Enigmatic Sex Ratio in a Nearly Extinct Bird

Pterodroma magentae is the Magenta Petral (also known as the Chatham Island Taiko). There are between 8 and 15 breeding pairs in the New Zealand home range of this species. Indeed, this bird was thought extinct for quite some time before it was rediscovered in 1978.A recent study indicates something funny is going on with sex ratio and mating strategies in this bird, which may, although I’m not quite sure how, lead to improved conservation efforts. From a BirdLife press release:

A study into one of the world’s rarest seabirds provides knowledge that could help avoid extinction. Molecular analysis of the Critically Endangered Magenta Petrel Pterodroma magentae … discovered that 95% of non-breeding adults were male. This suggests that critically low population levels may be causing male birds difficulty in attracting a mate. Their calls are too spread out to attract the infrequent females which pass by. Conservationists are planning to increase the male Magenta Petrel’s pulling power by creating a new breeding colony within a predator-proof fence….Male and female Magenta Petrels look extremely similar, and are difficult to distinguish by sight alone. Scientists collected blood samples from almost the entire known living population over a 20 year period. This allowed the team to distinguish gender accurately using DNA sexing techniques.The sex-ratio of males to females was approximately even in petrel chicks and breeding adults. However, 95% of non-breeding birds were found to be male. This finding suggests that unpaired males may be having difficulty in attracting females to burrows.Conservationists are helping to increase the petrel’s density by focusing birds within the Sweetwater Secure Breeding Site. This is being achieved by translocating chicks, and by using calls to attract adult petrels to the refuge. Eight chicks were successfully moved and fledged last year, and The Chatham Island Taiko Trust was established in 1998 to provide legal status to the continuing work.Scientists are hoping to use knowledge of male behaviour traits to make the plan work. “It has been found in other petrel species – such as Manx Shearwater Puffinus puffinus, Cory’s Shearwater Calonectris diomedea, and Wandering Albatross Diomedea exulans – that males return most frequently to the site where they were reared as a chick”, commented Ben Lascelles BirdLife’s Marine IBA Research Assistant. By using the DNA sexing technique to slightly favour male chicks for translocation, the team hope to increase the numbers of birds returning as adult breeders to the refuge.

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One thought on “Enigmatic Sex Ratio in a Nearly Extinct Bird

  1. Something is odd here. The fact that most of the non-breeders are male would suggest that other males had harems, but on the other hand we are told that the sex ratio of breeding adults is even so this can’t be the case. The sex-ratio of chicks is the same too so it isn’t skewed from the start. The only explanation I can think of is higher female mortality leaving an excess male population unable to find a mate.But then we are told “Their calls are too spread out to attract the infrequent females which pass by”. If there is an excess of males there won’t be enough females for everyone, no matter the population density! The fact that the ratio of unmated adults is as skewed as it is suggest that they are very efficient at pairing up. Almost all females do after all find a mate.

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