Simple!!! Someone may have killed it by throwing a stone at it.
me
I'm not sure about the UK, but in 2007 there were about 100,000 birds killed by wind turbines
It killed and endangered many birds.
Assuming that the question refers to a dream in which birds are being killed, the birds represent something in your own life, such as problems that you need to solve. The emotional tone of the dream is important; if the birds represent problems, then there would be feelings of relief as they are "shot down." On the other hand, they might represent ideas that are being "killed" by others, or plans of things you want to do that seem to be "killed" before they are realized.
he spotted 12 birds
Some people are foolish idiots that break the law and like death, yes killing birds is against the law depending on the bird and very few birds can be killed.
they are placed on roadsides and they float there
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The cast of Roadsides and Pillowcases - 2013 includes: Maddison Tebbutt as Worried Friend Natalia Urquiza as Sick Friend
Geese do not have to be killed for pillows. Sometimes the down comes from slaughtered birds. Other manufactures pluck the birds while still alive. This is painful to the bird.
Endangered birds are those birds who are becoming less in quantity in the whole world. Most probably, killed by humans for food etc.
the Black Death killed animals and birds as well as humans.