28th winter bird-population study. Ryder, R. 1975.
bibtex   
@article{ryder_28th_1975,
	title = {28th winter bird-population study},
	volume = {29},
	number = {3},
	author = {Ryder, RA},
	year = {1975},
	keywords = {SGS, bird populations, winter}
}

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