A large‐scale survey of bird plumage colour aberrations reveals a collection bias in Internet‐mined photographs

Abstract

Birds with plumage colour aberrations are of interest to both the general public and scientists. However, due to their rarity in nature, information on the presence of colour aberrations is rarely found in the peer‐reviewed literature. Exploration of public observations using modern information technologies such as Internet‐based search engines could facilitate cost‐effective and rapid broad‐scale collection of data on phenotypic aberrations in animals but may also be prone to the same problems as fieldwork, including systematic collection bias. We used Google Images and also asked birdwatchers and ornithologists, via naturalists' forums and social media, to collate a unique dataset of photographs of 936 aberrantly coloured birds of 74 species from Poland. Phylogenetically informed analyses, which included species both with and without reported colour aberrations, revealed that the number of colour aberrations was higher in species with larger populations in Poland, those with larger body size, and those associated with human settlements. Colour aberrations were also more often reported for species with a wider habitat breadth and those which do not migrate over long distances. Habitat openness and diet type were not related to the number of colour aberrations across species in multivariate models. Our study emphasizes not only the power of novel sources to collect large datasets on relatively rare phenotypic aberrations in animals but also the importance of vigilance when using data mined from public sources because the observed patterns may reflect collection bias rather than the nature of the studied phenomena.

Leucistic plumage as a result of progressive greying in a cryptic nocturnal bird

“…Nevertheless, the frequency of leucism in nightjars is close to the ones recorded for nocturnal mammals (bats) 9 . The smaller prevalence reported in most other bird studies could reflect a detection bias resulting from the use of indirect approaches, such as citizen science data and online search tools 21 . But, in any case, the direction of this bias should act to increase, rather than decrease, the actual frequencies of leucism, because rare plumage patterns tend to be overrepresented in non-systematic surveys and image repositories 21 .…”

“…The smaller prevalence reported in most other bird studies could reflect a detection bias resulting from the use of indirect approaches, such as citizen science data and online search tools 21 . But, in any case, the direction of this bias should act to increase, rather than decrease, the actual frequencies of leucism, because rare plumage patterns tend to be overrepresented in non-systematic surveys and image repositories 21 . Our estimates are unlikely to be confounded by such methodological bias, since they are based on the close examination of individuals captured during a systematic field-based survey.…”

“…Leucism in birds is defined as the complete or partial absence of eumelanin and pheomelanin in one or more feathers, but not in the eyes, bill or feet, resulting in white patches in all or part of the plumage 4 . Leucism studies of birds are usually based on observations of unmarked animals 10 , 11 , 20 or information from potentially biased sources, such as online photographs 21 and animals shot for specimens 5 , 18 . Field studies of individually marked birds, although representing a small proportion of the literature, are generally more reliable and usually include information on the age and sex of individuals 22 , 23 .…”

Community-sourced sightings of atypical birds can be used to understand the evolution of plumage color and pattern

“…In contrast, these sightings get a much more muted interest in the scientific literature (e.g., Schreiber et al 2006, Shawkey andHill 2006). The rise in community-driven science has made it possible to study these sightings in large numbers (Izquierdo et al 2018, Zbyryt et al 2021) in a way that is nearly impossible with museum holdings or an individual's own sightings alone. Here, we argue that viewing atypically colored birds through an evolutionary lens represents a unique opportunity to understand the evolution of plumage coloration in birds.…”

United colours of the city: A review about urbanisation impact on animal colours

“…2018; Zbyryt et al . 2020). A study comparing urban and non‐urban areas in countries of Eurasia and Oceania found a higher number of leucistic individuals of the Eurasian blackbird ( Turdus merula ), the house sparrow ( Passer domesticus ) and the common starling ( Sturnus vulgaris ) in urban than in rural areas (Izquierdo et al .…”