The avifauna of Ramanathapuram, Tamil Nadu along the Southeast coast of India: waterbird assessments and conservation implications across key sanctuaries and Ramsar sites

Study area

The study area included five bird sanctuaries from Tamil Nadu on the southeast coast of India. They are (i) Chitrangudi Bird Sanctuary (CBS) (9°19′N and 78°28′E) has an area of 47.63 ha. (ii) Melselvanoor Kelaselvanoor Bird Sanctuary (MKBS) (9°13′–9°12′N and 78°32′–78°34′E) has a total ayacut area of 429.15 ha. (iii) Kanjirankulam Birds Sanctuary (KBS) (9°21′N and 78°30′E) has an area of 98 ha. (iv) Sakkarakottai Birds Sanctuary (SBS) (9°21′N and 78°48′E) has an estimated area of 230.49 ha. (v) Therthangal Bird Sanctuary (TBS) (9°27′N and 78°46′E) an area of 29.295 ha (Fig. 1). All the sanctuaries are seasonal water holding community tanks with one meter to five meters depth. Elevation of all sanctuaries ranges between 30 m to 100 m mean sea level. Between the tank embankments and the vegetation, there is approximately a 15–100 m wide water holding region. Agricultural lands surround the sanctuaries. The prominent tree, Acacia nilotica (Babul) offers conducive breeding and feeding grounds for the waterbirds. Besides this, trees like Prosopis juliflora, Tamarindus indica, Ficus spp., Thespesia populnea, Albizzia amara, and Palmyra (Borassus flabellifer) are also found (Byju & Raveendran, 2023a; Byju & Raveendran, 2023b; Byju & Raveendran, 2024a; Byju & Raveendran, 2024b; Byju et al., 2024a). The main habitat types observed in the sanctuaries included: Open-water habitat, agricultural land, trees like Babul, mesquite Prosopis juliflora, palmyra Borassus flabellifer, and tamarind Tamarindus indica trees on the bund bordering the wetland, grassland on the wetland area, and shrub habitat.

Three distinct seasons are experienced in all the sanctuary areas. The winter (November–February); summer (March–June); scanty rains (July–September); and monsoon (October–December) as monsoon and winter seasons overlap with an average rainfall of 350 mm to 900 mm annually. Most of the water collected in the tanks is from the Northeast monsoon. Ramanathapuram Wildlife Division gave verbal permission to conduct the work.

Data collection

A study on avifaunal diversity was conducted between April 2022 and March 2023. Twelve field visits (one per month) to assess the bird diversity, residential status, and breeding activity. Surveys were conducted in the morning (07.00 h–10.00 h) and the evening (16.00 h–19.00 h), during peak bird activity, following direct count and block count methods (Bibby et al., 2000; Howes & Bakewell, 1989). Three experienced observers and two field assistants conducted counts at vantage points: eight at CBS, five at KBS, seven at TBS, eight at SBS, and eight at MKBS separated by 100–200 m, depending on the landscape and visibility. Birds were counted for 15 min at each point, after a five-minute acclimation period. The observations recorded while moving from one scanning point to another were entered as incidental records. Birds were observed using Nikon binoculars (10×50) and photographed using a Canon 100–400 mm lens. Breeding activity of waterbirds was documented through fortnightly surveys, during the breeding season (the time the birds occupied the nests during incubation). Identification of active nests was achieved through monitoring of flights of adult birds between nests and feeding grounds. The number of nests was estimated employing standard methods such as ground counts or nest counts using binoculars and spotting scopes (Gibbs et al., 1988; Dodd & Murphy, 1995). Other methods were impractical for sites with few nests or those completely inaccessible, therefore we used perimeter counts based on visible nests and observed foraging flights from the colony boundary (Dodd & Murphy, 1995). The common name, scientific name, IUCN Red List status, and migratory status are followed (Praveen & Jayapal, 2023). Species residential status as Resident (R), Passage Migrant (PM), or Winter Visitor (WV) depending on the temporal patterns and duration of occurrence (Grimmett et al., 2011) while global Red List status (IUCN, 2024), Convention of the Conservation of Migratory Species of Wild Animals (CMS) protection status (The Convention on Migratory Species, 2024) and national bird abundance trends (State of India’s Birds, 2023) were assessed for the recorded species. According to SOIB, the current population trend of the bird species in India is the average annual change in the species abundance over the past eight years (2015–2022). Different categories of current population trend indices are, Insufficient Data means too few reports, Trend Inconclusive means 95% confidence interval >2%, Rapid Decline is decline >2.7%, Decline is >1.1%, Increase is >0.9% and Rapid Increase is >1.6% (State of India’s Birds, 2023). Potential threats to birds based on impact of threat index across sanctuaries were noted for management recommendations (Borgman, 2011).

