Biology subjects

Srinivasan, U.

Publications and source records attributed to Srinivasan, U..

11 recordsLinked to original sources

bioRxiv2026

Resource abundance and dietary specialization predict elevational migration in a hyperdiverse montane bird community

Migration is a well-described behavioural strategy that allows species to track variation in resources and climatic conditions by moving in response to seasonality. A common form is elevational migration, an annual short-distance movement undertaken by many mountain bird species globally. While studies show that the timing of migration may relate to food availability, the mechanisms determining which species migrate remain unclear. Our study investigated if the degree of dietary specialization explains why some high-elevation bird species in seasonal environments migrate downslope for the winter while others remain resident at high altitudes despite the apparent scarcity of their preferred food resources. We mist-netted birds along a 2300-m elevational gradient in the Eastern Himalaya and collected blood and faecal samples from 261 individual birds belonging to 18 species of high-elevation residents (ten) and elevational migrants (eight) in their breeding and wintering ranges. Using stable isotope ratios of carbon and nitrogen in whole blood and faecal DNA metabarcoding, we compared their seasonal trophic levels and dietary niches. Nitrogen isotope ratios showed that residents had a substantially lower trophic position in the winter compared to summer (-0.35 [-0.52, -0.17]), whereas migrants had a slightly higher trophic position in the winter (0.15 [-0.02, 0.32]). This trophic shift in residents was likely due to a decrease in insectivory and an increase in frugivory in the winter. The frequency of key insect orders (Lepidoptera, Hemiptera, and Coleoptera) declined by 20-35% in their winter diets alongside an increase in fruit, particularly from the family Polygonaceae (0.33 [0.18, 0.46]). Additionally, compared with residents, migrants showed greater overlap in their dietary niches between summer and winter (98% vs 80%). Because arthropod abundances in the Himalayas peak at high elevations in the summer and decline in the winter, we suggest that elevational migrants are likely dietary specialists that track resources, while high-elevation residents are dietary generalists that supplement their winter diet with fruit and nectar because of the scarcity of arthropods. These findings indicate that a species dietary specialization is linked to its migratory behaviour, providing a potential mechanistic explanation for how different species solve the challenge of seasonal resource limitation.

ecology↗

bioRxiv2024

Vocal species are more central in Eastern Himalayan Mixed-Species bird flocks

Mixed-species flocks (MSF) represent an important form of social organisation in bird communities worldwide. Despite its likely importance in flock formation and cohesion, the role of vocal communication in the formation and maintenance of MSF in birds is hitherto understudied. In this study, we examine if a species centrality within a mixed-species flock is influenced by its vocal behaviour during the dawn chorus, i.e., the time of MSF formation. Using acoustic sampling and field observations, we studied the bird species found in MSF in the Eastern Himalayas. Our results show differential vocal activity patterns among MSF-forming bird species and suggest a positive correlation between calling rates and closeness centrality (species importance in a flock) in understory MSFs. We also found a more synchronised vocalisation pattern in the understory MSFs, with a consistent peak in vocal activity in the early morning hours, whereas no consistent vocal pattern was found for canopy flocks. Overall, our results suggest a potential mechanism that drives MSF formation wherein the vocal activity of central species precedes and likely attracts participation from other attendant species.

ecology↗

bioRxiv2024

Impact of land cover and climate change on habitat of Indian birds

Understanding species vulnerability to global change requires approaches that integrate projections of future environmental change with empirical population trends; such integrated assessments remain limited in biodiverse, data-limited tropical regions. In this study, we develop and apply a scalable, decision-oriented framework that synthesises climate and land-cover projections with existing contemporary population data to assess species-level vulnerability and conservation, monitoring, and research priorities for Indian birds. Using an ensemble of machine-learning habitat-suitability models, we projected current and future habitat suitability for 955 species under two 2100 scenarios (SSP3-7.0 and SSP5-8.5). We then integrated these projections with population trends from the State of Indias Birds assessment. We found habitat responses were highly heterogeneous: 507 species were projected to lose suitable habitat under both future scenarios, 327 species were projected to gain habitat, and 121 species exhibited mixed or uncertain responses. The direction and magnitude of projected change varied substantially within and among habitat-specialisation guilds, with overall losses exceeding gains. The synthesis of habitat projections with population trends revealed 121 (mostly wetland) species facing ongoing population decline and future habitat loss, representing high-priority targets for immediate conservation action. A further 300 species, largely forest-associated, were projected to lose habitat but lacked reliable population trend data, highlighting substantial monitoring gaps, while species with mixed or uncertain habitat trajectories emerged as key research priorities. By explicitly integrating projected habitat change, contemporary population trajectories, and data availability, this study advances a transferable framework for prioritising conservation action, monitoring, and research under accelerating global change.

