The evolution of masturbation in birds

The few studies examining masturbation in non-human animals have used an array of definitions to describe masturbation, and these are often geared towards the study organism. We define masturbation as sexual stimulation of the genitals by self-manipulation. This definition includes individuals using inanimate object(s). Reports of masturbation in birds are most commonly recorded from non-experts using a variety of platforms. As such, we used several approaches to determine how widespread instances of masturbation were documented across avian taxa.<i>Literature review</i>We first performed a literature review to compile published and peer-reviewed accounts of masturbation across avian taxa. We performed electronic searches of the literature in line with the methodology from Mengist, Soromessa and Legese (2019). The search terms used were: “masturbation”, “solo-sex”, “bird”, “mating+behaviour”, “mating+system”. After identifying articles of interest using our search terms, all articles within the final library were read in full and included in the data collection if they met the final criteria and documented or discussed instances of masturbation in avian taxa.<br><i>Avian experts</i>We produced a questionnaire as a tool to survey avian experts in order to quantify how widespread masturbation is across avian taxa. We surveyed experts from academia who either currently or previously conduct research in birds, as well as other professionals and practitioners such as zookeepers. In association with the British and Irish Association of Zoos and Aquariums (BIAZA), this questionnaire was disseminated to all Zoos registered within this organisation. This approach was selected as bird keepers have extensive experience of working in close quarters with their study organisms and could therefore confidently state whether or not they had observed masturbation in their study species. As such, we requested data only for species that the respondent felt they had substantial experience and time observing. Equal attention was paid to respondents who stated that masturbation had never been observed in their study species, however this behaviour was classed as present in species for which a single confirmation of masturbation has been recorded. A species was classed as non-masturbatory if the respondent had never observed this behaviour before and there were no submitted responses to say the contrary.<br><i>Online reports and communities</i>There is a stark disparity between the number of published accounts of masturbation in avian species, in comparison to its widespread documentation from bird keepers, breeders and enthusiasts, particularly within online communities. As such, a vital component of this research was to integrate the extensive knowledge from these individuals and groups with published accounts, to determine how widespread this behaviour is observed across avian taxa. We therefore used a citizen science approach to survey online communities of bird keepers, breeders and enthusiasts for evidence of this behaviour, using the same questionnaire that was sent to avian experts. Further, we extended this methodology to conduct a comprehensive review of documented accounts of masturbation in avian taxa across online communities. We reviewed forums, social media sites (Facebook) and the online video sharing platform, Youtube. For all online data collection accounts, we had set inclusion criteria that posts had to reach in order to be included in the data collection. The minimum requirements were species, sex (either clearly stated or apparent through photographic evidence, such as in sexually dimorphic species), and an accurate written, photographic or video description of the individual(s) masturbating. Any posts that did not reach these minimum requirements were not included in the data collection or any subsequent analysis.A database was created that collated information from all sources regarding this behaviour across avian taxa. We included information detailing species, age (adult/juvenile), sex (male/female), origin (wild/captive), species mating system, social environment and condition (good/bad). For captive species we also recorded the rearing origin which we classed as either parent or hand reared (using human intervention). Social environment was used to record if individuals were housed with others of the same/different sex and the same/different species, and how many were in the group housing situation. Mating system information was gathered from the scientific literature and referenced accordingly.<b>Description:</b> A comprehensive excel spreadsheet with two tabs containing relevant data concerning the presence/absence of masturbation in birds.VariablesOrderSpecies latin and common namesIndication of whether masturbation has been observed in this species beforeIndication of confidence of respondentSex of birdNumber of individuals observed masturbatingWild or captive originGroup sizeHousing conditions with list of four potential optionsRearing origin (hand or parent reared)Age (adult or juvenile)Condition (good or bad)Any applicable notesReliability of data (scored 1-4 with being highest score)There is a second tab on the excel spreadsheet which details the breeding system of birds that were reported in our data.