Six-month field study links Citreoline Trogon abundance to seasonal fruit availability in an Oaxaca dry forest

A six-month field study in a regenerating tropical dry forest in Oaxaca found that Citreoline Trogons were more abundant in months with greater fruit availability. Researchers recorded 159 observations of the birds and 41 foraging events involving 88 individuals, documenting fruit consumption from 14 plant species in 10 families. The source gives inconsistent labels for the sampling years, describing the November–April field period variously as 2020–2021 and 2021–2022.

The study was conducted in the 16.7-hectare Puerto Escondido Botanical Garden of the Universidad del Mar, in the coastal plain of Oaxaca in southwestern Mexico. The area had been used to grow corn, beans, sesame and pumpkin from 1972 to 1983 before being donated to the university in 2004. At the time of the research, roughly 20 years of secondary succession had produced tropical dry forest on hills ranging from 70 to 160 metres above sea level. Common tree species included Bursera simaruba, Ceiba parvifolia, Cochlospermum vitifolium, Comocladia engleriana, Guapira macrocarpa and Tabebuia chrysantha.

To estimate abundance, researchers established 15 fixed-radius point counts separated by 150 metres. A single observer spent 10 minutes at each point recording all Citreoline Trogons seen or heard, including their number and activity. Counts took place from 6:00 to 10:00 and from 16:30 to 18:30. The overall relative abundance was 2.6 ± 2.3 individuals per point count. April had the largest number of records, 46 of 159, while December had the smallest, with 16 records. The reported mean was 3.9 ± 2.5 individuals in April and 1.1 ± 1.0 in December. A Friedman test indicated significant monthly differences, and a Tukey post hoc comparison identified April as significantly higher than December; the source does not report other pairwise comparisons as significant.

Diet was determined through focal and opportunistic observations of foraging birds. For each record, observers noted the date, time, number of individuals and tree or shrub species being used. A new foraging record was created when birds moved to another resource on the same tree or to a different tree. Across the observations, all recorded feeding involved fruit, with no insect consumption documented. Ficus cotinifolia accounted for 24 individuals, or 27.3% of those observed feeding; Comocladia engleriana accounted for 17, or 19.3%; and Randia gentlei accounted for 14, or 15.9%. Together, those three plants contributed more than 60% of the observed diet.

The researchers calculated a standardized Levins food-niche breadth of 0.420, describing resource use as moderately broad, with six plant species contributing more than 5% each. The Shannon–Wiener index was H′ = 0.94, classified by the study as low dietary diversity, while the Simpson index was λ = 0.85, indicating strong dominance by particular food resources. Of the 14 plants consumed, 57.2% were described as species of mature deciduous tropical forest and 42.8% as common in regenerating forests. The authors also identified 10 plant species not previously reported in the Citreoline Trogon diet, including R. gentlei, Passiflora sp. and Ampelocissus acapulcensis.

To measure fruit availability, the team established 10 phenological transects, each 100 metres long and 4 metres wide, and visited them monthly. All fruiting trees and shrubs were marked and identified, their diameter at breast height measured, and the proportion of each crown bearing fruit placed into one of four categories. The investigators then combined these measures in a fruit abundance index and also tracked the number of species and individual plants fruiting. The transects contained 266 trees from 28 species and 13 families. Food availability varied significantly by month for the number of species fruiting, the number of trees fruiting and the fruit abundance index. More species and more trees were fruiting in November and April, while the fruit abundance index increased from March to April and reached 6.1 ± 2.4 in April, compared with 1.9 ± 0.8 in January.

Monthly trogon counts were positively associated with fruit availability. The total fruit abundance index had a correlation of r = 0.61 with the number of trogons recorded per point count, P < 0.001. The relationship was stronger when only fruit from known dietary trees was considered, with r = 0.68, P < 0.001. These correlations support an association between seasonal fruit availability and Citreoline Trogon abundance but do not by themselves establish that fruit availability caused the observed changes. The observed diet also changed over time: birds ate R. gentlei, Diospyros verae-crucis and Tabernaemontana grandiflora during November and December; D. verae-crucis and Coccoloba liebmannii during January and February; and Comocladia engleriana, Spondias purpurea and Ampelocissus acapulcensis during later parts of the dry season.

The study provides a first account of Citreoline Trogon abundance in a regenerating tropical dry forest on Mexico’s Pacific coast, but its scope was limited to one small site, one six-month dry-season period and relatively few foraging observations. Earlier work has reported insect consumption by the species during the wet season, so the absence of insects here cannot be treated as representative of its year-round diet. The authors note that the species was not legally protected by Mexican law and that published abundance information was lacking before this study. They call for continued monitoring of the population and for wet-season research to obtain a fuller account of the bird’s foraging strategy in regenerating forests.

Subject of Research: Seasonal abundance, fruit diet and resource availability of Citreoline Trogons in a regenerating tropical dry forest in Oaxaca, Mexico

Article Title: Abundance, diet and food availability of Citreoline Trogon ( Trogon citreolus ) in a regenerating tropical dry forest in the Central Pacific, Mexico

Article References: Floreán-Díaz, E., & De Labra-Hernández, M. A. (2024). Abundance, diet and food availability of Citreoline Trogon (Trogon citreolus) in a regenerating tropical dry forest in the Central Pacific, Mexico. Neotropical Biology and Conservation, 19(4), 411-425. https://doi.org/10.3897/neotropical.19.e128529

Image Credits: AI Generated

DOI: 10.3897/neotropical.19.e128529

Keywords: Citreoline Trogon, Trogon citreolus, tropical dry forest, forest regeneration, seasonal abundance, frugivory, fruit availability, Oaxaca, Mexico, bird ecology, Abundance, diet