Hornbill abundance and habitat correlates in Kali Tiger Reserve, Western Ghats, India-Insights from collaborative monitoring

This collaborative study in Kali Tiger Reserve, Western Ghats, documents the distribution and habitat correlates of three hornbill species, revealing that Malabar Grey Hornbills are the most abundant and positively associated with food-tree density, while noting that low detection rates for Pied and Great Hornbills likely reflect seasonal breeding behaviors.

The Gist

Imagine the Kali Tiger Reserve in India as a massive, ancient library. Usually, when we think of libraries, we worry about the books (the trees) or the librarians (the tigers). But this study was about the messengers who fly through the aisles, carrying seeds from one shelf to another: the Hornbills.

These birds are like the "delivery drivers" of the forest. Without them, the forest can't grow new trees because they carry the seeds far and wide. However, like any delivery service, if the drivers stop showing up, the whole system breaks down.

Here is the story of how researchers and forest rangers teamed up to count these delivery drivers and figure out what they need to stay happy.

1. The Big Question: Are the Drivers Still Working?

The scientists knew that climate change and human activity are making life harder for wildlife. Even inside a "Protected Area" (a safe zone like a fortress for nature), things can go wrong. They wanted to know: Are the hornbills still thriving in Kali Tiger Reserve, or are they quietly disappearing?

There are three main types of hornbills in this region:

• The Malabar Grey Hornbill: The most common, like the "standard delivery van" of the forest.
• The Malabar Pied Hornbill: A bit rarer, like a "specialized courier."
• The Great Hornbill: The giant, majestic one, like a "heavy-duty truck," but very hard to spot.

2. The Detective Work: How They Counted

Instead of hiring a whole new army of scientists to run around the forest, they used a clever trick. The forest rangers were already walking specific paths (called transects) every day to count tigers and their prey (like deer).

The researchers asked: "Can we count the birds while you're counting the tigers?"
It was a perfect team-up. The rangers became the eyes and ears of the study. They walked 57 paths across the forest, listening for calls and looking for birds, just like a detective looking for clues.

3. What They Found: The "Who, Where, and Why"

The Popularity Contest:

• The Grey Hornbill was the star of the show. They found them everywhere, in every corner of the reserve. It's like finding a popular coffee shop on every street corner.
• The Pied and Great Hornbills were much harder to find. They were like rare, exclusive clubs; you only saw them in specific neighborhoods (mostly near rivers or in drier, deciduous forests).

The "Why" (The Secret Sauce):
The researchers wanted to know why the birds were in some places and not others. They looked at the "ingredients" of the forest:

• For the Grey Hornbills: They loved areas with lots of fruit trees. It's like a delivery driver preferring a route with plenty of customers. The more fruit trees, the more birds.
• For the Pied Hornbills: They seemed to prefer areas with fewer giant trees (lower "basal area"). They liked the open, deciduous forests more than the dense, dark evergreen ones.
• For the Great Hornbills: They were so rare and quiet that the scientists couldn't find a clear pattern yet. They might be looking for something very specific that the study didn't catch, or they might just be very shy.

The Numbers:
They estimated there are about 7,700 Malabar Grey Hornbills living in the reserve. That's a healthy population! However, the density (how many birds per square kilometer) was lower in dry areas compared to the wet, lush forests.

4. The Catch: Why the Numbers Might Be Low

The study noted that the survey happened during the breeding season (March to May).

• The Analogy: Imagine trying to count people in a busy office building, but everyone is locked in their private meeting rooms.
• During breeding, female hornbills seal themselves inside tree hollows to raise their babies. They stop flying around and stop making noise. So, the "low" numbers for the Pied and Great Hornbills might not mean they are disappearing; it might just mean they are hiding at home taking care of their families.

5. The Big Takeaway: Teamwork Makes the Dream Work

The most important part of this paper isn't just the bird count; it's how they did it.

• The Metaphor: Think of the Forest Department as the "owners of the building" and the Researchers as the "architects." Usually, they work in separate offices. Here, they built a bridge.
• The forest rangers helped collect the data, and the researchers analyzed it and shared the results back with the rangers. This "co-production" of knowledge means that conservation isn't just something scientists do to the forest; it's something they do with the people who guard it every day.

In a Nutshell

This paper tells us that the Kali Tiger Reserve is a healthy home for hornbills, especially the Grey ones, as long as there are plenty of fruit trees. The study proved that you don't need expensive, separate surveys to monitor wildlife; you can piggyback on existing work (like tiger monitoring) if you get the rangers and scientists to work together.

It's a reminder that nature is like a complex machine: if we keep the gears (the trees) oiled and the workers (the rangers) and engineers (the scientists) talking to each other, the machine keeps running smoothly.

Technical Summary

1. Problem Statement and Context

• Conservation Challenge: Large-bodied forest specialist species, such as hornbills, are critical indicators of ecosystem health due to their role in seed dispersal and their dependence on large, intact forest patches. However, their populations are threatened by habitat loss, degradation, fragmentation, and climate change.
• The Monitoring Gap: While Protected Areas (PAs) like Tiger Reserves are intended to mitigate these threats, they are not immune to global changes. There is a significant lack of baseline data regarding hornbill abundance and distribution even within these PAs. Previous studies often lack systematic spatial coverage or rely on opportunistic sightings rather than rigorous density estimation.
• Specific Context: The Kali Tiger Reserve (KTR) in the Western Ghats of India is known to host healthy hornbill populations (Malabar Grey, Malabar Pied, and Great Hornbills), yet no systematic survey had previously estimated their densities or habitat correlates across the entire reserve.
• Objective: The study aimed to:
  1. Map the distributions of the three hornbill species in KTR.
  2. Identify habitat correlates (vegetation structure and food resources) influencing their encounter rates.
  3. Estimate the population density of the most abundant species, the Malabar Grey Hornbill (Ocyceros griseus).

