Understory bird taxonomic and functional diversity during secondary forest succession in the tropical Andes

Diversidad taxonómica y funcional de las aves de sotobosque durante la sucesión secundaria del bosque en los Andes tropicales

Abstract

Understanding understory bird taxonomic and functional diversity patterns across successional stages can be an essential criterion for conservation and ecological restoration strategies in the humid forests of the tropical Andes. We evaluated understory bird taxonomic and functional diversity in the secondary successional forests of the tropical Andes. Three successional stages were analyzed: young secondary forest (10–15 years old), mid-successional forest (25–30 years old), and mature forest (~100 years old). Birds were captured using mist nets across these successional stages. We defined functional traits for the captured species, including morphological, life history, and behavioral characteristics. The results showed that the alpha taxonomic diversity of understory bird species was similar across the three stages, with a total of 126 species recorded. However, alpha functional diversity varied, being lower in the mature forest. Beta taxonomic and functional diversity revealed that species turnover was the principal cause of the differences between successional stages. Specifically, species turnover between young and mid-secondary forests resulted from the colonization of mid-successional bird species and the local extinction of shrubland species. Functionally, generalist understory birds dominated young forests, while mid-secondary forests exhibited a greater variety of birds with foraging adaptations to different vegetation strata. In mature forests, the presence of understory insectivores, such as ant followers, was characteristic of this stage. These bird functional patterns reflect the environmental filter imposed by vegetation structure and resource availability at each successional stage. The functional diversity of the communities associated with each successional stage would have differential effects on the functioning of tropical Andean ecosystems. Conservation efforts should emphasize functional diversity and the protection of mature forests to maintain ecosystem resilience and support specialized bird species.

Keywords
Bird conservation; ecological restoration; environmental filtering; functional traits; successional stages; understory

Resumen

Comprender los patrones de diversidad taxonómica y funcional de las aves de sotobosque a lo largo de las etapas sucesionales puede proporcionar información esencial para las estrategias de conservación y restauración ecológica en los bosques húmedos de los Andes tropicales. Se evaluó la diversidad taxonómica y funcional de las aves de sotobosque en los bosques secundarios sucesionales de los Andes tropicales. Se analizaron tres etapas sucesionales: bosque secundario joven (10–15 años de edad), bosque secundario intermedio (25–30 años de edad) y bosque maduro (~100 años de edad). Las aves fueron capturadas utilizando redes de niebla en estas etapas sucesionales. Definimos rasgos funcionales para las especies capturadas, incluyendo características morfológicas, de historia de vida y de comportamiento. Los resultados indican que la diversidad taxonómica alfa de las especies de aves de sotobosque fue similar en las tres etapas, con un total de 126 especies registradas. Sin embargo, la diversidad funcional alfa varió significativamente, siendo más baja en el bosque maduro. La diversidad beta taxonómica y funcional reveló que el recambio de especies fue responsable de la mayoría de las diferencias entre las etapas sucesionales. Específicamente, el recambio de especies entre los bosques secundarios jóvenes e intermedios se debió a la colonización por parte de especies de aves de estadios secundarios intermedios y a la extinción local de especies dependientes de los arbustos. Desde el punto de vista funcional, las aves de sotobosque generalistas dominaron los bosques jóvenes, mientras que los bosques secundarios medios exhibieron una mayor variedad de aves con adaptaciones de forrajeo a diferentes estratos de vegetación. En los bosques maduros, la presencia de insectívoros del sotobosque, como los seguidores de hormigas, fue característica de esta etapa. Estos patrones funcionales de las aves reflejan el filtro ambiental impuesto por la estructura de la vegetación y la disponibilidad de recursos en cada etapa sucesional. La diversidad funcional de las comunidades asociadas con cada etapa sucesional puede tener efectos diferenciales sobre el funcionamiento de los ecosistemas andinos tropicales. Los esfuerzos de conservación deben enfatizar en la diversidad funcional y la protección de los bosques maduros para mantener la resiliencia del ecosistema y apoyar a las aves especializadas.

