Study species and sample collection
Tinkerbirds are African barbets (Lybiidae) that while omnivorous, feed primarily on fruit, and have a particular preference for mistletoe (Godschalk, 1985; Dowsett-Lemaire, 1988). Species in the genus differ subtly in plumage characters in spite of millions of years of divergence in mitochondrial DNA (mtDNA) between currently recognised sister species (Kirschel et al., 2009; Kirschel et al., 2018; Nwankwo et al., 2018; Nwankwo et al., 2019). P. pusillus pusillus (hereafterpusillus ) and P. chrysoconus extoni (Layard, 1871) (hereafter extoni ) are the subspecies of red-fronted and yellow-fronted tinkerbird, respectively, that meet at a contact zone in southern Africa (Monadjem et al., 1994). They differ in plumage characters beyond the forecrown colour in the yellower supercilium and dorsal streaking in pusillus – whiter in extoni , the deeper amber and greater percentage of carotenoid pigment over melanin pigment on the wing coverts in pusillus – paler yellow and more black in extoni (Fig. 1). The two species might also differ more subtly in underpart coloration, such as in throat and breast colour (Hockey et al., 2005).
Fieldwork was performed at 16 sites across the contact zone where subspecies extoni and pusillus meet in southern Africa. Fieldwork took place in Tanzania June – September 2013, and in South Africa and Eswatini in March and December 2015, and January – February 2017. Methods used to obtain DNA and feather samples have been previously described (Nwankwo et al., 2019). Briefly, 79 birds were caught in mist nets and ringed, of which 37 were from within the contact zone. Blood samples were obtained via venepuncture of the brachial vein, and crown feathers were plucked and placed in envelopes for spectral reflectance analysis in the lab. Photographs of 78 birds were taken from several angles using either a DSLR Camera or smartphone for subsequent plumage scoring. Fieldwork was performed with ringing permits obtained from SAFRING, Eswatini’s Big Game Parks, Mpumalanga Tourism and Parks Agency, the Game and Fauna Service of Cyprus, and with research and collecting permits from the respective authorities (see Acknowledgements).