| dc.description.abstract | Obligate interspecific avian brood parasites do not build a nest but lay eggs in the host nest and avoid the cost of parental care by transferring it to hosts. The high breeding cost forces hosts to evolve effective defence mechanisms against parasitism, while brood parasites develop better trickery against host defence, leading both parties into a refined coevolutionary arms race. In this thesis we examined the coevolutionary interactions of two poorly studied cuckoo host systems, the Asian koel (Eudynamys scolopaceus) with its three hosts, namely, the house crow (Corvus splendens), the common myna (Acridotheres tristis) and the long-tailed shrike (Lanius schach), and the plaintive cuckoo (Cacomantis merulinus) with the common tailorbird host (Orthotomus sutorius).
Brood parasitism is considered to cause an increase in the risk of nest predation. We found that nest parasitism significantly increased the risk of nest predation in nests of three host species that contained koel nestlings, but not in nests with koel eggs. In addition, this study revealed that Asian koel parasitism reduced the host reproductive rate by lowering host fledgling success. The number of host fledglings decreased with the number of Asian koel nestlings. Human disturbance is considered a potential threat in avian conservation, and different species respond to it in different ways. We found that human disturbance influenced the risk of brood parasitism and that nest predation differed between host species. The influence of factors affecting the risk of parasitism and predation differed between host species, except distance to the food source, which significantly influenced the nest site in all host species on risk of parasitism.
We investigated Asian koel egg mimicry in terms of egg size and shape, eggshell colour and pattern in relation to three host species and found a significant difference between koel eggs laid in different host nests with regard to egg volume, egg shape and some egg spotting pattern variables, but not in colors. Furthermore, there were significant differences in the pairwise comparisons between host eggs and host-specific koel eggs in most of the egg traits. Our results revealed that the variation among the host-specific koel eggs was not the result of host egg mimicry because the variation was not consistent with the variation between the host eggs. Hosts of avian brood parasites often lay polymorphic eggs to outpace parasitic egg mimicry. We revealed that some house crows lay immaculate blue eggs, which is clearly different from the regular morph with a bluish-green ground colour with black or brown blotches.
The plaintive cuckoo is widely distributed and one of the smallest brood parasites in Asia but details regarding host utilization for this parasite is not known in Bangladesh. We identified the common tailorbird as the only potential host of this cuckoo species in our study area. The tailorbird laid two different egg morphs in this area; one was blue with brown spots, while the other was white with brown spots at the blunt end. However, the plaintive cuckoo laid only a single egg type, with a light blue ground colour and brown blotche | nb_NO |