Palavras-chave
Conservación de aves; restauración ecológica; filtrado ambiental; rasgos funcionales; etapas sucesionales; sotobosque

Introduction

The expansion of agriculture and livestock, and the subsequent use and abandonment of land dedicated to these activities, has resulted in secondary succession forests in a large part of the tropical region (Walker et al. 2010). Globally, forest cover extends to approximately 4,343 million hectares, with secondary forests accounting for about 64% (~2,956 million hectares) of this total. Notably, in South America, secondary forests cover approximately 723 million hectares (Coradini et al. 2022, FAO 2025). In particular, tropical rainforests of the northern Andes (Hotspot) have lost approximately 60% of their area; most of them are in early secondary successional stages (Armenteras & Moreno 2007, Armenteras et al. 2011, Hurtado et al. 2022). Therefore, secondary successional stages could be relevant in species conservation (Wright 2005, Vargas-Daza et al. 2023). Secondary succession is the ecological development of a community in a disturbed site to a relatively stable state. It is a dynamic process that involves changes in community composition, structure, and ecosystem functions over time (Guariguata & Ostertag 2001, Rozendaal et al. 2019). Through secondary succession, changes in the richness and composition of taxonomic and functional diversity in various vertebrate groups have been reported (see Acevedo-Charry & Aide 2019). Understory birds are a sensitive group for understanding the long- and medium-term community responses during the secondary succession (Laurance et al. 2002, Borges et al. 2021, Coddington et al. 2023).

Changes in vegetation structure, floristic composition, and resource supply occur during secondary succession in tropical forests (Dewalt et al. 2003, Ruiz et al. 2005). These changes in forest characteristics act as environmental filters that can determine differences in the taxonomic identity of understory birds inhabiting these forests and, consequently, in their functional traits (Luck et al. 2013, Acevedo-Charry & Aide 2019, Espejo & Morales 2019). Changes in bird species richness and composition at the taxonomic or functional level have been described during secondary succession (Acevedo-Charry & Aide 2019, Borges et al. 2021). For example, rapid increases in bird species richness have been observed in the early stages of secondary succession, mainly accompanied by changes in composition due to species turnover (Borges et al. 2021). Furthermore, an increasing differentiation of functional traits has been identified during the succession process (Manzoli et al. 2023) as well as changes in the functional diversity of birds related to the supply of resources (Espejo & Morales 2019, Acevedo-Charry & Aide 2019, Borges et al. 2021). Nevertheless, there are gaps in information and knowledge of the changes in the taxonomic and functional diversity of understory birds during secondary succession in areas with high bird richness, such as the neotropical region and specifically Colombia (Espejo & Morales 2019, Echeverry-Galvis et al. 2022). Therefore, this study contributes to the knowledge of the changes at the taxonomic and functional level during secondary succession in understory avifauna, which can provide some essential criteria for conservation and ecological restoration strategies in the humid forests of the tropical Andes.

This study evaluated the differences in the alpha and beta taxonomic and functional diversity of understory bird assemblages associated with forests at three successional stages (i.e., young, mid-successional, and mature secondary forests) in the tropical Andes. As secondary succession advances, there is an increase in heterogeneity, complexity of vegetation structure, and resource availability (Purschke et al. 2013, Espejo & Morales 2019), providing diverse habitats that can harbor a wider range of understory birds (Terborgh 1985, Castaño-Villa et al. 2014, Betancurt-Grisales et al. 2021). We expect alpha taxonomic and functional understory bird diversity to increase across secondary succession, enabling the coexistence of species with diverse traits. Additionally, species turnover will drive beta taxonomic and functional diversity, as changes in vegetation structure and resources create new opportunities for colonization by forest-specialist species.