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bioRxiv2023

Bamboozling Interactions: Interspecific associations within mixed-species bird flocks in bamboo in the Eastern Himalayas

O_LIBamboo, although widespread, is a globally understudied habitat, but supports significant biodiversity, including specialist bird species. Mixed-species bird flocks (flocks) are an important and regular feature of forest bird communities worldwide and have been well-studied. However, how and why flocks might differ between bamboo and non-bamboo habitats is unknown. C_LIO_LIWe studied flocks and their arthropod prey in rainforest and bamboo stands across two seasons in the Eastern Himalaya. Using network analysis, we compared interspecific associations within flocks, and asked if resource availability (arthropod abundance) drives flocking. C_LIO_LIBamboo and rainforest flocks were significantly different, with bamboo flocks being more cohesive, less modular and their species more interconnected. Further, bamboo flocks were more consistent in their composition across seasons whereas rainforest flocks partially disintegrated in spring, probably because of increased arthropod abundance. In bamboo, the loss of arthropods in an entire substrate and the increase in other substrates was potentially insufficient to cease flocking in spring. C_LIO_LIResource availability and the unique foraging behaviour of the bamboo specialists likely drive flocking in the Eastern Himalaya bamboo. Our study is among the few to describe bamboo flocks and we emphasise conserving bamboo stands to ensure persistence of this unique flock in the Eastern Himalaya. C_LI

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bioRxiv2023

Ant impacts on global patterns of bird elevational diversity

Across the worlds mountains, elevation-species richness relationships are highly variable. Here, using data on bird species elevational distributions from all 46 of the worlds major mountain ranges, bird species dietary traits, and the distribution of the low-elevation ant genus Oecophylla, we show that global patterns in bird elevational diversity are likely to be affected by competition with ants. Oecophylla is an exceptionally abundant and aggressive predator of invertebrates, which preys on the same species that sympatric invertivorous bird species feed on. In mountain ranges with Oecophylla present in the foothills, maximum species richness of invertivorous birds occurs, on average, at 960m, [~]450m higher than in mountain ranges without Oecophylla. Further, in mountain ranges with Oecophylla, species richness of invertivorous birds increases initially with with elevation to produce a mid-elevation peak in invertivore bird species richness. Where Oecophylla is absent, invertivore bird species richness generally shows monotonic declines with increasing elevation. We attribute the pattern to the following mechanism: first, Oecophylla reduces prey density for invertivorous birds; second, low invertebrate prey abundance reduces invertivorous bird density and third, lower bird density is correlated with lower bird species richness. Because invertivores dominate montane bird communities, global elevational bird diversity patterns are also driven by Oecophylla. The findings emphasize how competitive interactions between distantly related taxa set geographical range limits.

ecology↗

bioRxiv2023

Bird communities in a selectively logged tropical montane forest are dominated by small, low-elevation species

Tropical montane forests are critical centres of terrestrial biodiversity and endemism, and face a range of threats, including selective logging and climate change. However, few studies have explored the joint influence of these threats, particularly in tropical mountains, despite how crucial it is to understand the cumulative impact of forest loss and climate change on biodiversity. We used mist-netting and bird ringing data from a long-term (10-year) community and population monitoring program to examine how the composition of the mid-elevation Eastern Himalayan understorey bird community changed in the mid-elevation primary and logged forest. Because logged forest is warmer than primary forest, we hypothesised that the bird community would shift towards lower elevation species, with a faster transition in logged forest. Further, because logged forest has lower arthropod abundance, we hypothesised that the primary forest community would have larger species than the logged forest community. Our study shows that the bird community in the logged forest shifted towards lower-elevation species than in the primary forest (although this was not statistically significant). Moreover, we found that smaller species were better able to colonise warmer logged forest, whereas larger species reached higher densities in primary forest. These trends are likely to be driven by climate change causing upslope range shifts and logged forest selecting for species that are likely to tolerate the consequently altered abiotic (higher temperatures) and biotic (lower resources) environment. These findings have significant implications for understanding the impacts of forest loss and climate change on biodiversity, particularly in tropical montane forests.