2. Methodology

The study employed a collaborative approach between researchers (Nature Conservation Foundation) and the Karnataka Forest Department, utilizing existing infrastructure for cost-effective monitoring.

• Study Area: Kali Tiger Reserve (1,425 km²), Uttara Kannada district, Karnataka. The area encompasses diverse habitats ranging from South Indian Moist Deciduous to West Coast Semi-Evergreen forests, with elevations from 27m to 1,059m.
• Sampling Design:
  • Transects: 57 line transects (originally established for tiger prey monitoring) were used. These were 1–2 km long, laid in N-S orientation, and uniformly distributed across seven ranges of the reserve.
  • Timing: Surveys were conducted between March and May 2025, coinciding with the breeding season (when vocal activity is high, though females are often nesting).
  • Data Collection: Two observers walked transects (06:00–10:00 hrs) recording species identity, detection mode (visual/auditory), flock size, and perpendicular distance (for visual) or distance bands (for auditory).
• Vegetation Sampling: 56 plots (50m × 10m) were established to measure vegetation structure. Metrics included stem density, basal area, canopy height, and specifically the density of "food-tree" stems (species known to be consumed by hornbills).
• Statistical Analysis:
  • Distributions: Mapped using GPS coordinates via Epicollect5 and QGIS.
  • Correlates: Pearson's correlation coefficients were used to test relationships between encounter rates (per km) and vegetation variables (stem density, food-tree density, basal area, canopy height).
  • Density Estimation: Distance sampling (Multiple Covariate Distance Sampling - MCDS) using the R-package Distance. Detection probability was modeled as a function of forest ranges. The best-fit model was selected based on Akaike Information Criterion (AIC).

3. Key Results

A. Distribution and Relative Abundance

• Malabar Grey Hornbill (Ocyceros griseus): The most abundant and widely distributed species, detected in all seven ranges. Kumbarwada and Gund ranges were hotspots.
• Malabar Pied Hornbill (Anthracoceros coronatus): Relatively rare with sparse sightings. Highest detections occurred in Kadra, Gund, and Phansoli ranges. Notably, they were observed utilizing riverine areas and even foraging on non-native Acacia auriculiformis.
• Great Hornbill (Buceros bicornis): The rarest species, with very few visual detections (mostly auditory). Detections were concentrated in Kadra and Phansoli (moist deciduous forests), contrary to the expectation of higher prevalence in evergreen forests.

B. Habitat Correlates

• Malabar Grey Hornbill: Encounter rates showed a significant positive correlation with food-tree stem density ($r=0.84, p=0.02$). This suggests these birds track fruit resources at the patch level.
• Malabar Pied Hornbill: Encounter rates showed a significant negative correlation with basal area ($r=-0.78, p=0.04$). This aligns with their preference for moist deciduous forests (which typically have lower basal area than evergreen forests) over dense evergreen patches.
• Great Hornbill: No significant correlations were found with any measured vegetation variables, suggesting they may track resources at broader spatial scales or rely on specific large fruiting trees not captured by plot-level density metrics.

C. Population Density Estimates (Malabar Grey Hornbill)

• Overall Density: Estimated at 5.1 birds/km² (95% CI: 4.0–6.4).
• Range-wise Variation:
  • Highest density: Gund (7.8 birds/km²) and Kumbarwada (7.2 birds/km²).
  • Lowest density: Kulgi (1.1 birds/km²), a range dominated by drier deciduous forests.
• Total Abundance: The study estimates a total population of approximately 7,686 individuals (95% CI: 6,020–9,812) within KTR.
• Flock Size: Mean flock size was 1.2 birds.

4. Key Contributions

1. Baseline Data: This is the first systematic survey to provide robust density estimates and distribution maps for all three hornbill species in Kali Tiger Reserve.
2. Methodological Innovation: The study successfully demonstrated the utility of repurposing tiger prey monitoring transects for bird population estimation. This "co-production of knowledge" model significantly reduces logistical costs and increases the feasibility of large-scale monitoring.
3. Habitat Specificity: The research clarifies the distinct habitat preferences of the three species within the same landscape, highlighting that while the Grey Hornbill thrives in evergreen/semi-evergreen forests with high food-tree density, the Pied Hornbill prefers deciduous/riverine habitats.
4. Collaborative Model: It establishes a framework for transdisciplinary partnerships between researchers and forest department staff, leveraging local indigenous knowledge and field staff for data collection.

5. Significance and Implications

• Conservation Priority: The estimated population of ~7,700 Malabar Grey Hornbills underscores KTR as a critical stronghold for this endemic species, which has faced declines in other parts of the Western Ghats.
• Management Insights: The negative correlation between Pied Hornbills and basal area suggests that management practices favoring dense evergreen structures might inadvertently disadvantage this species, which prefers more open deciduous or riverine zones.
• Scalability: With 58 other Tiger Reserves in India, this methodology offers a scalable template for monitoring hornbill populations across the country, filling a critical data gap for conservation planning.
• Climate Resilience: By establishing baseline densities and habitat associations, this study provides the necessary data to monitor how hornbill populations respond to future climate shifts and habitat degradation within protected areas.

Conclusion: The study confirms that Kali Tiger Reserve supports a substantial and diverse hornbill population. It validates the effectiveness of collaborative monitoring using existing forest infrastructure and provides essential baseline data to guide future conservation strategies for these ecologically vital birds.