Birds Project

Feeding preferences of granivorous birds

Information on granivorous birds preferences for different types of seeds allows us to propose hypothesis on the consequences of seed consumption on soil seed banks and, ultimately, on the abundance and composition of plant communities.

Birds may prefer certain seed species or sizes due to their morphological and/or nutritional traits as well as to the presence of secondary metabolites (for example tannins, phenols or alkaloids). However, these preferences might not be reflected on birds' diet, as there are other factors that could modify them. For instance, once they arrive at the soil, seeds decay as a result of being exposed to humidity, wind, changes in temperature, and the attack by fungi and bacteria. Consequently, seeds which have been on the soil for some time become moldy and this may lead to birds' avoidance. The location of seeds within the habitat affects the availability for birds in a similar way because seeds on the soil surface, in the litter, on plants or buried seeds represent very different situations for many of them. In fact, birds might be able to access to seeds present in just some of those conditions. Finally, seed use can be affected by the relative abundance of seed species in the habitat, at least for those birds which show no restrictive preferences and feed at random or in microhabitats with high densities of certain seeds.

Therefore, the study of birds' diet (i.e., of the proportion of the different types of food in digestive tracts) is not enough to establish feeding preferences not even to reveal the mechanisms that constrain these preferences and determine the diet. In order to understand the mechanisms involved, it is necessary to carry out experiments under controlled conditions to evaluate the composition of the diet when equal amounts of seed types or species are offered (Cueto et al. 2001). The results of these experiments compared to the effective diet in the field can help us to understand the factors conditioning birds' feeding behaviour and its degree of plasticity.