Data processing

Alpha diversity Indices like Shannon-Weiner Diversity Index (H), Pielou’s Evenness Index, Menhinick’s Index for richness and Dominance Index (D) which implies presence of one or few dominating species in the site, were performed to understand the overall community structure of breeding waterbirds in each sanctuary (Kitikidou et al., 2024). Whittaker’s Beta-diversity Index measures the similarity in diversity between the study sites (Whittaker, 1972). Kruskal–Wallis test was performed to understand whether there was a statistically significant difference in the abundance of breeding waterbirds among the five sanctuaries (Kruskal & Wallis, 1952). The correlation between sanctuary area and the breeding waterbirds diversity was confirmed by Pearson correlation analysis. Further, principal component analysis (PCA) was performed to assess whether some waterbird families were more strongly associated with specific sanctuaries, using abundances of each waterbird family recorded in the sanctuaries as a variable (Jolliffe, 2002). Bray–Curtis cluster analysis, and individual rarefaction analysis were done for breeding waterbirds to understand the similarity in diversity and species accumulation among the sanctuaries and overall comprehensive community structure for each bird sanctuary (Gotelli & Colwell, 2001). All the analysis was done using Paleontological Statistics (PAST) Software version 4.17. Heatmap of species abundnace and other graphs were plotted using R version 4.4.1. Threats to breeding waterbirds were analysed for their extent of impact based on severity score, scope score and irreversibility score of the threat categories. Severity was assessed based on potential impacts of the threats or degrees of damage to the species; scope score was assessed based on the geographical extent to which the threat affects the landscape, whether the threat impacts the entire sanctuary, half of the sanctuary or create localized effect on a small portion of the sanctuary; based on whether the effects of these threats can be reversed or they create a permanent damage, irreversibility score was assessed. Four-scale measurements were used for all these scores as follows: 4 = extremely high, 3 = high, 2 = medium, and 1 = low. Total was calculated using the formula: Total = 2*(severity + scope) + irreversibility (Margoluis & Salafsky, 1998; Rajashekara & Venkatesha, 2017). Based on the results, threats were classified as High, Medium, and Low categories.

Avifauna

We recorded 131 species of birds compiled from all the sanctuaries together. The maximum number of bird species was recorded from KBS (123) followed by CBS (122), MKBS (117), SBS (116), and TBS (96). Based on the residential status, Resident birds (R) were predominant with 78% (n = 102) of the total species recorded whereas, Winter Visitors (WV) contributed 21% (n = 28). One species, Rosy Starling Pastor roseus, a Passage Migrant (PM) was common to all the sanctuaries. Table 1 summarizes the avifaunal list (order & family wise) compiled from all five sanctuaries with residential status and IUCN Red List categories. Twenty-seven bird species in the sanctuaries are protected under the CMS (Table 2). The avifaunal species listed in Appendix II of CMS correspond to migratory species that need international cooperation and international agreements for conservation and management (The Convention on Migratory Species, 2024). The national trends of the avifauna recorded from all sanctuaries, 40.4% of species populations are stable; rapid increase (4.5%); increase (4.5%); decline (12.9%); rapid decline (11.4%); trend inconclusive (23.6%) as per assessment by State of India’s Birds (2023) (Table 3).