Material and Methods

1. Study area

The study was carried out in a tropical humid forest of the central Colombian Andes, located in the municipalities of Norcasia (5° 34′ 30″N, 74° 53′ 19″W) and Samaná (5° 31′ 59″N, 74° 57′ 0″W) in an elevation range between 533 and 810 m (Figure 1). The study area has a mean annual temperature of 25.5 °C and precipitation of 5500 mm (Gil-Ospina & Moreno-López 2020). The forests in the study area are part of the protected area within the Amaní reservoir and the Manso River basin. Based on the information provided by the inhabitants (age of the forest) and characteristics of the vegetation structure observed in the field (canopy height), two forest patches were identified for each of the three successional stages evaluated. 1) young secondary forest (hereafter YSF): these forests are between 10 and 15 years old, with two vertical strata of vegetation and a canopy height of 9 m; 2) mid-successional secondary forest (hereafter MSF): these forests are between 25 and 30 years old, with three strata of vegetation reaching a canopy height of 30 m; 3) mature forest (hereafter MF): forests up to 100 years old, with five vertical strata of vegetation, and a canopy up to 35 m. Young and mid-successional forests were previously used for agricultural production (Vargas-Daza et al. 2023). The denomination of the successional stages of the forests followed the criteria of Acevedo-Charry & Aide (2019).

Figure 1
Forests location where bird captures were conducted. (A) Study area location in Colombia, (B) and Caldas, (C) successional stages location: Young Secondary Forest (YSF), Mid-successional Secondary Forest (MSF), and Mature Forest (MF) within the study area.

2. Bird data

We used information on understory bird species (i.e., traits), abundance (number of individuals captured), and species richness (number of species captured during each sampling event) collected in the field between 2014 and 2017 (Agreements 47/180 and 47/623 Universidad de Caldas and ISAGEN). We sampled each forest patch 12 times for four years (except for one patch of young secondary forest that was sampled 11 times). In each sample, five mist nets (12 m * 2 m * 30 mm) were placed inside each forest patch from 06:00 hours to 11:00 hours. Sampling was carried out regularly every four months. We used mist nets to capture avifauna because it is a widely used technique for sampling bird communities in the tropics with less error in bird identification and allows direct collection of morphological information of the species (Barlow et al. 2007, Castaño-Villa et al. 2014, Betancurt-Grisales et al. 2021). Mist nets were not open in heavy rain or heat to reduce the risk of accidental bird death. Sampling effort for young secondary forest was 1500 hours/net, for mid-successional secondary forest 1800 hours/net, and for mature forest 2100 hours/net. Captured individuals were temporarily marked using a small cut on the first rectrix or with metal bands to avoid recounts (Martínez-Sánchez et al. 2018). All capture and marking procedures followed established ethical guidelines for the use of wild birds in research, and were conducted by trained personnel to minimize handling time, stress, and risk of injury. This research was conducted under the framework permit granted to the University of Caldas by the National Environmental Licensing Authority of Colombia (ANLA) through resolution 1166 of October 09, 2014. Birds were released at the site of capture. Birds were taxonomically identified according to Remsen et al. (2025).

3. Bird functional traits

The functional traits of understory bird included in the analyses were selected to be sensitive to changes in habitat structure, vegetation composition, and resource supply (Luck et al. 2012). Five morphological functional traits were defined, wing length (RW), tail length (T), tarsus length (Ta) and total culmen length (TC) measurements were taken in millimeters (mm), and body weight (Wg) in grams (g), as proposed by Betancurt-Grisales et al. (2021) and Velásquez-Trujillo et al. (2021). Measurements were made with a spi 2000 analog plastic caliper, an Avinet wing measuring ruler, and a digital scale. The morphological traits of each bird species were obtained from field records and completed by measuring specimens from the ornithological collection of the Natural History Museum of the University of Caldas and with morphological information published by Cardona-Salazar et al. (2020). Two life history traits were established: diet (frugivorous [Fr], granivorous [Gr], insectivorous [In] and nectarivorous [Ne]) and nest type (cup [Cu], enclosed [En], hole [Ho], and platform [Pl]). Two feeding behavior traits were also included in our analyses: foraging strata (ground [Grn], understory [Un], subcanopy [Sca], and canopy [Ca]) and foraging strategies (bark [Ba], flower-visiting [Fv], flycatching [Fy], searcher [R], and gleaner [Gl]). Life history and behavioral traits were consulted from Hoyo et al. (1992–2011) and defined according to Betancurt-Grisales et al. (2021), Velásquez-Trujillo et al. (2021), and Fontúrbel et al. (2022). The bird functional traits subsequently were scored binomially (present = 1 or absent = 0). The categories of functional traits by bird species are presented in Table S1.

