Roosting behaviour of harriers at Wiang Nong Lom

The harrier roost at Wiang Nong Lom, located between Mae Chan and Chiang Saen district in Chiang Rai, is the most well known roost of wintering harriers in Thailand. Since I visit Chiang Saen almost monthly, I always visit the roost during winter months. I have observed 4 species of harriers roosting at the site regularly including Pied Harrier, Eastern Marsh Harrier, Western Marsh Harrier, and Hen Harrier. The first 2 species are abundant, with Pied Harrier being the most numerous species, while the latter 2 species are scarce and require thorough observation to find one.

The most unique characteristic of Wiang Nong Lom is the dense grassland that is actually floating above the water. This makes it a safe place for harriers to roost, and also a safe breeding ground for the little known Eastern Grass Owl. It’s extremely difficult to go into the grassland, and quite risky as you can easily sink into the water below. However, recent development on the eastern side of the lake has been shrinking the size of the floating vegetation. Burning and draining of water for agriculture are also the main threats to this valuable but overlooked habitat.

It was estimated that more than 400 harriers roosted at Wiang Nong Lom in the past, but the current number appears to be much fewer than that. The most abundant species is the Pied Harrier (Circus melanoleucos) which is estimated to be approximately 200 individuals each winter. Unlike other harriers, the Pied Harriers often perch together in big flocks on open ground before flying into the actual roost once it gets completely dark. The most frequented pre-roosting congregation site is near the watch tower where a permanent hide has been set up for many years.

Although it is the most abundant species at the site, Pied Harriers usually arrive much later in the evening than the larger and less numerous Eastern Marsh Harrier. The main flock of Pied Harriers usually arrives after sunset. Once they arrive at Wiang Nong Lom, they usually gather in big flocks either on the ground or on treetops. Apart from the pre-roosting site at the permanent hide, they also like to gather on the ground inside the pineapple plantation nearby. They usually spend 15-30 minutes at the pre-roosting site waiting for it to get completely dark, then fly out altogether to roost inside the floating grassland. I’m still unsure of the purpose of such congregation. They don’t seem to do anything much, just standing and looking at each other. Some might preen or call just a little, but there’s usually not many interactions between each bird. Then at one point when it gets nearly completely dark, they would just fly out all at once.

Eastern Marsh Harrier (Circus spilonotus) is the second most numerous harrier at Wiang Nong Lom. The number seems to vary but there are probably less than 100 birds in recent years. Unlike the Pied Harrier, the first Eastern Marsh Harrier usually arrives much earlier in the afternoon starting from around 15.30 onward. I also notice that Eastern Marsh Harriers don’t tend to gather in flock, but rather fly in and perch separately. There’s also no pre-roosting congregation like in the Pied Harriers, although some odd birds might join the standing flock of Pied Harriers briefly.

Unlike Pied Harriers, the Eastern Marsh Harriers usually land on shrubs and treetops separately across the floating grassland. They like to sit in the open for a long while before dropping into the denser cover when it gets nearly dark. The best way to observe perching Eastern Marsh Harriers is by using a telescope to scan across the floating grassland from a higher angle like from the watch tower or on the nearby hill.

Western Marsh Harrier (Circus aeruginosus) is one of the two rare harriers that regularly roost at Wiang Nong Lom. The maximum number of birds seen together is about 4 birds, but there can be a few more. Adult male and juvenile are the two most often seen plumage types. Behaviour-wise, I don’t see any difference from the previously described Eastern Marsh Harrier. The first Western Marsh Harrier can also appear quite early in the late afternoon as well.

The last species of harrier that can be found roosting at Wiang Nong Lom is the Hen Harrier (Circus cyaneus). It is the rarest of all 4 regular harriers in Thailand (not including Pallid Harrier which has only 1 dubious historical record). However, I’ve noticed a slight increase of Hen Harriers in the recent year, particularly at Wiang Nong Lom where I’ve already spotted at least 4 individuals this winter including an adult male which is the rarest plumage in Thailand.

Since there are only few birds and can be quite difficult to identify in the field, the behaviour of Hen Harriers at Wiang Nong Lom is the least understood. Compared to other harriers, I often see Hen Harriers flying much lower and closer to the ground, both while hunting and when coming back to roost. They can be easily overlooked, particularly when it’s a female or juvenile flying low and fast against the dark background. They also rarely perch in the open. I’ve observed a few birds that were coming back to roost and they all dropped immediately into thick grass, not allowing any view while perching unlike the Pied and Marsh Harriers. However, I still need to see them more often to fully understand how they behave when coming to roost.