Except for some species, most bird species observed are common to all the five sanctuaries. Passeriformes order was predominant with 22 families and 41 species in CBS, MKBS, KBS, and SBS, except for TBS which is only represented by 18 families and 29 species. Family Ardeidae predominated in all the sanctuaries (represented by 11 species in CBS, MKBS, and KBS; 10 in SBS and TBS respectively). According to the IUCN Red List status, two Near Threatened species namely, Asian Woolly-necked Stork Ciconia episcopus (only recorded in TBS) and, Spot-billed Pelican Pelecanus philippensis and one Vulnerable species Indian Spotted Eagle Clanga hastata are commonly found in all the five sanctuaries. Migratory species such as Northern Shoveler Spatula clypeata, Baillon’s Crake Zapornia pusilla, and resident birds like Watercock Gallicrex cinerea, were recorded only in TBS. Similarly, in KBS, we recorded the winter visitor Green Sandpiper Tringa ocropus which was not recorded from other sanctuaries. Table 3 summarizes the avifaunal list of all sanctuaries depicting the presence or absence of species with SOIB trends.

Breeding waterbird diversity

We recorded 55 waterbird species recorded from all sanctuaries. Out of which 51% of species (n = 28) are breeding in these wetlands. Grey Heron Ardea cinerea, Black-crowned Night Heron Nycticorax nycticorax, Purple Heron Ardea purpurea, Asian Openbill Aanastomus oscitans, Black-headed Ibis, and Little Cormorant Microcarbo niger were found to breed in all the five sites. Five species namely Common Moorhen Gallinula chloropus, Pheasant-tailed Jacana Hydrophasianus chirurgus, Grey Headed Swamphen Porphyrio poliocephalus, White-breasted Waterhen Amaurornis phoenicurus, and Little Grebe Tachybaptus ruficollis were least common, found to breed only in SBS (Fig. 2). Except for SBS, Asian Openbill nested predominantly in the other four sanctuaries followed by Painted Stork, Black-headed Ibis, and Spot-billed Pelican. It was observed in all the sanctuaries that Spot-billed Pelicans and Painted Storks occupy the large Acacia nilotica trees which are dominantly cultivated as fuel wood for nearby villagers in all the tanks. Trees like Prosopis juliflora are occupied by Asian Openbill while Black-headed Ibis occupy the canopy. Cormorants, egrets, and pond herons occupy the lower strata of the habitat. Abundance of all breeding waterbird species in each sanctuary in the breeding months is represented in Fig. 3. The overall trend in the monthly abundance of breeding waterbirds in each sanctuary showed that in TBS, highest abundance was recorded in November and December, whereas in SBS it was in January and February. In MKBS, CBS and KBS, the abundance increased slightly from October to November and remained almost similar until February (Fig. 4).

Chitrangudi Bird Sanctuary

CBS supports 42 species of waterbirds and 13 species are recorded to be breeding. The waterbirds breeding season (incubating in nests) started from November 2022 to February 2023. For two species, Glossy Ibis and Oriental Darter, the breeding season started in December 2022.

Kanjirankulam Bird Sanctuary

In KBS, 49 species of waterbirds were recorded and 14 species are breeding. For all the species breeding season starts from November 2022 and extends until February 2023 except for Spot-billed Duck and Knob-billed Duck which breeds during January and February 2023. Interestingly, Oriental Darter, Cattle Egret Bubulcus ibis, Little Egret, Glossy Ibis Plegadis falcinellus, Eurasian Spoonbill and Black-winged Stilt Himantopus himantopus which were found to nest at the nearby sanctuaries were not reported from here.

Melselvanoor–Kelselvanoor Bird Sanctuary

MKBS provided a breeding habitat for 19 waterbird species. The breeding season starts in November 2022 and extends until February 2023 for all the species with no exception. Four species—Oriental Darter, Painted Stork, Black-headed Ibis, and Spot-billed Pelican—nest together in this sanctuary.

Sakkarakottai Bird Sanctuary

SBS recorded 43 species of waterbirds and 17 species breed here. The breeding season in this sanctuary is from October 2022 to February 2023.

Therthangal Bird Sanctuary

TBS recorded 40 species of waterbirds and 23 breeds here. The breeding season (birds incubating in nests) started in September 2022 and extended until February 2023. Three species, like Black-headed Ibis, Oriental Darter, and Spot-bellied Pelican, nest together in this sanctuary. Red-naped Ibis is found to breed only in TBS.