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bioRxiv2023

Understanding inter-individual social networks in mixed-species bird flocks

Mixed-species flocks (MSFs) are an important form of social organisation in forest bird communities worldwide. MSFs provide participants with the benefits of reduced predation risk and/or enhanced foraging efficiency. Recent work has shown that participation in MSFs confers long-term survival benefits in the face of anthropogenic change. However, our understanding of MSFs mainly comes from studies that examine species-level networks, where each node is a unique species and the edges or connections between nodes are associations/interactions between species. While valuable, such approaches might not allow us to understand and investigate the mechanisms that drive MSF formation and structure because social interactions and their effects occur at the individual-level. Empirical studies on multi-species, individual-level MSF social networks have seldom been undertaken due to the various complexities and logistical challenges involved. In this study, we use mist-netting and colour-ringing followed by a standardised observation protocol to construct individual-level social networks in MSFs at 2000m ASL in Eaglenest Wildlife Sanctuary, Arunachal Pradesh, Eastern Himalaya, India. First, we found two separate flocktypes at our study site, comprising two distinct sets of understorey species. The mechanisms contributing to individual-level co-occurrences are likely to differ between these flocktypes, and with MSFs in the Neotropics. The Rusty-fronted Barwing (the nuclear species of one flocktype) shows spatially disjunct territories for each flock while the Yellow-throated Fulvetta (nuclear species of the other flocktype) shows large spatial overlap in its MSF networks, which is likely driven by non-individual-specific benefits such as predation risk dilution. Further, the addition of associating individuals to the social networks has opposite impacts on the two networks. The addition of Coral-billed Scimitar Babblers to the barwing networks greatly reduces network modularity because the associating individuals bridge two modules of barwings (both spatially and in the social network). On the contrary, territorial Rufous-capped Babblers and Grey-cheeked Warblers increase the modularity of the spatially overlapping fulvetta network. Our study provides novel insights into flock formation mechanisms in the Eastern Himalaya, likely applicable to other multi-species flock systems in the Old World.

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bioRxiv2023

Poles apart - the structure and composition of the bird community in bamboo in the Eastern Himalaya

Bamboos (Poaceae: Bambuseae) are a unique and diverse group of plants (> 1500 species) that are found in tropical and subtropical regions of the world. Various birds, mammals, and insects use bamboo either directly or indirectly, and some species are obligate bamboo specialists. For instance, several species of Neotropical birds are found in bamboo alone. However, the mechanisms underlying dependence on/associations with bamboo are almost entirely unknown. We studied bird communities in bamboo and rainforest across two seasons in the Eastern Himalaya, where we explored a possible mechanism for bird associations with bamboo - dietary resource specialization. We calculated use of bamboo versus rainforest by bird species and described the structure and composition of the bird and arthropod communities in both habitats and seasons. We provide the first systematic evidence of bamboo specialist bird species from the Eastern Himalaya. Although the species richness of birds and arthropods in both habitats and seasons did not vary starkly, we found that the composition of bird and arthropod communities in both habitats and both seasons to be distinct. Interestingly, we found arthropod communities in different substrates of bamboo to also be distinct. Bird specialization in bamboo in the Eastern Himalaya could be because of their dietary specialization to the unique arthropods found in bamboo. The results from this study emphasise the importance of bamboo in the Eastern Himalaya and provide baseline information that might aid in conservation.

