I've done the same sort of analysis here in Hong Kong and get exactly the opposite result - the colder the spring, the earlier the migrants.
This diagram compares the average earliest arrival date for 35 species over the period 1994 to 2010 with the average temperature in the months of March and April for that year - each dot shows one year's data and you can see that as the average temperature drops, the average earliest arrival date tends to get earlier.
This relationship still applies if the species are restricted to just passerines or even just flycatchers.
It's not to say the American study is wrong, or that Hong Kong is not affected by global warming. It's just that in Hong Kong, our migration seasons and especially spring are very much affected by weather - more cold fronts = more and earlier spring migrants, as well as colder weather.
This would make it very difficult to find a 'global warming' effect in Hong Kong data.
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Last edited by wgeoff at 24/02/2012 13:04 ]
