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Citizen science projects invite members of the public to gather or classify data for professional researchers, often through smartphone apps or simple online tools. Early sceptics questioned whether untrained volunteers could produce reliable observations, but large-scale studies comparing amateur and expert bird counts found close agreement once obvious errors were filtered out statistically. The real advantage of these projects lies not in individual accuracy but in scale: a single ecologist might survey a few dozen sites in a season, whereas thousands of volunteers can cover an entire continent simultaneously. This breadth has allowed researchers to detect shifts in species migration timing that would otherwise have required decades of dedicated fieldwork to notice.