4. Data analysis

4.1. Taxonomic and functional alpha diversity

To describe understory bird species richness across functional trait categories, stacked bar charts were created to represent the number of species recorded in each successional stage for four traits: diet type, foraging strata, nest type, and foraging strategies. Species that consume two or more food items were grouped as omnivores. Similarly, species that use two or more strata for foraging or have two or more foraging strategies were grouped as several strata and several strategies, respectively. We estimated the bird taxonomic diversity for each successional stage of the forest as the mean observed richness of species recorded in each successional stage. The abundance of each bird species captured for each successional stage (Matrix L) and information on morphological, life history, and behavioral functional traits (Matrix Q) were used to calculate functional diversity. The functional life history and behavioral traits included in Matrix Q were previously transformed from binomial data to numerical data by Spearman’s rank correlation (Céréghino et al. 2018). This method involves assigning ranks to the original trait values, appropriately handling ties (repeated values). For each trait, values are ordered from lowest to highest, and consecutive ranks are assigned. In case of ties, the average rank is calculated for those tied values. This approach facilitates their use in quantitative analyses of functional diversity (Legendre & Legendre 2012). With the information from the L and Q matrices, the RaoQ index quadratic entropy-based (Rao 1982) was calculated. This index is one of the most widely used estimators of functional diversity because it includes species abundance and more than one functional trait (Botta-Dukát 2005). The RaoQ index indicates the expected dissimilarity of a trait between two random individuals in a community (Ricotta 2005, Velásquez-Trujillo et al. 2021). The RaoQ index was calculated with the SYNCSA package (Debastiani 2018) in R version 4.3.3 (R Development Core Team 2024).

To compare taxonomic (mean observed richness) and functional (RaoQ) diversity of understory birds among successional stages, we fit Generalized Linear Mixed Models (GLMMs, hereafter) with a Gaussian error distribution for each response variable. We included the successional stages (YSF, MSF, and MF) as a fixed effect, and forest patch identity as a random effect to account for the hierarchical sampling design. It should be noted that the models yielded a singular fit for the random effect (variance estimated at or near zero), indicating no detectable variability between the paired patches within each successional stage relative to the within-patch temporal variability. To fitted the GLMMs, we used the lme4 package (Bates et al. 2014), and for the posterior multiple comparisons, the multcomp package (Hothorn et al. 2016), in R version 4.3.3 (R Development Core Team 2024).

5. Taxonomic and functional beta diversity

To compare understory bird beta taxonomic and functional diversity between successional stages, we divided total beta diversity (β_jac) into two additive components: dissimilarity derived from species turnover (β_jtu, turnover) and dissimilarity derived from differences in the number of species (β_jne, nestedness) according to Baselga (2010). We estimated beta taxonomic diversity using Jaccard’s dissimilarity index from incidence data (absence: 0 and presence: 1) of bird species present in each successional stage. We calculated functional diversity beta using the L matrix modified to incidence data and Q matrix. Principal coordinate analyses were then run. Following Magneville et al. (2022), we retained the first five axes to optimize the representation of the functional space. (Table S1), to gain the transformed coordinates, which were then used to calculate beta functional diversity of birds among successional stages. This analysis was performed with Jaccard’s dissimilarity index. Beta taxonomic diversity was calculated using the betapart package (Baselga et al. 2017), and beta functional diversity was estimated using the mFD package (Magneville et al. 2022) in R version 4.3.3 (R Development Core Team 2024).

