Owls are the hardest birds to find. They often roost in dense vegetation or thick pines. Tell-tale signs of owls include, white wash - their excrement  (it actually looks like white paint) and pellets (regurgitated organic materials from their prey such as bone and fur). 

NYBG's Lena and Louis. These owls resided in the garden for almost 25 years. They were notoriously friendly and quite visible delighting birders for years.

Great Horned Owl

NYBG Great Horned Owls - 12/21/13

"Cat Eyes" the new addition to the NYBG owls.

New Owl In Town - NYBG 11/23/2013

Offspring of Junior and Mrs. Robusto- NYBG 9/7/13

Great Horned Owl on Nest - NYBG

Immature Great Horned Owl- NYBG

Immature Great Horned Owl-NYBG

My beloved late Lena and Louis-NYBG

Barred Owl- Pelham Bay Park

Great Horned Owl - "Junior" NYBG

Mother and baby great horned owls, The New York Botanical Garden, 2010

Junior, Great Horned Owl, New York Botanical Garden

Long Eared Owl, New York Botanical Garden

Saw-Whet Owl, Pelham Bay Park

The most requested bird on the Saturday bird walk at The New York Botanical Garden are owls. They are the hardest bird to find. Owls like to hide and finding them can be like looking for a needle in a haystack. 
Clues: Resident owls that establish a roost (a tree limb or nook they are most often found) are a bit easier to locate. 
White Wash on the ground or tree limb.
Pellets (bones, fur, teeth, hair, feathers, tails) are pieces of prey that they cannot digest. So they regurgitate the matter as a pellet. Looking on the ground for pellets will often indicate an owl is roosting above.
Mobbing- Often smaller birds, such as blue jays or crows will harass or mob a roosting owl, thus giving away its hiding spot. 

Owl Pellets

Barn Owl, New York Botanical Garden

Saw-whet owl - The New York Botanical Garden

Burrowing Owls

Burrowing Owl, Ft. Lauderdale, FL

Juvenile Burrowing Owls, Ft. Lauderdale, FL

Juvenile Burrowing Owls, Ft. Lauderdale, FL

Burrowing Owl, Ft. Lauderdale, FL