Community structure of breeding waterbirds in the five sanctuaries

The Shannon-Weiner Index (H) revealed the highest breeding waterbird diversity at MKBS (2.54) followed by KBS (2.39), SBS (2.3), CBS (2.29), and the lowest at TBS (2.23). Dominance indices indicated balanced communities in CBS, KBS, and MKBS, while SBS and TBS, were dominated by a few species like Asian Open-bill, Painted Stork and Spot-billied Pelican. The Evenness Index revealed high evenness in CBS (0.7) and KBS (0.7), followed by MKBS (0.6), SBS (0.5), and TBS (0.4). Menhinick’s Index ranked MKBS highest in species richness (0.8), followed by CBS (0.6) and KBS (0.6), with SBS and TBS showing the lowest richness (0.3) (Fig. 5 and Table S1). Overall, MKBS exhibited the highest diversity, and species richness with low dominance, whereas, TBS showed the lowest diversity, richness, and evenness dominated by a few species. These indices together provide a comprehensive view of the community structure for each bird sanctuary. Whittaker’s beta diversity profile revealed that CBS and KBS are highly similar to MKBS. MKBS and TBS showed high similarity in diversity with each other. SBS showed distinctness in diversity and more similarity to TBS than to other sanctuaries (Table 4). In general, lower values in Whittaker’s beta-diversity index indicate high similarity between the sites. Kruskal–Wallis test showed that there is no statistically significant difference (p = 0.406 > 0.05) in breeding waterbird diversity and richness among the sanctuaries. Further, the positive correlation between the area of the sanctuaries to the breeding waterbird diversity (r = 0.8, p = 0.074) and richness (r = 0.4, p = 0.424) was confirmed by Pearson correlation analysis although the correlation was not statistically significant.

The PCA analysis showed the association of families like Ciconiidae, Pelecanidae, Anhingidae, Ardeidae, Threskiornithidae specifically within TBS, whereas families Jacanidae, Anatidae, Rallidae, Podicipedidae, Phalacrocoracidae and Recurvirostridae more associated with SBS and comparatively less to TBS. CBS, MKBS and KBS did not show specific associations with any particular families and showed a negative correlation with other families (Fig. 6). Eigenvalues of the components are given in Table 5. Rarefaction curves (Fig. 7) indicated TBS had the highest curve, (20 species, 3,500 individuals) suggesting further sampling may uncover more species. MKBS had 19 taxa, and SBS, though lower in richness (17 taxa), appeared sufficiently sampled. KBS and CBS showed the lowest species richness with only 13 and 14 taxa respectively, and fewer than 500 individuals. Bray–Curtis cluster analysis (Fig. 8) showed SBS and TBS shared the most similar species, while KBS and MKBS formed a closely related cluster. CBS is closely related to the cluster of KBS and MKBS. The largest dissimilarity is between the SBS and TBS and the combined cluster of CBS, KBS, and MKBS.

Potential threats

The present study recorded potential threats to the wetlands due to anthropogenic activities. Threats were categorized to assess the severity on the breeding waterbirds in all the sanctuaries. Water unavailability and nesting tree unavailability were the high ranked threats to the breeding waterbirds. Firewood collection, recreation and other human disturbances were the medium category threats whereas livestock grazing, presence of invasive species, overfishing and feral dogs were the low impact threats, considering the severity, extent of impact and degree of irreversibility of the damage (Table S2).

Diversity and community structure across sanctuaries

Our hypothesis—that wetland area positively affects bird diversity and species richness—was broadly supported by the results, though nuances were evident in the distribution patterns among sanctuaries as MKBS, the largest sanctuary had the highest breeding waterbird diversity and the smallest sanctuary TBS had the lowest. These findings underscore the ecological importance of wetland heterogeneity, structural diversity, and habitat size in supporting complex waterbird communities. The Shannon-Weiner index (H) indicated that MKBS exhibited the highest diversity (H = 2.54) and suggesting a well-balanced avian community (Magurran, 2004), followed by KBS, SBS, CBS and TBS. Conversely, TBS showed the lowest diversity (H = 2.23) indicating a community structure dominated by a few species, likely due to habitat degradation or limited resources (Ludwig & Reynolds, 1988) This is further reinforced by the dominance indices, which revealed balanced communities in CBS, KBS and MKBS, but community dominance by certain species in SBS and TBS. These patterns confirm that habitat complexity within larger wetlands likely fosters diverse avian assemblages through niche diversification. In terms of total area, MKBS and TBS are the largest and the smallest sanctuaries, respectively. It is evident by the correlation analysis that there is a strong positive relationship between sanctuary area and breeding waterbird diversity and richness. Larger the wetlands, more are the microhabitats and hence, they support greater bird diversity (Paszkowski & Tonn, 2000).