Results

1. Taxonomic and functional alpha diversity

A total of 2509 individuals of 126 understory bird species distributed in 26 families were recorded. The families with the highest number of species in the three successional stages were Trochilidae, Tyrannidae and Thraupidae. The total species richness observed for young secondary, mid-successional secondary, and mature forests was 83, 99, and 93 species, respectively. The richness of omnivorous species (including two or more food items in their diet) was higher in the mid-successional secondary forest (S = 72) than in the young secondary forest (S = 59) or mature forest (S = 64) (Figure 2A, see Table S2). Whereas in the mature forest, there was a higher richness of insectivorous species (S = 25) than in the other two successional stages of the forest (e.g., Hafferia immaculata, Gymnopithys bicolor, Terenotriccus erythrurus, Myiobius atricaudus, Pheugopedius spadix and Microbates cinereiventris) (Figure 2A, see Table S2). In mid-successional secondary forest, there was a higher richness of foraging species in various vegetation strata (S = 38) than in the other two forests (Figure 2B; see Table S2). In contrast, in mature forest there was a higher richness of understory-associated species (S = 31) than in the other successional stages (Figure 2B, see Table S2). In mid-successional secondary, there was a higher richness of cup nesting species (S = 56) and closed nests (S = 29) than in the other two successional stages of the forests. While cavity-nesting species richness was higher in young secondary forest (i.e., 14 species of the families Momotidae, Capitonidae, Picidae, Furnariidae, Hirundinidae and Troglodytidae) (Figure 2C, see Table S2). Searcher foraging species had the highest richness in mid-successional secondary (S = 48), followed by mature forest (S = 45) and young secondary forest (S = 38) (Figure 2D, see Table S2). While bark foraging species richness decreased with increasing forest age (Figure 2D, see Table S2). Taxonomic richness was similar among the three successional stages (Table 1). On the other hand, functional diversity (RaoQ) was significantly lower in mature forest compared to young secondary forest and between mature and mid-successional secondary forest (Table 1). We included the tables of intercepts and the multiple comparisons for taxonomic (observed richness) and functional diversity (RaoQ index) as supplementary material (Table S3).

Figure 2
Species richness recorded in Young Secondary Forest (YSF), Mid-successional Secondary Forest (MSF), and Mature Forest (MF), according to their diet type (A), foraging strata (B), nest type (C), and foraging strategy (D).

Table 1
Taxonomic and functional diversity (RaoQ) of birds associated with secondary forests in the Tropical Andes. Ln average bird richness refers to the natural logarithm of the average bird richness. Different letters indicate significant differences (p < 0.05).

Variables	Young secondary forest	Mid-successional secondary forest	Mature Forest	Pairwise comparison (vs. Mature Forest)
Total observed bird richness	83	99	93	–
Taxonomic diversity (Ln average bird richness)	2.25 ± 0.39a	2.24 ± 0.58a	2.39 ± 0.34a	YSF vs. MF: t = -1.083, p = 0.282, df = 68 MSF vs. MF: t = -1.125, p = 0.264, df = 68
RaoQ Index	0.43 ± 0.03a	0.42 ± 0.05a	0.39 ± 0.02b	YSF vs. MF: t = 4.173, p < 0.001, df = 68 MSF vs. MF: t = 3.195, p = 0.002, df = 68

2. Taxonomic and functional beta diversity

Species turnover was the main component of understory bird taxonomic and functional beta diversity. The greatest dissimilarity in taxonomic diversity was between young secondary forest and mid-successional secondary forest (β_jac = 0.403), with 75.9% of the dissimilarity explained by species turnover (Table 2). Likewise, species turnover explained 91.7% of the taxonomic dissimilarity between mid-successional secondary and mature forests (Table 2). Conversely, the greatest dissimilarity in functional diversity was between mid-successional secondary and mature forests (β_jac = 0.280; Table 2). The functional dissimilarity between mid-successional secondary and mature forests was mainly due to species turnover (β_jtu = 0.276; Table 2). In addition, functional dissimilarity between mid-successional and young forest was low (β_jac = 0.181; Table 2).

Table 2
Jaccard dissimilarity index (total, turnover, and nestedness) of taxonomic and functional beta diversity of birds in three successional stages: Young Secondary Forest (YSF), Mid-successional Secondary Forest (MSF), and Mature Forest (MF).

Beta Taxonomic Diversity
Forest	Total dissimilarity	Turnover	Nestedness
YSF-MSF	0.403	0.306	0.097
YSF-MF	0.324	0.253	0.071
MSF-MF	0.386	0.354	0.032
Beta Functional Diversity
Forest	Total dissimilarity	Turnover	Nestedness
YSF-MSF	0.181	0.084	0.097
YSF-MF	0.255	0.165	0.090
MSF-MF	0.280	0.276	0.004

