Are hornbills in danger due to extreme weather conditions?

Researchers monitoring the breeding habits of three hornbill species in the Pakke Tiger Reserve in Arunachal Pradesh for 22 years observed that extreme weather conditions this year disrupted the timing of nesting; there was late nesting and fewer active nests, several of which have already failed.

Great hornbill male. | Photo Credit: Aparajita Datta

Zazu, the diminutive red-billed hornbill depicted in the movie The Lion King, is a familiar endearing character. Recently, its cousin, the southern yellow-billed hornbill in the Kalahari Desert, made world headlines. A study found that this small desert-dwelling African hornbill species has been impacted by climate change and is likely to be facing a collapse in breeding by 2027. The study found that from 2008 to 2019, the percentage of occupied nests declined from 52 per cent to 12 per cent, while the percentage of successful breeding attempts declined from 58 per cent to 17 per cent. The mean fledglings produced per nesting attempt used to be more than one but by 2019 had declined to fewer than one.

Poor breeding effort and performance were linked to external factors such as higher temperature, drought during the breeding season, late nesting, and shorter time spent by females in the nests after chick hatching.

The consequences of rising temperatures because of climate change on hornbill breeding are very clear for an African hornbill in an arid region. However, the Asian hornbills in tropical forests in Arunachal Pradesh seem also to be feeling the heat, and the story is messy and complicated for the larger-bodied hornbills in these wet forest habitats.

Hornbills are especially vulnerable to the effects of climate change because of their unique breeding habits. They nest in tree cavities (which they cannot create), with the female entering the nests and sealing herself inside for nearly the duration of the nesting cycle. They have long nesting cycles, which can go up to four to five months for the largest forest hornbill species. The male provides food for the female and the growing chick or chicks. They breed only once and must time their breeding to coincide with periods of higher food availability. There is also evidence that not all adult pairs breed every year.

Oriental pied hornbill male. | Photo Credit: Aparajita Datta

While many savannah-dwelling African hornbills are largely insectivores, Asian forest hornbills depend mainly on fruits of many rainforest tree species.

In the tropical forests of the Pakke Tiger Reserve in Arunachal Pradesh, we have monitored breeding by the great hornbill (3–4 kg), the wreathed hornbill (2.5 kg), and the Oriental pied hornbill (a much smaller-sized species, weighing 800–900 g) for 22 years. The initial monitoring was from 1997 to 2000, when I was carrying out my PhD study on hornbills. Then, we restarted monitoring from 2004. There are a few gaps in the intervening years. Hornbill nests and habitats in the forests adjoining Pakke are also protected through a community-based conservation programme (known as the Hornbill Nest Adoption programme) by the Nyishi tribe. Since 2012, when this programme began, the team of Nyishi nest protectors monitors the breeding of these species in the adjoining reserved forests near their villages.

We have also tracked the patterns in ripe fruit availability for 17 years. In 2011, we set up a weather station in Seijosa (near the south-eastern part of the reserve) to record temperature, rainfall, humidity, solar radiation, and wind speed.

Hornbill conservation has taken off in other States, too, with local environmental activists and forest departments taking active part. This Kadar tribesman in Kochi, Kerala, climbs up with fruits for a Malabar grey hornbill chick and its mother nesting in the tree, in March 2018. The male hornbill, which feeds its mate and offspring, had been found dead on the roadside. | Photo Credit: THULASI KAKKAT

We track how many nests are active or have attempted breeding in each year. For all active nests, we record the date of female nest entry or date when nesting is initiated. We monitor the nest for the next few months to see whether the nest remains active with the male visiting to feed the female (and the chicks, once they have hatched). We monitor whether the breeding was successful by checking the nests before the expected fledging to record the date when the chick exits the nest. In the larger two hornbills, there is usually only one chick that fledges successfully. Although they are known to lay two eggs, one is only laid as an insurance in case the first one fails. In the Oriental pied hornbill, there can be more than one chick fledging. Knowing both female entry and chick exit dates allows us to calculate the length of the breeding cycle.

The breeding season

The breeding season is from March to July and mid-August. The two larger hornbills start nesting in March, while the Oriental pied starts nesting in the first to second week of April and has an 80- to 90-day nesting cycle. The nesting cycle of the great hornbill is on average four months long. Although the wreathed hornbill is smaller than the great hornbill, it generally has a mean nesting cycle of 130 days. When the great hornbill chicks fledge in early to mid-July, it is the middle of the monsoon, while most of the wreathed hornbill chicks come out some time later, some in early to mid-August.

This was the general pattern over the years, with some variation. In some years, we noted somewhat earlier breeding in the first week of March, but it was usually from the second to the last week of March.

In 2017, we first noted early breeding in the larger hornbills. They initiated nesting in February. The earliest date was February 14, 29 days earlier on average than what we had observed for 17 years. Before 2017, the median nest entry date was March 15.

Wreathed hornbill male. | Photo Credit: Aparajita Datta

During my PhD study, I had found that the peak ripe fruit availability of bird-dispersed tree species was from May to July, which coincided with the post-hatching and chick development period. However, the peak ripe fruit availability of 14 important hornbill food tree species shifted from 2011 to 2017. The peak is now in April, and the fruiting is more staggered, from February to July. This pointed to the possibility that the earlier breeding observed in 2017 was linked to the changing availability of fruit. The timing of hornbill breeding coincides with peak fruit availability of bird-dispersed species, and now that the peak has shifted to earlier months, hornbills might be responding to this shift by starting to breed early. Hornbill nesting and fruiting patterns also seem to show some linkage with El Nino/La Nina weather phenomena, but we are yet to fully understand this.

