Coevolution of acoustical communication between obligate avian brood parasites and their hosts

The mutually antagonistic processes producing adaptations and counter-adaptations in avian brood parasites and their hosts provide a model system for the study of coevolution;this topic has long been an area of focus in ornithology and evolutionary biology.Although there is an extensive body of literature dealing with avian brood parasitism,few empirical studies have considered the effects of the coevolutionary processes associated with brood parasitism on the acoustic characteristics of parent–offspring communication.Under the strong selection pressures associated with brood parasitism,parasitic birds may,for instance,produce deceptive songs.The host may in turn evolve the ability to recognize these sounds as deceptive.At present,the mechanisms underlying the different competitive strategies employed by hosts and parasitic birds remain unclear.Here,we reviewed previous studies that investigated acoustic traits in scenarios of brood parasitism,highlighting possible adaptive functions.Using a meta-analysis,we identified no heterogeneity among studies of begging call adaptations in parasitic nestlings.However,our results may have been affected by the small number of applicable papers available for analysis.Our meta-analysis also suggested that studies of acoustic communication and transmission in adult hosts were highly heterogenous,suggesting that research methods were inconsistent among studies.Finally,we identified knowledge gaps and proposed several lines of future research.

Do swallows(Hirundo daurica)use the visual cue of hatchling down-feathers to discriminate parasite alien nestlings?

Egg recognition is a variable but common anti-parasitism defense among different species of birds with brood parasites.In contrast,nestling recognition is rare.Very few studies have found nestling recognition in brood parasite hosts and determined the rejection mechanism behind this behavior.Hosts may use the number of hatchling downfeathers to reject parasite nestlings.We tested whether hatchling down-feathers is a visual cue for the red-rumped swallow,a host that can recognize and reject parasite nestlings.Our results indicated that red-rumped swallows do not recognize foreign nestlings based on hatchling down-feathers.The closed nest structure and hatchling morph may explain the absence of such a mechanism.None of the rejection mechanisms found in previous studies could explain the nestling recognition in swallows.Olfactory cues,tactile cues,or other visual cues,except for single nestling or hatchling down-feathers,may provide nestling recognition in red-rumped swallows.More study is needed to evaluate these possibilities.

Impact of nest sanitation behavior on hosts’egg rejection:an empirical study and meta-analyses

Egg rejection in birds is a specific adaptation toward avian brood parasitism,whereas nest sanitation is a general behavior for cleaning the nest and avoiding predation.However,both behaviors refer to the action of ejecting objects out of the nest,and nest sanitation has been proposed as a preadaptation for egg rejection.Here,we tested the eliciting effect of nest sanitation on egg rejection in the red-whiskered bulbul Pycnonotus jocosus,a potential host species that are sympatric with parasitic cuckoos.We conducted meta-analyses of previous studies on both nest sanitation and egg rejection,in order to evaluate the consistency of our conclusions.Our results showed that nest sanitation did not elicit egg rejection in P.jocosus.The conclusions concerning such an eliciting effect from previous studies were mixed,whereas the methodologies were inconsistent,making the studies unsuitable for comparisons.However,the ejection frequency of nest sanitation was consistently higher than the frequency of egg rejection across different host species or populations.These results suggest that nest sanitation,which is an ancient behavior,is more fundamental than egg rejection,but the effect of the former on the latter is complex and needs further study.Standardized methodologies and the integration of behavior,physiology,and modeling may provide better opportunities to explore the relationship between nest sanitation and egg rejection.

Coevolution between Himalayan cuckoos and 2 sympatric Pycnonotidae hosts

Selection due to cuckoo parasitism is responsible for the evolution of anti-parasitism defenses in hosts.Different host species breeding sympatrically with a single parasitic cuckoo may evolve different strategies to reduce the risk of counter cuckoo parasitism,resulting in different interactions between cuckoos and hosts in areas of sympatry.Here,we studied the coevolutionary interactions between Himalayan cuckoos Cuculus saturatus and 2 sympatric and closely related potential hosts belonging to the family Pycnonotidae,the brown-breasted bulbul Pycnonotus xanthorrhous and the collared finchbill Spizixos semitorques.We investigated parasitism rates and nest-site selection(nest height,nest cover,human disturbance,perch height,forest distance,and degree of concealment)related to parasitism risk,nest defense against a cuckoo dummy,and egg rejection against cuckoo model eggs.Bulbuls used specific nest sites that were further away from forests than those of finchbills,and they behaved more aggressively toward cuckoos than finchbills.In contrast,bulbuls possessed moderate egg rejection ability,whereas the finchbill rejected 100%of cuckoo model eggs.We suggest that selection of a nest site away from forests by the bulbul explains the absence of parasitism by Himalayan cuckoos.We suggest that these interspecific differences in nest-site selection and nest defense indicate alternative responses to selection due to cuckoos.