Malabar Gray Hornbill Ocyceros griseus
Version: 2.0 — Published July 9, 2020
Demography and Populations
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Demography and Populations
The population of the species in its entire range along the Western Ghats has not been estimated. Populations were believed to be declining in the northern part of the range in the early 1990s or even earlier (S. A. Hussain in litt. in 4). Comparisons of density estimates and abundance indices from 2017-18 with available historical data suggest a significant decline in populations, range-wide (see below) and locally (see Population Status).
Abundance indices generated from eBird data used to generate range-wide population trends for the State of India's Birds 2020 report indicates population declines (18). This includes a significant long-term (from pre-2000 to 2018) as well as recent (for the period 2014-2018) population decline. The report estimates a long-term decline of -66.8% (95% CI = -43.6 to -89.9%) and an annual decline of -3.3%. Based on their criteria, the species would merit categorization as High Concern based on the average estimates, but as Moderate Concern due to width of the confidence interval of the population index. The data indicate a 54% decline in the population index over an approximately 10-year period till 2018 (supplementary temporal data provided by the authors of the report, 18).
Measures of Breeding Activity
Age at First Breeding
Age at first breeding is not known from primary field studies, but model estimates based on avian life history traits suggest 2 years (35).
Intervals Between Breeding
Breeding is annual.
Clutch Size and Number of Clutches per Season
Clutch size ranges from 1 to 4 eggs (9, 31, 8). Usually, a single chick hatches in most nests, with 2 chicks noted only in 2 cases so far (20). Although clutch sizes of up to 4 have been reported in Asian hornbills, most species that are not cooperative breeders manage to raise only a single chick (10). Only a single clutch is known to be produced per season.
Annual and Lifetime Reproductive Success
Few measures of breeding success (nests that successfully fledged young) are available for the species. One study reported, of 26 nests monitored in 1994, 24 were considered successful as either the chick had fledged or the nest was active at the end of the breeding season (31).
Indices of Breeding Activity
Breeding activity in Malabar Gray Hornbills has been indexed using (a) the percentage of pairs in the population that are nesting, and (b) the percentage of known nests that are active.
The sex ratio of adults (S = number of adult females/ number of adult males) in the non-nesting season tends to be relatively even or slightly female-biased. When females are confined in tree-cavity nests, the sex ratio of free-ranging birds becomes male-biased as expected. The change in sex ratio from non-nesting to nesting season can be used to estimate a breeding activity index as the proportion of pairs in the population that are breeding. One study from the Anamalai Hills (P. Pawar, D. Mudappa, and T. R. S. Raman, unpublished ms) indicated that during the nesting season, the adult sex ratio of Malabar Gray Hornbill was male-biased in tropical rainforests within a protected area (2.76 males per female, S = 0.36) and in an adjoining plantation landscape (1.44 males per female, S = 0.70). During the non-nesting season, the adult sex ratio was female-biased in both protected area (S = 1.53) and in the plantation landscape (S = 1.20), possibly because some sub-adults (which resemble females) may have been included in the count. The estimated proportion of breeding pairs in the protected area was 64%, higher than the proportion of breeding pairs in plantations (30%), indicating that the latter may be a sub-optimal habitat for breeding hornbills.
In the Anamalai Hills, a study from 1993-1994 reported that of 27 nests, 26 were active (31). A follow-up study of 46 nests monitored annually for two to eight years in the Anamalais, found on average 63% of the nests active each year, with 4.9% of the nests becoming defunct due to enlargement or closure of cavity entrance, colonization by bees, fire, predation, and treefall (33). In the Nilgiris, of 52 nests found in 2000, 67% were re-used in 2001, but only 40% were used in 2002 as a number of nests remained unoccupied in a fire-affected area (32).
Life Span and Survivorship
No primary information is available on lifespan, survivorship, and generation length. Modeling analysis using life history traits of the world's birds estimates the age of first breeding as 2 years, adult survival of 75%, and maximum longevity 16 years for the Malabar Gray Hornbill. Using these values, the generation length was estimated as approximately 5 years for the species (35).
Disease and Body Parasites
Causes of Mortality
Besides natural predation (see above), other causes of mortality include roadkill (probably of low-flying birds), and poaching by humans.
Population Spatial Metrics
Surveys of a number of wildlife reserves and reserved forests along the Western Ghats in 2004-2005 reported estimates of relative abundance (36) and population density (5). These surveys documented widespread occurrence of the species and reported significant populations in mid-elevation (600-1,100 m) moist forest habitats in Agasthyamalai and Anamalai Hills, and the Periyar landscape in the southern Western Ghats (5). Population density estimates from more northern sites, such as the Anshi – Dandeli – Goa landscape, in the states of Karnataka and Goa, were lower than estimates from the landscapes south of the Palghat Gap in the southern Western Ghats (see Table 7).
Comparing density estimates for the Anamalai landscape from 2017-2018 (P. Pawar, D. Mudappa, and T. R. S. Raman, unpublished data) with estimates from 2004-2005 , indicates a 39% decline in population density inside the protected area and a 55% decline in the Valparai Plateau (see Table 7).
Populations may be regulated by limitation of nest sites as the birds are dependent on large trees with suitable nest cavities for breeding.