Reevaluation of dialect boundaries in the Yellowhammer (Emberiza citrinella) based on citizen science data: Should we split or lump?

We are pleased to publish the second special issue on avian brood parasitism and to be responsible guest editors for the two special issues of Chinese Birds (Vol. 3, No. 4, 2012 and Vol. 4, No. 1, 2013), entitled "Avian Brood Parasitism — A Growing Research Area in Behavioral Ecology". The first issue was published in December 2012.

The goal of the two special issues is to publish accumulated knowledge and some of the recent developments in the fascinating research occurring in avian brood parasitism and to stimulate further research in this attractive field. The papers in the two special issues are based on "The 3^rd International Conference on Avian Brood Parasitism" that was held at Hainan Normal University, China, during November 15–19, 2012, where more than 40 scientists on avian brood parasitism met and shared their knowledge. We share some of the discussions from the conference with the readers of Chinese Birds in the two special issues.

In the first issue we showed examples of brood parasite adaptations (Aidala et al., Antonov et al.), host adaptations (Avilés and Parejo, Lee and Jablonski, Samaš et al., Sealy and Underwood), as well as host-parasite interactions in time and space (Møller and Soler, Moskát et al.). While that issue mainly focused on hostcuckoo interactions in Europe, the second issue covers a broader range of host-parasite systems. Hence, the present issue contains four papers dealing with cowbirds from North and South America (Peer et al. a, b, Reboreda et al., Robinson et al.), four papers focusing on cuckoos in Europe (Moksnes et al., Thorogood and Davies), Australia (Langmore) and Asia (Yang et al.), and one paper on the Black-headed Duck (Heteronetta atricapilla) in South America (Lyon and Eadie).

While scientists and conservationists are worried about general decline in Common Cuckoo (Cuculus canorus) populations in Europe, we can in this issue read that Brown-headed Cowbirds (Molothrus ater) in some areas in North America are regarded as pests. Studies focusing on a combination of co-evolutionary interactions, landscape structure, management and conservation are therefore important to shed more light on the possibility that cowbirds are responsible for the decline and extinctions of some of their host species (Peer et al. a, b, Robinson et al.). We can also read an interesting review of anti-parasite defenses in hosts of three South American cowbird species (Reboreda et al.).

The only obligate brood parasitic duck, the Blackheaded Duck, is found in South America. Lyon and Eadie contribute a paper on the life history of this fascinating duck.

Cuckoos are found on most continents, although not all of the species are parasitic. This issue includes papers dealing with a review of adaptations evolved in Australian cuckoos to overcome host defenses (Langmore), egg polymorphism in Common Cuckoos and one of their hosts in Asia (Yang et al.), a review of 30 years of studies on Common Cuckoos and some targeted hosts in Europe (Moksnes et al.), and last but not least the evolution and adaptive value in a co-evolutionary scenario of polymorphism in plumage of adult Common Cuckoos (Thorogood and Davies).

Despite many years of scientific interest in avian brood parasitism, the Brown-headed Cowbird and the Common Cuckoo are still by far the most studied species. However during the last decade we witness an increasing trend in number of published papers on other parasites. Although Africa is a fascinating continent with regard to avian brood parasites, we failed to inspire somebody to write a paper on African brood parasites like cuckoos, honeyguides, indigobirds or whydahs. However, there are several brilliant researchers studying various parasites and hosts on that continent. Hopefully such papers will be published in the forthcoming issues of Chinese Birds.

The conference as well as the two special issues have been devoted to five persons who have been especially important to our field over the last 30–40 years, namely Nicholas B. Davies, Arne Moksnes, Anders Pape Møller, Stephen I. Rothstein, and Spencer G. Sealy. We are pleased to acknowledge their contributions to these two special issues of Chinese birds. Each of them has contributed one or two papers.

We would like to acknowledge Hainan Normal University (HNU), Norwegian University of Science and Technology (NTNU), the Research Council of Norway, China Ornithological Society (COS) and Beijing Forestry University for their roles in initiating the two special issues. We thank many reviewers for their kind revision of one or several of the papers in the two special issues: Peter Adamík, Jesús Avilés, Frode Fossøy, Tomáš Grim, Daniel Hanley, Mark Hauber, Rebecca Kilner, Naomi Langmore, Jin-Won Lee, Bruce Lyon, Anders P. Møller, Csaba Moskát, Juan Carlos Reboreda, Scott K. Robinson, Spencer Sealy, Juan J. Soler, Martin Stevens, Fugo Takasu, Canchao Yang and Ning Wang.

We would furthermore, like to thank Guangmei Zheng, the Editor-in-Chief, Chinese Birds, for making the two special issues possible, and Zhengwang Zhang, Ping Ding, Fumin Lei, Yuehua Sun and Pengjun Cheng, for their valuable support.