Cannabis can germinate from bird seed. It is illegal to grow (Brian Shamblen / publicdomainpictures.net; Kevin Casper / flickr.com).
Guy Barter, RHS Chief Horticulturist, told The Telegraph: "Yes, this is a Cannabis sativa plant. It is illegal to possess/cultivate it in the UK without a special licence. Some legal cultivation is allowed under licence both for pharmaceutical use and, using low cannaboid content cultivars, for seed, oil and fibre.
Her answer read: "It looks like hemp (Cannabis sativa). It is, of course, illegal to grow and probably germinated from bird seed. Although it has obviously thrived in the warm summer, you had better destroy it. It is safe to put on the compost heap but I would advise against a bonfire."
Barry Thorp sent in an image asking for identification of a mysterious plant in his garden and was immediately warned to destroy it as soon as possible. According to The Telegraph, the September 2019 issue of the magazine contains an email from Barry who sent a picture of the plant and asked: "Can you identify this mystery plant from my garden?".
Wild bird seed can cause illegal cannabis plants to grow in your garden, a BBC Gardeners' World expert has warned readers of the programme's magazine.
Male plants will adversely affect the four female plants you have carefully grown. The male plant pollinates the female plant and thus results in the abundance of seeds, which turns your plant into a hermaphrodite. Your wild bird food is perhaps not a good thing for the cannabis farmer.
So although your intentions were good in filling that bird seeder for the different birds in your neighbourhood, your carefully planted cannabis plants have perhaps been compromised by the cannabis seeds in the bird food. The cannabis seeds are not the only seeds that will sprout from the birds discarded the seeds while eating. Some of the seeds that grow from the wild bird seed can produce a hemp plant.
Growing Marijuana- Tips and tricks to maintain a healthy plant
Sterilizing wild bird seed
The microwave method
What seeds are you feeding the birds?