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bioRxiv2022

Abiotic niche predictors of long-term trends in body mass and survival of Eastern Himalayan birds

The synergistic impacts of climate change and habitat degradation threaten tropical species worldwide. However, how species abiotic niches affect their demographic vital rates and phenotypic changes under anthropogenic change remains poorly understood. Using an 11-year mark-recapture dataset from primary and selectively logged forest in the Eastern Himalayas, we investigated how temperature-humidity niche characteristics predicted body mass and survival trends in understorey insectivorous birds over time in each habitat. Our results show that logged forest is hotter and drier than primary forest, and the arthropod community shows dramatic shifts in composition upon selective logging. In understorey insectivores, the degree of dissimilarity between species-specific primary and logged forest niches was strongly and negatively correlated with survival and body mass trends in logged forest. Here, we show that temperature-humidity niche shifts in response to anthropogenic habitat modification can impact demographic vital rates and body condition crucial for population persistence. This work has the potential to inform prompt, targeted conservation efforts toward species that are the most threatened in a warmer and more degraded world.

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bioRxiv2022

The effect of selective logging on microclimates, arthropod abundance and the foraging behaviour of Eastern Himalayan birds

1. Selective logging--the practice of removing a subset of commercially important trees from a forest--is a globally pervasive form of forest degradation. Selective logging alters both the structure and function of forests and the composition of ecological communities. 2. Tropical insectivorous birds are highly vulnerable to microhabitat alterations in logged forest. Such altered microhabitats might affect the foraging of forest birds by altering (a) resource availability, and (b) foraging behaviour. 3. We investigated the effect of selective logging on microclimates, prey availability, foraging behaviour and the foraging success of eastern Himalayan birds in the breeding season. 4. Selective logging alters temperature-humidity microclimates and the composition of arthropod communities, both of which are likely to then collectively alter foraging behaviour by birds. We show that birds spent a lower proportion of their time foraging in primary compared with logged forest. Further, selective logging interacts with species traits such as body mass, preferred foraging stratum (understorey, midstorey or canopy) and foraging manoeuvre to influence foraging success. Gleaners generally foraged more successfully in primary forest and salliers in logged forest, although these patterns were modified by body mass and foraging stratum. 5. Synthesis and applications: Our study shows how altered microclimates in anthropogenically modified habitats can influence resource availability and have downstream impacts on the behaviour of species at higher trophic levels.

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bioRxiv2020

Preliminary evidence for upward elevational range shifts by Eastern Himalayan birds

The ongoing climate crisis is one of the most significant threats to biodiversity globally. As the Earth warms, species have adapted by shifting their geographical ranges either polewards, or in mountainous regions, upslope towards higher elevations, presumably to continue to inhabit a suitable thermal environment. Upslope range shifts are of particular concern in tropical mountain ranges because: (a) tropical species are particularly thermally sensitive, (b) as species move upwards, they can run out of habitable space, leading to local extirpations, and (c) tropical mountains harbour a disproportionately high fraction of the planets terrestrial biodiversity - rapid upslope range shifts can therefore result in significant biodiversity losses. We used citizen science data over a 13-year period to evaluate whether 39 eastern Himalayan bird species might be shifting to higher elevations over time, by analyzing changes in the frequency of reporting of these species at birdwatching hotspots. For these species, we find evidence consistent with upslope range shifts, with species with the bulk of their elevational ranges below the hotspot elevation showing increases in their reporting frequency over time, and those with most of their elevational ranges above the hotspot elevation declining in reporting frequency. Our findings are suggestive of rapid responses to climate change by eastern Himalayan birds. We caution that eastern Himalayan bird species might be at special risk from increasing global temperatures because of their heightened thermal sensitivity coupled with particularly high rates of warming in the region. Eastern Himalayan birds are likely to require large tracts of undisturbed natural habitat across entire elevational gradients to be able to track changing temperatures by moving to higher elevations to remain resilient to climate change. SUMMARYO_LIOne of the most fundamental responses of species to changing temperatures is to change their geographic ranges, possibly to track the range of temperatures that is ideal for their survival. C_LIO_LIWith increasing climate warming, this shift could happen by moving towards the poles or in mountainous areas, towards the summit. C_LIO_LIDue to the high thermal sensitivity of tropical species and the decrease in space as species move up mountains, the extremely biodiverse bird communities of tropical mountains are particularly vulnerable. C_LIO_LIUsing citizen science data from birding hotspots along an elevational gradient in the Eastern Himalayas over a 13-year period, we measured the change in reporting frequency of 39 common bird species. C_LIO_LIChanges in reporting frequency are generally consistent with the fact that upslope shifts are taking place in the Eastern Himalayas, similar to results for birds from other tropical mountains. C_LI

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