27th winter bird-population study. Ryder, R. 1974.
bibtex   
@article{ryder_27th_1974,
	title = {27th winter bird-population study},
	volume = {28},
	number = {3},
	author = {Ryder, RA},
	year = {1974},
	keywords = {SGS, bird populations, winter}
}
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