There's state law which has a prohibition against possessing wildlife found dead (below). Wildlife, though, has an exclusion for the family Muridae, Old World rats and mice, which might apply to the parts likely to be found in an urban owl pellet. Last I knew, the family Muridae included much more than the old world rodents that have been naturalized here. I included many of ournative rodents. If that's still the case, I'm not sure if the modifier, "Old World rats and mice," provides an adequate distinction between the introduced species that are exempted here and the native species.

Kelly McAllister

72) "Wildlife" means all species of the animal kingdom whose members exist in Washington in a wild state. This includes but is not limited to mammals, birds, reptiles, amphibians, fish, and invertebrates. The term "wildlife" does not include feral domestic mammals, old world rats and mice of the family Muridae of the order Rodentia, or those fish, shellfish, and marine invertebrates classified as food fish or shellfish by the director. The term "wildlife" includes all stages of development and the bodily parts of wildlife members.

WAC 220-400-040 Possession of dead wildlife. (1) Except as authorized by permit of the director or as otherwise provided by this
section, it is unlawful to possess wildlife found dead. This rule does
not prohibit the possession of naturally shed antlers of deer, elk, or
moose.
(2) Except as provided in subsection (4) of this section, an individual may remove and dispose of wildlife found dead on his or her
property or an adjoining public roadway. Before removing the wildlife,
the individual shall, by telephone, notify the department or the Washington state patrol communications office, and shall provide his or
her name, address, telephone number, and the description and location
of the wildlife. The individual may remove the wildlife for disposal
only, and may not retain the wildlife for personal use or consumption.
Other laws and rules may apply to the disposal, including rules of the
department of health (WAC 246-203-121). Wildlife removed under this
subsection remain the property of the state.
(3) Possession of wildlife in violation of subsections (1) and
(2) is punishable under RCW 77.15.130, 77.15.400, 77.15.410, and
77.15.430 depending on the species involved in the violation.
(4) It is permissible to salvage and transport a deer or elk that
is accidentally killed by a motor vehicle collision except for any
deer killed by a motor vehicle collision in Clark, Cowlitz, and Wahkiakum counties.
(a) Big game licenses and tags cannot be used for the purpose of
salvaging motor vehicle-killed deer or elk.
(b) A salvage permit must be obtained from the department within
24 hours of taking possession of the animal. Permits may be obtained
on the department's web site or at department regional offices.
(c) The entire carcass, including entrails, of the animal must be
removed from the road right of way.
(d) Any meat an individual deems unfit for human consumption or
unusable animal parts must be disposed of pursuant to WAC 246-203-121.
Individuals salvaging and consuming this meat do so at their own risk.
The department makes no guarantee as to the fitness for consumption of
deer or elk collected under a salvage permit.
(e) An individual may not kill an injured or wounded animal that
they encounter for the purpose of salvage. Only a law enforcement officer or individuals or entities authorized by the department may euthanize an animal injured in a motor vehicle collision and that deer
or elk may be taken for salvage.
(5) Possession of wildlife in violation of subsection (4) of this
section is punishable under RCW 77.15.750.