Hawk mimicry does not reduce attacks of cuckoos by highly aggressive hosts

Background: Resemblance to raptors such as hawks(Accipiter spp.) is considered to be an adaptive strategy of cuckoos(Cuculus spp.), which has evolved to protect cuckoos against host attacks. However, the effectiveness of the mimicry remains controversial, and is not yet fully studied for highly aggressive hosts.Methods: We evaluated the effectiveness of sparrowhawk(Accipiter nisus) mimicry by common cuckoos(Cuculus canorus) in oriental reed warblers(Acrocephaus orientalis), which are highly aggressive hosts. Using a both the single and the paired dummy experiment, defense behaviors and attack intensities of oriental reed warblers against common cuckoos, sparrowhawks and oriental turtle doves(Streptopelia orientalis) were assessed.Results: Oriental reed warblers exhibit strong nest defense behaviors, and such behaviors do not change with breeding stage(i.e., egg stage and nestling stage). Furthermore, assistance from conspecific helpers may increase attack intensities. However, they were deterred from mobbing overall by the presence of the hawk.Conclusions: Oriental reed warblers are able to distinguish cuckoos from harmless doves. However, they may be deterred from mobbing by the presence of the predatory hawk, suggesting hawk mimicry may be ineffective and does not reduce attacks of cuckoos by highly aggressive hosts.

Adaptation or ecological trap? Altered nest-site selection by Reed Parrotbills after an extreme flood

Background:Floods and other extreme events have disastrous effects on wetland breeding birds.However,such events and their consequences are difficult to study due to their rarity and unpredictable occurrence.Methods:Here we compared nest-sites chosen by Reed Parrotbills(Paradoxornis heudei) during June-August 2016 in Yongnianwa Wetlands,Hebei Province,China,before and after an extreme flooding event.Results:Twenty-three nests were identified before and 13 new nests after the flood.There was no significant difference in most nest-site characteristics,such as distance from the road,height of the reeds in which nests were built,or nest volume before or after the flood.However,nests after the flood were located significantly higher in the vegetation compared to before the flood(mean ± SE:1.17 ± 0.13 m vs.0.75 ± 0.26 m,p < 0.01).However,predation rate also increased significantly after the flood(67% vs.25%,p = 0.030).Conclusions:Our results suggested that Reed Parrotbills demonstrated behavioral plasticity in their nest-site selection.Thus,they appeared to increase the height of their nests in response to the drastically changing water levels in reed wetlands,to reduce the likelihood that their nests would be submerged again by flooding.However,predation rate also increased significantly after the flood,suggesting that the change in nest height to combat the threat of flooding made the nests more susceptible to other threats,such as predation.Animals' response to rare climatic events,such as flooding,may produce ecological traps if they make the animals more susceptible to other kinds of threats they are more likely to continue to encounter.

Egg rejection and egg recognition mechanisms in Oriental Reed Warblers

Background:Nest parasitism by cuckoos(Cuculus spp.)results in enormous reproductive failure and forces hosts to evolve antiparasitic strategies,i.e.,recognition of own eggs and rejection of cuckoo eggs.There are often sexual conflicts between male and female individuals in the expression of antiparasitic behavior due to the differences in reproductive inputs and division of labor.Methods:By adding a foreign egg made of blue soft clay to the host nest during early incubation period in the field,and by removing several host eggs and adding experimental eggs to control the proportion of two egg types in the nest,we examined egg rejection ability,egg recognition mechanism and sexual difference in egg rejection of the Oriental Reed Warbler(Acrocephalus orientalis),one of the major hosts of Common Cuckoos(Cuculus canorus).Results:Our results indicated that Oriental Reed Warblers can recognize and reject nearly 100%(73/75)of the nonmimetic eggs made of blue soft clay,and they could reject foreign eggs with 100%accuracy,regardless of the ratio of experimental eggs and its own eggs in the nest.Furthermore,all cases of egg rejections recorded by videos were only carried out by females.Conclusions:Oriental Reed Warblers have a high egg recognition ability and show a true recognition mechanism.Only female warblers perform egg rejection,suggesting that the sex for host egg incubation seems to play an important role in the evolution of egg recognition mechanisms.

Comparison of head size and bite force in two sister species of parrotbills

Background: Brain size is associated with morphological evolution and behavioral flexibility because animals with large brain size tend to utilize new resources and colonize novel environments more successfully.Therefore, animals with larger brain size should possess larger distribution ranges.Brain size is strongly positively correlated with head size in birds, but also with prey size and vocalizations, because individuals with large heads eat large food items and produce high frequency calls.Methods: To test if there exists an association between head morphology and bite performance, we compared head size and bite force in two sympatric sister species of parrotbills, Ashy-throated Parrotbill(Paradoxornis alphonsianus) and Vinous-throated Parrotbill(P.webbianus), which both originated from the Himalayan area, but differ significantly in their distribution ranges.Results: In Guizhou, southwestern China, the Ashy-throated Parrotbill with a restricted distribution range had smaller heads whilst the Vinous-throated Parrotbill with a large distribution range had larger heads.However, there were no differences in head size between Ashy-throated Parrotbills and allopatric populations of Vinous-throated Parrotbills(Jiangxi and Hebei).Furthermore, the tendency of variation in bite force was opposite to that in head size with populations with larger head size having weaker bite force.Conclusions: We showed that there are no differences in head size between the Ashy-throated Parrotbill and allopatric populations of Vinous-throated Parrotbill, which provides evidence for the hypothesis that differences in head size in the Guizhou populations of two sister species are probably the result of local adaptation rather than species-specific.Our study has implications for avian dispersal and adaption related to head size such as diet ecology and vocalizations.

Comparative urbanization of birds in China and Europe based on birds associated with trees

Urbanization effects on living organisms are spatially heterogeneous.Here we quantified the abundance of birds per tree in forested urban and rural habitats for 85,829 trees mainly in China and Europe.A population model was based on the assumption that:1)birds have a normally distributed habitat preference;2)an increase in population size linked to the habitat preference;3)a population size dependent on the habitat preference;and 4)the removal of a certain fraction of individuals giving rise to extinction.We tested for large-scale differences in the impact of urbanization on the frequency distribution of the difference in abundance between urban and rural habitats in China and parts of Europe.The difference in the frequency distribution of urban population density of birds in trees minus rural population density of birds in trees in China and Europe was statistically significant suggesting that the abundance of birds differed between trees in urban and rural habitats,but more so in China than in Europe.We hypothesize that more pronounced differences in China than in Europe may have arisen due to the Four Pests Campaign in 1958-1962 that resulted in death of hundreds of millions of birds(mainly tree sparrows Passer monfanus,but also numerous other less common species that were starting to become urbanized around 1960).Species that were less common in 1960 could not sustain reductions in population size in urban areas and hence these species are still rare or absent in urban areas today 60 years later.