All parts of the plant can be poisonous, but the total amount of poisonous alkaloids varies significantly with the age of the plant and between parts. The roots of young, first-year plants contain only traces of poison or none at all. The leaves of seedlings have lower levels of poison than slightly older plants. In the spring of the second year the leaves are highly toxic, though not as poisonous as the flowers or seeds later in the season. The hollow stems remain deadly for up to three years after the plant has died. However, drying causes the plant to lose a large part of its toxic compounds. Plants which grew in sunny conditions can be twice as poisonous as plants that grew in wet and cloudy conditions. The main toxic alkaloids are coniine and γ-coniceine, also called gamma-coniceine. Intoxication is reported in diverse groups of animals including pigs, bovids, turkeys, equines, rabbits, cats, and dogs. However, songbirds are less sensitive and have reportedly become toxic from absorbing the coniine from hemlock. Ingesting more than 150–300 milligrams of coniine, approximately equivalent to six to eight hemlock leaves, can be fatal for adult humans. Grazing animals are most likely to be poisoned in the spring when other forage is unavailable. However, they may also be poisoned when hemlock has become mixed into grain, hay, or silage. Poison hemlock ingestion typically induces intense and fatal seizures in cattle. Even very small quantities have been shown to cause birth defects when ingested within 40–100 days of gestation. Hemlock was one of many plants suggested by medieval writers as a possible cause of coturnism, a disease caused by eating common quail in certain seasons.