We thought this unusual early nesting would impact their breeding success in some way. However, this did not result in any effect on the length of the breeding cycle and in nesting success in 2017. It appeared they had the flexibility to respond to a change in fruiting patterns.

We waited to see what would happen in 2018. The timing of breeding was “normal” in 2018. In 2019, breeding was again early for most nests: from the end of February to the first week of March. However, in 2019, five out of six great hornbill nests in Pakke failed, and nests were abandoned midway through the breeding season.

This was not normal. In general, once hornbills nest, breeding is successful. Average nesting success for hornbills is around 85 per cent. Nesting success can be affected by human threats in more disturbed habitats, but inside Pakke, owing to the protection provided by Forest Department staff, there are no threats due to poaching or tree felling. The natural causes of nesting failure can be external factors such as predation of chicks at nest (by yellow-throated martens, binturongs), tree falls due to storms, or problems with the cavity such as flooding or cavity floor sinking. There are other intrinsic causes that are hard to determine, such as failure of chick to hatch, death due to starvation or disease, or nest abandonment by the female/male. These failures are likely linked to weather patterns and food availability.

Sightings of the nine hornbill species found in India (as reported on Hornbill Watch in 2018). | Photo Credit: By Special Arrangement

In 2020, we could not monitor most nests because of COVID restrictions and no research permit for part of the time. In 2021, we could only begin nest monitoring in Pakke from April; therefore, we missed the nest entry dates for most nests. But this year, hornbill nesting is unusually late: most birds had not entered the nests in March, a few had entered by mid-April, and there are many that have not initiated breeding.

Out of 13 great hornbill nests, six pairs initiated nesting only by the second week of April. The mean nest entry date was April 2 compared with the long-term average of March 15. The Oriental pied hornbill usually starts nesting in the first week of April, but this year, its mean nest entry date was April 20, 18 days later than the long-term average of April 2. For the wreathed hornbill, the long-term mean nest entry date is March 17, while this year, the mean nest entry date was April 12, with entry dates ranging from March 29 to April 28.

In some nests, the female entered the nest, came out, and re-entered it, doing this several times. This is unusual behaviour. In one nest of a wreathed hornbill pair, the female entered the nest cavity on April 1. However, she was seen outside the nest with the male on April 8, 11, and 17. Finally, on April 28, the female re-entered the nest, and the male was seen feeding the female. In May, there was a heavy storm one night and the tree fell. The female died. There were eggshells inside the nest but no chicks had hatched. She had been inside the nest for just 12 days. It must have been difficult for her to break the seal when the tree fell suddenly. It is sad that the female had to die after being so reluctant to enter the nest. Maybe she had a sense that things would not go well this time.

A female Oriental pied hornbill entering a nest (picture takenfrom video). | Photo Credit: Sital Dako

Hornbills sometimes lose their nest trees during heavy storms. But such instances have been rare since we began monitoring in 1997.

This year, March was unusually hot, with record-breaking temperatures in India. There was unusual weather in Pakke too, with March being very hot and experiencing continuous rain from the end of March and most of April-May, which is not normal for the area. This extreme and unusual weather appears to have completely disrupted the timing of hornbill nesting. On my visit to Pakke in early April, I saw the unusual sight of several adult pairs still flying around inspecting the nests.

In normal years, when hornbills initiate breeding in March, the weather is dry but not too hot. The females need to seal the cavity after entering the nesting. They are often reluctant to enter the nest and must be coaxed by the male to enter. After they go in, they seal the nest, usually with their droppings. In the case of the Oriental pied hornbill, the male also brings clods of mud for sealing. It is no wonder that the females are hesitant as they must remain holed up inside a cramped space with a narrow opening for a very long time. It is dark, damp, and humid inside. The nesting period of the hornbills coincides with the hottest time of the year when maximum temperatures range from 30 to 33 degrees Celsius. It is common to see the female through the cavity opening, perspiring with its beak open. Even in “normal” years, they feel the heat and humidity during the breeding season.

The male Oriental pied hornbill with a clod of mud in his beak to seal the nest (pictures taken from a video).  | Photo Credit: Sital Dako

When I analysed the 2022 weather data, I found that the mean maximum temperature in March was 2 degrees warmer than the 10-year average from 2011 to 2021. The total rainfall in April and May was 49 per cent more than the average total rainfall in these two months in the past decade. The mean relative humidity usually ranges from 50 per cent to 70 per cent in March to May. This year, the mean humidity ranged from 70 per cent to 90 per cent.

With long-term data, it is always difficult because before I have completed the analysis, another year goes by and the story changes. The story is yet to be fully understood.

Low fruit availability

In 2017 and 2019, I had thought that the hornbills might be resilient and adapting to climate change by adjusting their timing of breeding, but after this year, I am not sure how they will cope when the weather is unpredictable and there are extreme changes in climate. Fewer hornbill pairs have attempted breeding: only around 40 per cent of nests are active this year and 20 per cent of these have already failed. There is low fruit availability of hornbill food plants this year.

A wreathed hornbill pair in the rain. Hornbills sometimes lose their nest trees during heavy storms. | Photo Credit: Aparajita Datta

Through our conservation work, we attempt to address other human-driven threats to hornbills such as hunting and habitat loss. These are challenging to accomplish. But how do we address the effects of climate change on hornbill breeding?

The hornbills must be feeling bewildered and helpless with the changing weather. I hope that they will find a way to adapt and be resilient unlike their African cousins. Or that they will have some “good” years in which they will be able to breed more easily and successfully.

Aparajita Datta is Senior Scientist, Nature Conservation Foundation, and Co-Chair (Asia), IUCN SSC Hornbill Specialist Group.