of the election of a feature at a broader scale, such as distance to the nesting site or refuge or other more general habitat features (see Marone 1991, Marone et al. 1997).

The main questions that we have posed are:

Do granivorous birds use the habitat (to feed) randomly? Do all the bird species within the guild respond homogeneously? Is this response seasonally variable?

If there is a selection (use < availability), is it related to the heterogeneity (relatively permanent) observed in the seeds on the soil bank at a microsite scale? Could this be evaluated through the characterization of the vegetation?

Is the use of feeding sites at the microsite scale constrained by (is a by-product of), or it can predict/explain, the selection at other (broader) scales?

Could other factors explain the use of feeding microsites (accessibility to food, predation risk, thermoregulation, social/interspecific interactions), better than the abundance of (preferred) seeds?

Which is the influence of the detected use on plant populations (distribution and abundance of seeds)?

In order to answer these questions, we have performed estimations of granivorous birds' activity by sampling transects running through different habitats (see Marone et al. 1997), systematic observations of individuals while feeding and seed offer experiments in the field to compare used sites at various scales with available ones, and microsite preference experiments in field aviaries.