Defense Strategies Of The Black-Browed Reed Warbler Against Cuckoo Parasitism

Reproduction is an important part of bird's life history,and to date there has been little work on breeding biology of the Black-Browed Reed Warbler(Acrocephalus bistrigiceps)(BRW).Nest-site selection has important impact on breeding success,with sympatric breeding birds being a hot spot of avian research.The cost of reproduction imposed by avian brood parasitism favors the evolution of antiparasitic strategies in host birds.Thus,arms race and interactions between the parasite and its host are model system for the study of coevolution.In Zhalong National Nature Reserve,the Common Cuckoo(Cuculus canorus)(CC),the Oriental Great Reed Warbler(Acrocephalus orientalis)(ORW)and BRW breed within the reed habitat,with ORWs being the main host of CC and BRWs being the rare host.In this study,breeding biology of BRWs,nest-site selection to cope with cuckoo parasitism,nest defense and egg recognition of BRWs were studied to explore the coevolutionary interactions between BRWs and CCs at different nest stages.The main results and conclusions are as follows:1.The nest is built by the female BRW alone,with nest building period being within two days.The nest is combined of dry grass,poplar cotton,dry reed flowers and reed roots.Each egg was laid every day between 03:42-05:16 am,and the clutch size is about two to five eggs with a mean of 4.45 ± 0.79.The female alone takes the incubation with the incubation period being 10-12 days(11.30 ± 0.67)and nestling feeding period being 8-10 days(9.31 ± 0.79).2.Although BRWs and ORWs breed in small groups within the same reed habitat,nest-sites of them were seldom overlapped.The neighbor nest distance of BRWs was 55.99 ± 61.46 m(n=47),while it was 48.90 ± 37.23 m(n=30)for ORWs.There is no significant difference of neighbor nest distance between BRWs and ORWs(P=0.871).For nest height,water depth,nesting plant height and plant height above the nest,BRWs were significantly less than those in ORWs.However,for nest coverage,distance to water edge,distance to reed edge,distance to cuckoo perch,distance to road and distance to human building,BRWs were significantly greater than those in ORWs.3.Dummy experiments showed that BRWs have a moderate level of nest defense,with 58%(21 out of 36 nests)of nesting parents attacking the dummies,and attacking behavior occurs in 55%(59 times out of 108 times)of dummy experiments.Only4.6%(5 times out of 108 times)of dummy experiments attracted 1-2 neighbors,with the neighbors never attacking dummies.The attacking behavior of BRWs was not impacted by dummy types and nest stages(Generalized Linear Mixed Model;dummy types: F=0.587,P=0.558;nest stages: F=0.917,P=0.340).4.Only one nest of BRWs was parasitized by CCs in 2019-2020 in Zhalong study area with 0.81% in cuckoo parasitism rate(n =123).Egg experiments showed that BRWs have a strong egg recognition ability and can reject most blue model eggs,CC and parrot eggs;up to 100% of eggs of ORWs and mannikins.The grasp index of BRWs is 125.14 ± 4.15 mm~2(n=10),with small eggs being rejected by ejection and most larger eggs being rejected by nest desertion due to its small bill(small grasp index).Importantly,BRWs never reject conspecific eggs and desert a nest with experimental conspecific eggs.The recognition clue for BRWs is spots on the blunt pole,because half of the own eggs dotted on the blunt pole was rejected,but never on the sharp pole.When there were three eggs of barn swallows(Hirundo rustica)and one own egg in its nest,BRWs can reject all foreign eggs and retain own eggs without recognition errors.When own eggs were all replaced by eggs of ORWs,BRWs can also reject 100% of the foreign eggs.Therefore,the recognition mechanism of BRWs is true recognition(template-based recognition).In summary,breeding biology including nest building,clutch size,incubation and nestling feeding of BRWs in Zhalong reed habitat was studied.BRWs and ORWs gathered in separated groups for breeding,but seldom overlapped in nesting area.There was significant difference in nest-site choice between BRWs and ORWs,which suggested that host selection by CCs was not random.Our results did not support the nest site hypothesis but support the host imprinting hypothesis.BRWs were seldom parasitized by CCs and have strong egg recognition ability to reject foreign eggs by either ejection or nest desertion.The recognition clue for BRWs is spots on the blunt pole of egg and the egg recognition mechanism of BRWs is true recognition(template-based recognition).Innovation:1.Host use by parasitic cuckoos has not been well understood.In this study,breeding ecology,nest defense,egg recognition and recognition mechanism of BRWs were intensively studied using dummy and egg experiments in the field.Our study made a contribution to research on evolutionary interactions between CCs and BRWs.2.Nest desertion is costly in birds and thus it has not been well confirmed as a strategy against avian brood parasitism.As a case study,our field experiments showed that nest desertion in BRWs occurred more often when they recognized but were unable to reject a larger parasitic egg,suggesting nest desertion in BRWs may be an effective anti-parasitism strategy.