Three plot sizes (36, 78, and 108 m²) were evaluated for testing blackbird seed repellents within a 0.2-ha flight pen. Groups of 12 to 20 red-winged blackbirds were observed as they foraged on pairs of plots within each size category. One plot from each pair was sown with 134 kg/ha of rice seed treated with 1.25-g methiocarb/kg of seed. Plot selection, avoidance, discrimination, and latency to discrimination were determined for each plot size. With the 36-m² plots, there were long delays in selecting untreated plots, treated plots were not avoided, the birds had difficulty discriminating between the plots, and results were inconsistent among replicates. The 78-m² plots were adequate for most purposes, but the clearest and most consistent results were from the 108-m² plots. These plots were readily found by the birds, were accurately discriminated, and had bird pressure similar to that observed in Louisiana rice fields.