这虽证明是学习北美鸟的一个有效的方法，但他仍要把他的未成熟的照片同他继续欣赏的那些出版的照片相比。
While this proved an effective method for learning about North American birds, he couldn't help comparing his rudimentary images to the published photos he continued to admire.
例如，四趾的始祖鸟先于三趾的始祖鸟出现，但北美的化石证据表明，这两种动物之间有一个急促而不均衡的转换过程。
The four-toed Eohippus preceded the three-toed Miohippus, for example, but North American fossil evidence suggests a jerky, uneven transition between the two.
最后一次在北美大陆上对象牙嘴啄木鸟无争议的观测是在1944年的美国路易斯安那州。
The last undisputed report of an ivory-billed woodpecker in continental North America was in Louisiana in 1944.
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