Willingness to pay for bird conservation

There is a large literature on the willingness to pay for non-market goods, such as security, bio-diversity or pollution. The typical way to elicit these amounts is by survey, asking people what they would be willing to pay, as opposed to what the community should be paying.

Pamela Kaval and Matthew Roskruge do such a study for the preservation of birdlife in the Waikato region of New Zealand. A few striking results: everyone is willing to pay, older people want to pay more, and native Maoris less. Kaval and Roskruge conclude that there is willingness to tax more for bird preservation.

Why would everyone be willing to pay? I suspect there is a selection boas in that only those who care about birdlife accept to participate in the phone interview. I certainly only pursue phone interviews I care about. Why would older people want to pay more? They would have fewer expected years to benefit from it. Is it because birds were more numerous in their childhood and nostalgia matters more? And the Maori? Aren't they supposed to care more about nature?