Discussion

1. Taxonomic and functional alpha diversity

Understory bird alpha taxonomic diversity was similar among the three stages of succession, while bird alpha functional diversity showed differences in the secondary succession. Contrary to our predictions, the RaoQ index values indicate a decrease in the functional alpha diversity of understory birds while the age of the secondary successional stages increases. This is consistent with the results found in the review by Sayer et al. (2017) but contrasts with other studies (Acevedo-Charry & Aide 2019, Espejo & Morales 2019). The similarity in bird richness between secondary successional forests and the reduction in functional diversity in older forests suggests greater functional redundancy in the mature forest (Borges et al. 2021). The number of species with different life-history and behavioral traits changed across the three stages of secondary succession. In young secondary forest, cavity-nesting and bark-foraging birds (e.g., Picumnus olivaceus and Xenops minutus) were dominant. The recovery rate of cavity-nesting and bark-foraging birds is limited by the recovery time of large trees and tree-related microhabitats (Larrieu et al. 2018). The presence of these species could be linked to the availability of other resources (e.g., invertebrate availability, refuge, and passage sites) and could be considered a new habitat for colonization (Bradfer-Lawrence et al. 2018, Pezda et al. 2021). Omnivorous and foraging species in various vegetation strata (i.e., generalist species such as Todirostrum cinereum and Coereba flaveola) exhibited higher abundance in mid-successional secondary forest. The adaptability of generalist birds to various forest types with high resource availability and diversity explains their presence in mid-successional forests (Chazdon et al. 2009, Acevedo-Charry & Aide 2019, Betancurt-Grisales et al. 2021). The mature forest had insectivorous and foraging species in the understory, including forest specialists (e.g., Thamnophilus atrinucha, Ceratopipra erythrocephala, and Tachyphonus delatrii). Acevedo-Charry & Aide (2019) report an increase in specialist species in secondary forests, occurring progressively from early to later successional stages. However, despite this increase in specialist species, the functional diversity (RaoQ) was lower in mature forests compared to younger stages. This suggests that while mature forests support a greater number of specialist species, these species share similar functional traits, such as insectivory and understory foraging, leading to reduced functional diversity compared with other successional stages. This indicates a high level of functional redundancy within the mature forest understory. In contrast, younger stages are dominated by generalist species that occupy a broader functional space (e.g., varied diets and strata), whereas the environmental conditions of mature forests act as a strong ecological filter, selecting for a narrow set of traits associated with insectivory and low-light foraging (Sayer et al. 2017). Consequently, although taxonomic richness is high, the functional distance between species is low because they belong to the same specialized guilds (e.g., ant-followers), performing similar ecological roles. The dominance of specialized traits in mature forests suggests that these ecosystems provide stable conditions for species with specific ecological requirements, which may mitigate the negative impacts of local species extinction (Borges et al. 2021, Bohada-Murillo et al. 2023).

2. Taxonomic and functional beta diversity

Species turnover explained understory bird taxonomic and functional beta diversity across secondary succession. The turnover of bird species between young secondary forests and mid-successional secondary forests likely results from two processes: colonization by forest generalist bird species adapted to the developing conditions of mid-successional forests, and the local extinction of shrubland species that thrived in earlier successional stages (Zaplata & Dullau 2022). Shrubland bird species, such as Legatus leucophaius and Tiaris olivaceus, were not found in mid-successional secondary forest and mature forest. Mid-successional bird species such as Malacoptila panamensis, Poecilotriccus sylvia and Henicorhina leucosticta appeared from mid-successional secondary forest. The presence of these species may be related to a positive response to secondary succession, allowing their colonization in older forests as vegetation structure changes and new resources become available (Sayer et al. 2017, Acevedo-Charry & Aide 2019, Zaplata & Dullau 2022).

