You face two legal perils when you use a firearm against a wild animal:
- Most wild animals are protected or regulated as game by state and/or federal law.
- Unnecessarily discharging a firearm is forbidden in many jurisdictions.
With respect to both charges, self defense is almost always a justification (assuming the possession of the weapon used was lawful).
The specifics vary a little by jurisdiction, but this Utah rule is pretty typical:
R657-63-3. Self Defense.
(1) A person is legally justified in killing or
seriously injuring a threatening wild animal when the person
reasonably believes such action is necessary to protect them self,
another person, or a domestic animal against an imminent attack by the
wild animal that will likely result in severe bodily injury or death
to the victim.
(2) In determining imminence or reasonableness under
Subsection (1), the trier of fact may consider, but is not limited to,
any of the following factors:
(a) the nature of the danger;
(b) the immediacy of the danger;
(c) the probability that the threatening wild animal will attack;
(d) the probability that the attack will result in death or serious bodily injury;
(e) the ability to safely retreat;
(f) the fault of the person in creating the encounter; and
(g) any previous pattern of aggressive or threatening behavior by the
individual wild animal which was known to the person claiming self
(3)(a) A person shall notify the division within 12 hours
after killing or wounding a wild animal under Subsection (1). (b) No
wild animal killed pursuant to Subsection (1) or the parts thereof may
be removed from the site, repositioned, retained, sold, or transferred
without written authorization from the division.
(4)(a) A person is not legally justified in killing or seriously injuring a threatening
wild animal under the circumstances specified in Subsection (1) if the
(i) has the ability to safely retreat from the threatening
animal and fails to do so, except when the animal enters a home, tent,
camper, or other permanent or temporary living structure occupied at
the time by the person or another person; or
knowingly, or recklessly provokes or attracts the wild animal into a
situation in which it is probable it will threaten the person, another
person, or a domestic animal.
Federal law is a little more terse: The Endangered Species Act includes the following:
Notwithstanding any other provision of this Act, no civil penalty
shall be imposed if it can be shown by a preponderance of the evidence
that the defendant committed an act based on a good faith belief that
he was acting to protect himself or herself, a member of his or her
family, or any other individual from bodily harm, from any endangered
or threatened species.
Defense of property
One can be fined for killing threatened or endangered animals in defense of property or livestock (see, for example, Christy v. Hodel).
These instructions from the Missouri Department of Conservation are typical:
If wildlife is damaging your property, you ... may shoot
or trap most damage-causing wildlife out of season and without a
permit to prevent further damage. Note: Wildlife you may not shoot or
trap under this provision are migratory birds, white-tailed deer, mule
deer, elk, turkeys, black bears, mountain lions, and any endangered
species. For conflicts with these species, contact your local county
conservation agent or nearest Department office. Control action may be
taken only on your property. Wildlife you take under this provision
may not be used in any way, and you must report it to the Department
within 24 hours, then dispose of it in accordance with Department
instructions. Check with local city or county authorities regarding
the use of traps and firearms in local jurisdictions.