Thursday, February 16, 2017


We know based on aural and/or visual detections that we have at least five species of owls on the Reserve; great horned owl, Northern pygmy owl, Northern saw-whet owl (NSWO, fall through spring?), California spotted owl (winter only), and Western screech owl (WESO). The screech owls are year-round residents and will sometimes come to investigate our NSWO caller, so they get caught in the nets. We have banded 25 WESO over 12 years of monitoring NSWO. Sometimes we are lucky to see them roosting in a cavity during the day, or perching at night. Banders Erika and Ken saw this owl perching in the barn, likely
Western screech owl in the barn at the Reserve
Possibly foraging on bats? He/she appears a little
perturbed at the disruption!
taking advantage of barn mammals. It may have been foraging on the bats that roost there! There happens to be a colony of wintering free-tailed bats, and potentially other bat species, living in the barn. I have seen great horned owls, peregrines, merlins, kestrels and red-tailed hawks capturing bats as they exit their roosts. It was quite an exciting flight dynamic as the predator bursts in a cloud of bats.  The success rate on capturing bats seemed pretty low. What we need to determine if this WESO has a palate for bats, is to find some pellets to identify prey remains.

Wednesday, February 15, 2017

A Winter Owl's Meal

Ken and team have continued monitoring owls through the winter and have found that three individuals have remained on site since fall. Three others were newly banded this winter and recaptured. It appears the wintering population of saw-whets on the Reserve is pretty good-which indicates that the prey base it sound. And proof is in the owl mass- all have increased since their first banding. Below is an example of this female's weight history, captured and weighed three times.
one wintering NSWO captured and weighed 3 times
01Jan2017 =  90.5 gr
02Feb2017 =  95.6 gr
11Feb2017 = 101.7 gr

NSWO are primarily rodent predators, but are also opportunistic, taking advantage of what's available (birds, insects, etc).From previous pellet analysis conducted by Julie Shaw (see earlier posts in 2010-2013) and visual observations we know Reserve owls eat field mice, deer mice and California voles. One owl even ate rain beetles! Saw-whets will often make two meals out of one prey item and roost with it or cache it for the next meal.

A great web photo of a NSWO with deer mouse prey