Environmental filtering could drive functional turnover understory in functional birds between mid-successional secondary and mature forests (Luck et al. 2013). This process could favor species with specialized morphological adaptations (e.g., culmen size), life-history traits (e.g., insectivory), and behaviors (e.g., foraging strategies) well-suited to the conditions in mature forests (Sayer et al. 2017). In mid-successional secondary forest, there are medium-sized species (between 10–100 g), with larger wings and tails than mature forest species, as well as fruit foragers in subcanopy and canopy (e.g., Euphonia minuta), which are exclusive to this successional stage. In contrast, the mature forest has medium-sized species, with larger tarsus and culmen length than those of mid-successional secondary forest, which are insect foragers in the understory and ground, such as H. immaculata, G. bicolor and P. spadix, which are recognized antfollower specialists (Martínez et al. 2021). In this way, the presence of specialist insectivorous birds that forage on or near the ground, such as ant-followers, could be determined by vegetation structure and the availability of understory resources in mature forest (Castaño-Villa et al. 2014, Betancurt-Grisales et al. 2021). Since ant-followers take longer to recolonize forests during secondary succession, they could serve as valuable indicators of successional progress (Powell et al. 2013). Bird communities across successional stages exhibit distinct functional patterns. In young secondary forests, generalist bird species dominate, characterized by broad diets, varied foraging strata, and flexible strategies. The mid-successional stage is distinguished by species with a wider range of foraging strategies and adaptations to diverse vegetation strata, typically medium-sized birds that include fruit foragers. Finally, mature forests are dominated by specialist insectivores with traits adapted for understory foraging, such as ant-followers.

3. Implications for ecological restoration

Our results show that functional diversity of understory birds provides a useful indicator for evaluating restoration trajectories in tropical Andean forests (Betancurt-Grisales et al. 2021). Higher functional diversity in young and mid-successional forests reflects functional complementarity driven by generalist species during early regeneration, whereas lower functional diversity in mature forests reflects increased functional redundancy among specialized guilds. Importantly, this redundancy represents a target condition associated with environmental stability rather than functional degradation (Borges et al. 2021). Therefore, restoration success should not be assessed solely by maximizing functional diversity metrics, but by recovering the functional composition characteristic of mature forests. Protecting remaining mature forest patches is essential, as they act as sources of specialist species that contribute to ecosystem resilience.

Conclusion

Our results suggest that the different stages of secondary succession evaluated in the Andes contribute similarly to maintaining understory bird species richness. Variations in the functional traits of species result from the environmental filters imposed by each successional stage. Changes in habitat structure could lead to a turnover of species; in the mature stages of succession, specialist forest bird species appear, exhibiting specialized foraging behaviors that even require indirect interactions with other animal species (e.g., ants). The functional diversity of the communities associated with each successional stage can have differential effects on the ecosystem functioning in the tropical Andes. Mature forests must be protected to conserve specialized bird species, such as understory insectivores. From a restoration perspective, our findings suggest that successful restoration should aim not only to recover species richness, but also to reestablish the functional composition characteristic of mature forest ecosystems.

Supplementary Material

The following online material is available for this article:

Table S1 – Principal Coordinates Analysis (PCoA, Gower distance method) to reduce the number of bird functional traits in the functional beta diversity. The bird functional traits were expressed in the first five PCoA axes and these explained 21.8% of the total variation.

Table S2 – Abundance and functional traits of birds assessed across three forest types: young secondary forest (YSF), mid-successional secondary forest (MSF) and mature forest (MF).

Table S3 – Results of the Generalized Linear Mixed Models comparing observed richness and RaoQ between young secondary forest, mid-successional secondary forest, and mature forest. Bold values denote significant comparisons.

Acknowledgments

The authors are grateful for the information provided by the project “Monitoreo de la fauna vertebrada silvestre en el área de influencia de los centros productivos de ISAGEN en el oriente de Caldas”, Agreements 47/180, 47/623, and 33/45 between Universidad de Caldas and ISAGEN. We thank R. Gil-Ospina, A. Henao-Murillo, and D. Moreno for their support in data collection during the fieldwork and the coordinators of the agreements, B. Toro, M. L. Bedoya, and V. Duque, for allowing the use of the data. Additionally, we extend our gratitude to M. Bohada-Murillo for their assistance during the preparation of this manuscript.

The collection of data and documentary information was supported by the Universidad de Caldas and ISAGEN (Agreements 47/180, 47/623, and 33/45).

Data Availability

Original data is available from the figshare digital repository: https://doi.org/10.6084/m9.figshare.28620896.v1

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Edited by

• Associate Editor
  Luis Fabio Silveira

Publication Dates

• Publication in this collection
  01 May 2026
• Date of issue
  2026

History

• Received
  18 Mar 2025
• Accepted
  17 Mar 2026