4.5 Importance of size overall
From my observations it appears that size is an important factor in determining which species gets to feed on fruit, only if the amount of fruit that is edible and/or accessible is limited. Given that the two species of trees I studied have very different fruiting strategies (abundant fruit available throughout the tree vs limited fruit being available sequentially), one might surmise that the size of a bird is more of a major factor in its success in feeding on fruits that are made available sparingly. However, one would have to examine many other species to see if that statement holds true, or whether bird feeding behavior in the fruiting trees I observed is an anomaly.
A second tentative conclusion can also be made. For both tree species larger birds could consume more than one fruit at a time, and often did so. Thus, it seems that my observations support the thesis that consumption of fruit is positively correlated with body size (Jordano 1986). One other possible explanation for the disparity in size between birds visiting the two trees is that larger-sized birds have higher energy needs which accounts for the relative difference in size between birds eating Huevos de Caballo fruits (lipid-rich) and those eating fruits of Gumbo Limbo (lipid-poor).
4.6 Lipid-rich fruits vs lipid-poor fruits
In their 2020 paper “Frugivory Specialization in Birds and Fruit Chemistry Structure Mutualistic Networks across the Neotropics” Pizo et al. analyzed 84 different studies of birds visiting various fruiting trees to feed. They also analyzed the different fruiting trees to determine whether their fruits were lipid-rich or lipid-poor, then charted the results to show which species were feeding on specific types of fruit and discussed how those feeding behaviors might ultimately affect seed dispersal of various plants. While my study only looked at two species of plants, their analysis shows that the family of one, Gumbo Limbo (Burseraceae ), is very low in lipids and high in sugars, while the family of the other, Huevos de Caballo(Aponcynaceae ), has one of the highest percentages of lipids in the fruit pulp. Thus, I thought it would be interesting to relate my findings to theirs regarding species feeding on these two different families. The results of these comparisons can be seen in Table 5 and Table 6. The order of each table is determined by the number of observations in which the species is noted feeding.
The Pizo study concluded that the family of flycatchers (Tyrannidae ) plays a major role in seed dispersal of fruiting trees in the tropics, and a recent study of Bursera tree species showed that family to be their major seed dispersers (Almazan-Nuñez 2016). Table 5, at first glance, seems to show the Thraupidaefamily feeding on Gumbo Limbo the most. However, if we combine all the observations of members of Tyrannidae , we find in fact that the percentage of both total observations of that family (32%) and observations when feeding was observed (37%) are larger than the amounts combined for members of Thraupidae (20% of total observations and 21% of observations when feeding was observed). Pizo’s study also found that the highest amount of frugivory was by members of the Thraupida e family with members of Tyrannidae being second. Thus, at least with regard to Gumbo Limbo, both our studies show these families to be the most frugivorous in the neotropics.
Despite this general agreement between our studies, I observed significant feeding activities in Gumbo Limbo by four members of bird families, which were not observed doing so in the studies analyzed by Pizo. These four species were Rose-breasted Grosbeak, Chestnut-capped Warbler, Rufous-naped Wren, and Rose-throated Becard. The extent of their feeding activities in Gumbo Limbo can be seen in Table 5. However, it is probable that the activities of Rose-breasted Grosbeak are undercounted. Like some other medium-sized species, Rose-breasted Grosbeak was a species that often fed numerous times each time it flew into a Gumbo Limbo. As evidenced both by photos and my own recall, it fed on these fruits many more times than the seven times indicated in the table.
Likewise, Table 6 (Birds Visiting Huevos de Caballo) shows more differences between my observations and the ones cited in Pizo et al. Overall, there were many fewer visits to this family of fruiting trees (Apocynaceae ) by birds in their studies. I also observed eight species of birds from bird families not observed feeding on the fruits of this family in their study. Five of those species (Rufous-naped Wren, Fiery-billed Aracari, White-winged Dove, Keel-billed Toucan and Groove-billed Ani) were observed feeding multiple times, and the number of their feedings were probably also undercounted due to my methodology.
What might account for these differences between my study and theirs? One might be the time of year when the studies were conducted. My studies were conducted at the beginning of the rainy season, when many species in my area are beginning to reproduce. Another occurrence at this time of year is that migrants are starting to move north. Could this possibly have influenced the great amount of feeding done by Rose-breasted Grosbeak on Gumbo Limbo fruits? A third possibility is that some species (either birds and/or trees), which are common where I live, are less common and/or absent from the areas where other studies were conducted.