The Asian koel rapidly locates hosts breeding in novel nest sites

Avian brood parasites depend upon finding host nests to lay their eggs. However, how brood parasites find the nests of their hosts and select the nests for parasitism remains mostly unresolved. Here, we examined how a non-evicting brood parasite, the Asian koel (Eudynamys scolopaceus) selects the hosts' nests to lay their eggs. We provided a novel habitat (nest box, n=100) to the Asian koel and its host, the common myna (Acridotheres tristis), at a field site in central Bangladesh. A total of 99 nests across 59 boxes were used by common mynas, of which 21.2% of these nests were parasitized by the Asian koel. Moreover, non-host species, the Oriental magpie robin (Copsychus saularis) (n=8) and jungle myna (Acridotheres fuscus) (n=6) also built nests in the boxes but none of these nests were parasitized. We found that active boxes were significantly more parasitized than inactive nest boxes. Among the active nest boxes, only common myna nests were parasitized by the Asian koel. We found a strong tendency for Asian koel selecting common myna nests more than the non-host species, Oriental magpie robin and jungle myna. Our results provide robust experimental support for the importance of host activity suggesting that Asian koels actively select occupied boxes. Our study shows support for the host imprinting hypothesis, where the Asian koel uses the host activity as a cue to search for potential nests, and then decide whether to parasitize.

Methods
We placed 100 nest boxes in the study area in March 2022 (Fig. 1). Nest boxes were placed on trees about seven meters above the ground. All the nest boxes were located in areas usually utilized by common mynas. The dimension of the nest boxes was length 20.5 cm, width 20.5 cm, height 30.5 cm, top cover 24.2 cm, and the entrance was 9.5 cm for 50 boxes and 12.7 cm for another 50 boxes. The nest boxes were tied to trees with metal wires. We checked the nest boxes regularly to monitor nest building process by the common myna, and later to examine whether a clutch was parasitized by the Asian koel. Asian koel eggs were easily distinguished from common myna eggs by color, pattern, and size (Nahid et al. 2021). Moreover, Oriental magpie robin and jungle myna eggs were also easily distinguishable from Asian koel egg (Begum et al. 2012; Karim and Ahsan 2016; Nahid et al. 2021).