Sunday, June 11, 2017

OWLETS In a Box and Another Occupant

Ken was able to check the owlet box this week; the chicks look healthy and mom is still roosting in the box with them. Both owlets are losing their down feathers and looking more and more like adults as their contour feathers grow. They were more curious about the camera then mom, who has the adult wisdom to avoid predators, lie flat and be still.

This owlet in the box is still pretty downy.
This owlet is slightly older, showing more contour feathers

The nestling phase is about 35 days so they should fledge between the 3rd week and end of June. We were hoping to put up some cameras around the nest box to watch the parents deliver food and the chicks fledge, but the supporting trees were too far away. We'll have to come up with other ideas.

Meanwhile, last week we found another occupant in nest box #2. We have deduced, based on the list of cavity nesting birds in the area, that it was an ash-throated flycatcher egg. Since there was only one egg and it had ants on it, we determined it was abandoned. This weeks' box check confirmed it, no new eggs, more ants.  Here's how Cornell Lab of Ornithology describes the egg, "creamy white with reddish brown streaks and elongated blotches." Click this link for more information about the ash-throated flycatcher. 

Monday, June 5, 2017

Western Screech OWLET UPDATE and Photo Essay!

#1! Lots of down but some contour feathers appearing
#2 of the two owlets
It was time to band the owlets after peeking into the box last week, I determined they were old enough, ~ 3 weeks to 1 month old and getting ready to "fledge." Typically owls leave the nest cavity and are called "branchers", because that's what they do, sometimes falling clumsily to the ground, climb up a tree, and perch on a branch. Their growing flight feathers are heavy with blood and they beg for food and begin to persistently follow their parents around before they are capable of even moderate flight. I wondered about this being a successful strategy because it seems safer remaining in a protected cavity. But after seeing and smelling the remains of this nest box, I think I figured out why it's best to get out of there (see below!)
At least 3 prey remains in this nestbox
Here is more evidence of the male being a good provider with three different prey remains that might be a deer mouse, vole or gopher and the tiny remains of a pocket mouse or other tiny rodent.

Ken climbs the ladder and hands me the mom for
weighing and checking the band we put on last week.
You can see the plug in the nest hole.
We begin with Ken removing the female from the box (the nest hole is plugged to ensure she doesn't flush out of the nest) and handing her to me. She is held in a bag until the nestlings are banded. As typical for screech owls, she is very calm through this process.The female has a lot invested with two owlets, so would unlikely abandon a nest at this stage. Both owlets were plump at 135-145 grams (the same weight as some adult males).  We banded and weighed them separately, so owlet #1 remained in the bag quietly waiting for the sibling.The mom was also well-fed, weighing in at 190 grams!
One owlet banded and held in a cloth bag until the second owlet is processed
The owlets are returned to the nest box, then mom is returned. We wait about five minutes to remove the plug before leaving the owls safely in their box. I have no doubt the male is roosting somewhere nearby, but we were unable to find him.

Saturday, May 27, 2017


Today Ken and I checked the boxes on the Reserve. We were particularly interested in Nest Box #15 (sort of like an address) where we discovered a nesting Western Screech Owl, 37 days ago in April. Today May 27, we saw a mom owl with her two owlets and a nice meal for the chicks, a deer mouse (peromyscus sp.)

A view from above: 3 screech owls and breakfast. Note the larger owlet on the left.
Owl eggs hatch asynchronously, so one will be a little older than the next.
Ken removed the female and lowered her down to me for banding. She remained still and calm through the process.We returned her to the box after taking some measurements and examining her condition.
Here I am taking the wing chord measurement of the owl.
Note the band on her right leg.

Then I looked at her brood patch. Hormones help release breast feathers providing direct heat exchange for incubating eggs and brooding young,until owlets can thermoregulate on their own. This female's brood patch is already getting pin feathers (two dark lines under her bare skin), because the owlet's feathers now keep them warm. By fall her breast will be feathered, which will help keep her warm for the winter months.

The brood patch, well past its use for warming chicks. Because the
chicks now have feathers they no longer need warming by the adult.
Leftover prey shows the male is a good provider. He was likely roosting in a cavity nearby, and maybe observing us. Stay tuned for the rest of the story!
A view with the mom removed. The owlets are about the size
of an adult deer mouse (right corner), and too small to band.
The Cornell Lab of ornithology is a great resource for bird natural history including screech owls. Also click here for a nice overview of owl courtship and reproductive behavior.

Friday, April 21, 2017


Western Screech Owl inside a nest box
Can you see the the owl in the box?  You are looking at the head and back of a Western Screech Owl inside nest box # 15. We believe she is incubating eggs due to her flattened posture and absolute stillness when we peeked the camera into the box. If she is incubating and all goes well, we can expect to see 2-7 nestlings within the next month.

So how did we see her? This winter Ken and crew installed nest boxes to determine potential use of the BCCER as breeding site for Northern Saw-whet owls, a cavity nesting species. To check the nests Ken attached a flexible cable with tiny camera and LED light to a bamboo pole, plugged in to his tablet. He can feed the camera quietly into the cavity entrance, see what is inside on his screen and then take a photo. We have conducted two box checks,

Ken's nest-cam set up. The camera is at the end of the pole
inside box #4. He is viewing the inside of the box on his tablet.

one on March 11 and one on April 19. In March, we found bird feces in two two boxes but no birds were present. We guessed the boxes were used by Northern Flickers roosting in the boxes at night. In April, we found identifying flicker feathers in the same boxes, and then we found the screech owl in a third box, What a fun surprise! Learn more about Western Screech Owls here.

Monday, March 6, 2017

SPRING MIGRATION, what do we know?

As with many of our questions, the answer is.....more study is needed! According to Project Owlnet, spring migration is poorly studied. And so, Ken and the owl monitoring team are making winter/spring efforts more standardized to identify migration timing on the Reserve. So far this year we have seen some new activity near the end of February with 10 new owls so far! Here's what it looks like:
February 25, 2 new owls
February 27, 5 new owls and 1 recap
February 28, 1 recap
March 2, 3 new owls
March 3, 0 owls
Bander Erika Iacona with one of the February 27 owls.
Is this a sign of spring migration? Possible, BUT, this year has been unusual, relative to the past 5 drought years. We have had record rainfalls and snow levels in the Sierra, which may hinder birds from moving to their breeding sites too early.So, we will continue our spring efforts to  enlighten our migration question this year. WAIT FOR IT!

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