Species richness and evenness among sanctuaries

Species richness, as quantified by Menhinick’s Index, was highest at MKBS (0.8), with CBS and KBS following closely. SBS and TBS exhibited the lowest species richness values, indicating a restricted avian community in these areas, potentially due to smaller habitat size or fewer habitat types available to meet varied ecological needs. The Evenness Index, showing high evenness in CBS and KBS, further suggests a balanced distribution of species in these sanctuaries, where resources or niches may be more evenly distributed. Conversely, TBS, dominated by a few species, showed lower evenness, aligning with expectations that smaller or less varied wetlands may support more specialized assemblages where particular species outcompete others. These indices collectively indicate that MKBS’s large area and CBS and KBS’s habitat structures contribute positively to both species richness and evenness. Studies by Fairbairn & Dinsmore (2001) and Riffel, Keas & Burton (2001) demonstrated that wetland area and heterogeneity are the important features affecting bird richness.

Beta diversity and inter-site community similarity

Whittaker’s beta diversity profile highlights community similarities and distinctions among the sanctuaries. MKBS was most similar to CBS and KBS, with TBS and SBS showing distinct diversity profiles. SBS shared more similarities with TBS than other sanctuaries, suggesting that these smaller sites may host more unique assemblages. The high similarity observed between CBS, KBS, and MKBS could be attributed to shared habitat characteristics or landscape connectivity, which allows species to exploit resources across these areas. Lower values in Whittaker’s index reflect this close similarity, suggesting possible species overlap or similar community dynamics facilitated by comparable habitat features.

Relationship between wetland area, bird diversity, and richness

A Kruskal–Wallis test confirmed no statistically significant difference in waterbird diversity and richness across the sanctuaries (p = 0.406), suggesting that each sanctuary supports comparable community richness and diversity levels despite some ecological and structural variations. However, a positive Pearson correlation (r = 0.8, p = 0.074) between sanctuary area and waterbird diversity suggests that larger sanctuaries tend to support more diverse communities. While the correlation between area and species richness was positive (r = 0.4) but not statistically significant (p = 0.424), the results nonetheless suggest a trend supporting the hypothesis that larger wetland areas tend to foster greater species richness.

Species assemblage patterns and habitat associations

The PCA provided additional insights into species assemblage patterns and habitat associations across sanctuaries. TBS was associated with families such as Ciconiidae, Pelecanidae, Anhingidae, Ardeidae, and Threskiornithidae, which may reflect its unique ecological attributes or niche specialization supporting these groups. On the other hand, families like Jacanidae, Anatidae, Rallidae, Podicipedidae, Phalacrocoracidae, and Recurvirostridae were more strongly associated with SBS, suggesting habitat characteristics or resources better suited to these families. CBS, MKBS, and KBS did not show specific family associations, indicating that their habitats may provide a broader, less specialized environment, allowing for a more generalized assemblage without strong affiliations. This generalist habitat structure might contribute to the observed balanced community structure in these sanctuaries.

Species status (CMS, SOIB)

As 20.6% of the species recorded from the study sites are enlisted in The Convention on Migratory Species (2024), the significance of these sanctuaries for avifauna is critical for conservation management as this is the first study from all sanctuaries which could initiate international collaboration for species conservation. SOIB status assessment was considered for a better undertanding of the species distribution in all the five sanctuaries as there was no previous studies and published literature from these important sites for waterbirds. However, SOIB assessment considered the frequency of reporting to calculate the indices of abundance, which does not directly measure the population size. A ‘complete checklist’ in the e-Bird citizen science platform is considered to assess the frequency of reporting (State of India’s Birds, 2023), which could be biased or contain errors due to bird detection and identification errors, inaccessibility of the birding locations due to seasonal changes or difficult terrains (Maitreyi, 2024). For the ‘Insufficient Data’ index, there are no measurable limits or range, hence this could be treated as a general trend and emphasize the value of our study due to data deficiency.

