Glasses raised, supporters toast Margaret Birch, the provincial secretary for social development, at victory party last night at Maple Creek Farm. Mrs. Birch defeated Liberal John Coates by 10,565 votes, the largest majority in the province, in a race that pundits declared was bitter and would be close. Mrs. Birch, who had been active in social planning and mental health work in the borough, defeated the Liberal incumbent, Tim Reid, by 670 votes in 1971.