The impact of human interference on Hill Mynahs Gracula religiosa breeding in Thailand

Abstract

SummaryThe breeding biology of Hill Mynah Gracula religiosa was studied in eight provinces of Thailand from 1991 to 1998 to evaluate how breeding behaviour has evolved to enhance reproductive success in the face of human interference. The northern race G. r. intermedia bred mostly during February to April whereas the southern race G. r. religiosa bred mostly later during April to June. Both races were cavity-nesting, non-excavating, monogamous and similar in breeding activities except for differences in body size. They nested at the bottom of deep cavities with a long entrance (mean 45 cm), in tall trees. Average nest height was 23 m. Nests were sometimes abandoned (10% of nests) during nest-building or incubation. Most Hill Mynahs (70%) had clutches of three eggs. Both parents shared incubation and feeding of young. The most detrimental factor to the reproductive success of this species was human theft of the young, rather than any natural factors. During 1991 to 1998, 80% of eggs laid were hatched, but 88% of all eggs hatched were lost before fledgling. Those illegally stolen by humans amounted to 61% of eggs hatched. Only 37 of 308 nestlings or 12% of eggs hatched survived to independence. In 1999, nests were experimentally guarded to prevent human interference, and fledgling success increased to 75% of eggs hatched. The remaining 25% of nestlings were lost to predators and unknown causes, close to the 27% lost to these causes in unguarded nests during 1991 to 1998. Although the breeding pattern of this species has evolved to promote survival of young, it does not seem to have evolved adaptations to human predation. The continuing decrease of wild Hill Mynah populations has prompted attempts at captive breeding. Data on breeding biology from this study should enhance its success.

Captive Hill Mynah Gracula religiosa breeding success: potential for bird conservation in Thailand?

“…Egg sizes of the captive-bred northern birds (24.7 ± 0.8 × 34.8 ± 1.1 mm, n = 38) were obviously smaller than those of the captive-bred southern ones (26.3 ± 0.8 × 37.1 ± 1.4 mm, n = 27). 'Captive' eggs were not different in size to 'natural' ones (northern race: 24.4 ± 0.8 × 34.1 ± 1.5 mm, n = 164; southern race: 26.9 ± 0.9 × 36.8 ± 1.2 mm, n = 92) (Archawaranon 2003). Clutch sizes of birds bred in the coconut trunks were the same as those bred in the nest-boxes.…”

“…After learning that the young left in the nest survived only 2-3 days, I removed nestlings from the nests the day after hatching. The body masses at hatching of nestlings (northern race: 11.9 ± 0.9 g, n = 32; southern race: 14.3 ± 0.9 g, n = 22) were not significantly different to those of wild young (northern race: 11.0 ± 0.7 g, n = 150; southern race: 13.9 ± 0.8 g, n = 88) (Archawaranon 2003). Southern nestlings were heavier in body mass and longer in wing length than their northern counterparts at all ages (Figure 1).…”

“…Predators are often the most important factor in determining nesting success in the natural populations (Lack 1954, 1968, Nilsson 1984, Martin 1988. A study of Hill Mynahs' fledging success in Thailand from 1991 to 1998 showed that the proportion of nestling loss before fledging was 88% (Archawaranon 2003). Out of this number, losses due to natural predators and to human depredation were 27% and 61% respectively.…”

“…Furthermore, in order to access birds, the poachers cut down trees or destroy nest-cavities, resulting in permanent habitat destruction. The case of the Hill Mynah Gracula religiosa is an example of how the pet trade directly affects a wild population (Bertram 1969, Archawaranon 2003. The birds' ability to mimic many sounds, and especially human speech, has created a great demand for them as pets.…”

“…Long-term collection of field data on Hill Mynah fledgling success from 1991 to 1998 in eight provinces of Thailand showed that only 12% of eggs hatched survived to independence while 61% of eggs hatched were lost to human theft (Archawaranon 2003). In contrast, in 1999 when nests were experimentally guarded 24 hours a day to prevent human interference, fledgling success increased to 75% of eggs hatched.…”