Sexual dichromatism in North Island Robins (Petroica longipes) is weakened by delayed plumage maturation in males and females
Åsa Berggren A B D and Matthew Low A CA Ecology Group, Massey University, Private Bag 11 222, Palmerston North, New Zealand.
B Present address: Department of Entomology, Swedish University of Agricultural Sciences, PO Box 7044, SE-75007, Uppsala, Sweden.
C Present address: Australian Antarctic Division, Southern Ocean Ecosystem Program, 203 Channel Highway, Kingston, Tas. 7050, Australia.
D Corresponding author. Email: Asa.Berggren@entom.slu.se
Emu 106(3) 203-209 https://doi.org/10.1071/MU05057
Submitted: 11 November 2005 Accepted: 8 May 2006 Published: 18 August 2006
Abstract
The North Island Robin (Petroica longipes) is an endemic New Zealand passerine with melanin-based plumage of grey-brown and black, with males typically described as having darker plumage than females. In this study we quantified the relationship of sex and age on plumage colour in 32 North Island Robins on Tiritiri Matangi Island, New Zealand. We objectively scored plumage colour in each individual relative to a colour identity chart created with reference to HTML code. Our results support the general assertion that males are darker than females in this species. However, we found that this is significantly confounded by the age of the bird in both sexes and that this relationship is not a simple dichotomy between first-year breeders and adults. This is the first study to document the existence of delayed plumage maturation in the female of this species, and that plumage changes occur over many years in both sexes. The plumage of most Robins lies within the colour-range overlap between the sexes, indicating that plumage colour is not a reliable criterion for sexing this species. We used a classification and regression tree (CART) analysis to determine which morphometric measures were the best classifiers of gender in birds within the colour-range overlap; a tarsus measure cut-off of ~36 mm accurately predicted gender in >80% of cases. We propose that delayed plumage maturation in this species results from social and physiological factors during moult, which affect melanin production and the colouring of the feathers.
Acknowledgments
We thank Barbara Walter, Ray Walter and Ian Price – the Department of Conservation rangers on Tiritiri Matangi Island – for help with logistics and practical issues. Thanks also to Mike Joy for help with analyses, and Askia Wittern for Robin monitoring during the 2003–04 breeding season. The Supporters of Tiritiri Matangi Island and the Department of Conservation made this study possible by allowing us access to the facilities on Tiritiri Matangi. We also thank Doug Armstrong for providing catching equipment and banding data before the 2000–01 breeding season. This study was undertaken with permission from the New Zealand Department of Conservation and ethics approval from Massey University.
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