Richness, distribution and conservation status of cavity nesting birds in Mexico
Introduction
In birds, nesting site selection (cavity nesting vs. open nesting) is known to influence an important number of life-history traits such as nesting success, clutch size, incubation time, nestling period length, and adult survival during incubation (Martin and Li, 1992, Martin, 1995, Eberhard, 2002). Worldwide, a substantial percentage of landbirds depend on tree cavities for their reproduction (Newton, 1998). Cavity nests provide important advantages, such as greater thermal insulation (Joy, 2000), lower risk of nest predation, and higher nest success compared to open nests (Scott, 1979, Nilsson, 1986, Martin and Li, 1992, Martin, 1995). Most cavity nesters select cavities in tall, large-diameter trees and snags to maximize brood space and avoid terrestrial predators; different studies have shown a decrease in predation rate with the increasing height of nest placement (Nilsson, 1984, Dobkin et al., 1995, Joy, 2000). Cavity nesting species have been classified in two groups, primary cavity nester species (which dig their own cavities) also called “excavators”, and secondary cavity nesters or non-excavators (which nest in pre-existing cavities) also known as “cavity adopter species”. The two types of cavity nesters differ in several important life-history traits, including nesting success, clutch size, nestling period length, and adult survival (Martin and Li, 1992, Eberhard, 2002, Arsenault, 2004, Martin et al., 2004).
Cavity nesting species constitute a structured community that interacts through the creation and competition for nesting sites in temperate forests. Analogous to food webs, some species depend partly or entirely on excavators to produce a key resource “tree cavities” (Martin et al., 2004). In absence of human interference, all natural forested vegetation provide usable cavities in large trees and snags, and their availability increases with age and decay of trees and forest patches, which augment the amount of dead wood and facilitates cavity formation (Newton, 1998, Joy, 2000). Persistence of cavity-nesting species depends on the continual recruitment of large trees and snags, where cavities are form as a result of the natural process of forest decay. Some studies have demonstrated that cavity nesters populations are limited by suitable nesting cavities availability, this situation is observed more frequently in highly managed temperate habitats, but also witnessed in tropical forests (Hilden, 1965, Scott, 1979, Mannan et al., 1980, Munn, 1992, Collar and Juniper, 1992, Gibbs et al., 1993, Newton, 1994, Holt and Martin, 1997, Saab et al., 2004).
The great majority of studies about cavity nesting species have been done in North America and Europe, showing that 4–5% of landbird resident species are obligate cavity-nesters, whereas in Australia approximately 11% of resident species are obligate cavity-nesters (Saunders et al., 1982, Newton, 1998).
In Mexico, this is the first published analysis focused on assessing the proportion of cavity nesting species and their status. This analysis is particularly important because the country has one of the highest rates of forest loss and degradation in Latin America. Agrosystem expansion and widespread intensive forest exploitation for timber purposes have contributed to the estimated loss of 90% of tropical rainforests, 67% of tropical dry forests, 54% of cloud forests, 50% of coniferous forests, and 99% of high-elevation old-growth forests in the last 50 years, with annual forest loss rates from 1% to10% depending on region and vegetation types (Lammertink et al., 1996, Challenger, 1998, INE-UNAM, 2002).
Secondary forests are rapidly replacing primary forests in temperate and tropical regions, most forestry practices include intensive and selective logging of large commercial trees and the removal of large standing snags as “sanitary procedures” (Lammertink et al., 1996, Challenger, 1998). Secondary forests do not posses the specific structural properties required by most cavity nesting species, due to the majority of standing trees have small dimensions for cavity formation (Newton, 1994). Management plans in forestry districts do not take into consideration habitat requirements of native fauna because wildlife protection laws are poorly enforced. The country has not implemented breeding bird survey programs, making difficult to asses the situation of most bird species. However, a decline of many obligate cavity nesting populations have been recently documented (Iñigo-Elías, 1996, Ceballos and Valdelamar, 2000, Monterrubio-Rico and Enkerlin-Hoeflich, 2004). In addition, all remaining breeding habitats for some endangered cavity-nesting species are inside parks and reserves, this situation is faced today by the scarlet macaw (Ara macao) and the resplendent quetzal (Pharomachrus mocinno) (Hernández-Baños et al., 1995, Iñigo-Elías, 1996, Ceballos and Valdelamar, 2000). The extinction of the endemic imperial woodpecker (Campephilus imperialis), the largest species of woodpecker in the world (Lammertink et al., 1996), is the most dramatic remainders of the negative effects that inadequate forestry practices are causing to the resident avifauna in Mexico.
Due to the sensitivity of cavity nesting species in terms of nesting requirements, the alarming rates of forest loss and degradation, and the absence of national breeding bird survey programs, it is necessary to asses the number of cavity nesting species potentially vulnerable. For those reasons, we conducted our study focused on the following aims: (a) to determine the percentage of the Mexican landbird avifauna requiring tree cavities for nesting, (b) to identify the habitats and relevant protected areas to conserve cavity nesting species, and (c) to analyze the percentage of species in status comparing cavity nesters and open nesters.
We hypothesized that the present high rates of forest degradation are affecting more severely cavity nesters than non-cavity nesters, if this is true, we can expect a higher proportion of cavity nesting species threatened or endangered. We also expected that forest degradation is not affecting equally the two groups of cavity nesting species, with a higher proportion of species at risk in “secondary cavity nesters” than “primary cavity nesters”. In addition, we predicted that the highest concentration of cavity nesting species would occur more frequently in habitats with greater structural complexity as tropical rain forests than in less complex habitats as temperate forests.
Section snippets
Methods
We gathered data about the distribution of resident landbird avifauna from 35 areas covering the major vegetation types and different regions of Mexico (Arriaga et al., 2000). Most areas analyzed correspond to Biosphere Reserves, National Parks or IBAS (Important Bird Areas). These areas posses several forested habitats; and relatively well-known avifaunas counting with published bird checklists. Some of these areas are the best preserved and last remaining forested habitats of considerable
Results
A preliminary species assessment resulted in a total of 657 resident landbird species that breed in temperate and tropical habitats of the country, hence were included in the analysis. This figure represents 65% of the 1007 bird species known to occur in Mexico based on the estimates of Escalante-Pliego et al. (1993). Of these 657 species, 112 (17%) have different tree cavity requirements for nesting, including 81 (12%) species that are obligate cavity nesters (Table 1 and Appendix A). From
Discussion
The resident landbird avifauna in Mexico can be considered rich and diverse but relatively vulnerable based on the high proportion of species requiring tree cavities for their reproduction and the widespread reduction of primary forests. The high number of cavity-nesting birds in Mexico is due, in large part, to the belonging of Mexico to both Nearctic and Neotropical regions. Cavity-nesting species include bird families with Neotropical and Nearctic affinities. Two families were particularly
Acknowledgments
We appreciate the help and advise of K. Renton, J. Van Remsen, Mark. S. Hafner, Lorena Tellez-García. We greatly appreciate the suggestions made by two anonymous reviewers. We thank to Sandy J. Andelman for the considerations to the manuscript. Financial support was provided by Fondo Sectorial Ambiental SEMARNAT-CONACYT 2002-C01-0021. We thank Coordinación de Investigación Científica and Facultad de Biología at Universidad Michoacana de San Nicolás de Hidalgo, and Instituto de Biología UNAM for
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