Implications on conservation and management

Many migratory birds’ flight paths depend on well-functioning wetlands (Ens, Piersma & Tinbergen, 1994; Ntiamoa-Baidu et al., 1998). Apart from resident and breeding birds, our study sites receive migratory birds majorly from families of Scolopacidae, Anatidae, Accipitridae, Charadriidae, Laridae, and Oriolidae. This remarkable diversity of avifauna in these study areas can be attributed to a wide range of feeding niches. The findings on threats have critical implications for conservation and habitat management in sanctuaries. As our results of threat ranking revealed, water unavailability is an extremely high threat in all the sanctuaries except TBS, where there is better availability of water as the area of TBS is much lesser. Another high ranked threat was nest tree unavailability which persists in all the sanctuaries. Although each of the five study sites has a large geographical area in water retention, the actual water holding area during the breeding season was much smaller due to reliance on rainfall, which is also impacted by climate change and altered monsoon patterns, finally affecting colonial breeding waterbirds (Cavitt et al., 2014). As the water availability decreased, trees wilted, reducing the number of suitable nesting sites (personal observations). The primary roosting and nesting tree, Acacia nilotica, is also declining due to altered monsoon patterns, emphasizing the need for planting native species that are more resilient to drought and excess water conditions, to support waterbird breeding populations. Fluctuating water levels influence ecological factors like the size, depth, and vegetation affecting the availability of suitable breeding and roosting sites (MacArthur, MacArthur & Preer, 1962; Karr & Roth, 1971; Pearman, 2002) and shaping waterbird populations and species distribution in wetlands (Wiens, 1989; Mukherjee, Borad & Parasharya, 2002; Ma et al., 2010).The neighboring agricultural lands play a significant role in providing food resources for all these varieties of bird species (Byju et al., 2024b), especially in April when the water level in the waterbody recedes, allowing for the cultivation of crops like paddy Oryza sativa, cotton Gossypium herbaceum, and chilly Capsicum annuum, which attracts numerous birds. Therefore, conservation strategies should prioritize habitat preservation and structural diversification, especially in smaller wetlands like TBS and CBS, where specialized or less diverse habitats may restrict community diversity.

This study also observed the other medium and low category threats common to habitat and breeding waterbirds in all the sanctuaries, like proximity to human settlements and urban expansion (Roshnath & Sashikumar, 2019) which create disturbances as firewood collection, fishing or recreation by people, cattle feeding on the bark of the nesting trees during dry season which decayed the tree, leading to tree death (Byju et al., 2024a), domestic dogs disturbing and preying on waterbirds as observed in all the sanctuaries (Mundkur & Langendoen, 2019), invasive species like Prosopis juliflora and Ipome a carnea taking over a major extent of these sanctuaries and the removal of Prosopis trees without a replacement strategy, has negatively impacted species like the Asian Openbill and cormorants, which preferred nesting in these trees.. Sometimes waterbodies are also seen choked with water Hyacinth Eichhornia crassipes, which reduces feeding and roosting habitat for ducks and other waterbird species in freshwater that reduces oxygen and sunlight for native plants and fishes (Lowe et al., 2000; Szabo & Mundkur, 2017).

Management recommendations

Human-wildlife conflicts over resources, such as fishing and water rights, further challenge bird conservation (Bosselmann, Engel & Taylor, 2008). To mitigate these threats, enhanced patrolling to limit overfishing, along with controlling firewood collection is essential, as dead trees provide crucial roosting sites. Collaboration with government agencies is needed to ensure perennial water in the sanctuaries, with occasional desilting and tank deepening to improve water-holding capacity (Anand, 1999). Raising public awareness and training local communities to monitor waterbird populations and threats can foster greater protection and conservation efforts (Szabo et al., 2016).