
On 11 January 2025, at a padi field in Perak, Malaysia, I observed an interaction between a Juvenile Pied Harrier Circus melanoleucos and a Greater Spotted Eagle Clanga clanga. While watching this, at a distance in the field, I thought this was interspecific kleptoparasitism; a feeding strategy where a bird steals food from another bird. But it was unexpected in view of the size difference of the raptors. On reviewing the images and reflecting, I realised that the behaviour I observed was more of a defence of a migratory feeding territory.
I observed a Greater Spotted Eagle fly past me, it had no prey in the claws or beak. It proceeded to land to acquire a prey. At the same time a juvenile Pied Harrier swooped in and harassed it (see the six image sequence collage). It appeared to land its feet on the back of the Greater Spotted Eagle. After a brief interaction, the Pied Harrier flew off and the eagle was on the ground. I am uncertain if prey was acquired or lost.
Temeles (1990), in describing the interspecific territoriality of Northern Harriers Circus hudsonius and the role of kleptoparasitism, observed that these harriers responded aggressively to larger raptor species. He suggested that “territorial aggression by harriers against larger heterospecifics may function primarily to reduce interference competition, rather than exploitative competition”.
Greater Spotted Eagle are known to kleptoparasitize other raptors (Wells 1999), including the Eastern Marsh-harrier Circus spilonotus in Malaysia and Thailand (Mallalieu 2007). Perhaps this is also true of Pied Harriers but yet undocumented.
What is surprising is the aggression by a Pied Harrier in migratory location; this could be expected in their breeding ground. Wells (1999) notes that Pied Harriers forage “not infrequently over the same habitat-patches and at the same times of day”. This may explain the behaviour I observed.
References
1. Mallalieu, M. (2007). Greater Spotted Eagles Aquila clanga in central Thailand. Forktail. 23: 167-170.
2. Temeles, E. J. (1990). Interspecific territoriality of Northern Harriers: the role of kleptoparasitism. Animal Behaviour 40:361-366.
3. Wells, D.R. (1999). The birds of the Thai-Malay Peninsula: Vol. 1 (Non-Passerines). Christopher Helm, London.