Date: 7/7/18 10:56 am
From: Jared Del Rosso <jared.delrosso...>
Subject: [cobirds] Nesting Cordies, But... - Cenntennial (Arapahoe)

For the better part of a month, Cynthia Madsen, Mary O’Connor, and I have
been dutifully watching a Cordilleran Flycatcher pair apparently nesting
along the Little Dry Creek in Centennial, CO (Arapahoe, approx. 5475’). Our
visits weren’t daily, but they were nearly so, and we’ve been rewarded with
the opportunity to observe the pair’s behavior as the apparent nest went

I say apparent because we never found the nest, which we think / thought
was under a bridge over the creek. We inferred from changes in behavior –
the male ceasing to sing regularly, the female remaining out of sight most
of the time, the female reappearing to make food runs, those food runs
increasing in frequency – that nesting was occurring. The timeline of these
shifts in behavior roughly corresponded to the timeline we calculated based
on Atlas data about Cordy nesting.

There was definitely a nest, we now know. Today, when I arrived at the
nesting location, I was thrilled to hear some apparent begging calls from
near the bridge. The pair continually visited the location with food. And
then, from some movement near the apparent parents, a fledgling emerged:

We’d worried about cowbirds but hoped that the likely location of the nest
– under the bridge – would protect against that. It’s possible that the
nest, in fact, was near but not under the bridge – or that it was under a
sort of “eave” around the bridge. Or it’s just possible that a female
cowbird found the nest under the bridge.

A small, not-quite-consolation: the Cordy feeding behavior was interesting
today. Previously, they had been flycatching in a Peachleaf Willow and
downstream from the bridge, out of view. Today, they did much more
flycatching nearer the bridge, where the fledgling cowbird was.
Specifically, I saw several efforts to nab insects from the ground.

Here’s the timeline of our observations, keeping in mind we didn’t make
daily visits, nor were our visits always at the same time of day. And some
were relatively brief (20-30 minutes).

5/24 – Male present

6/6 – Female arrives

6/7 – Female carrying nesting material

6/23 (or so) – Male singing slows

6/27 – Female carrying food – one run observed; not apparent if female ate
or fed food.

6/28 – Female leaving nest more frequently on food runs

7/7 – Cowbird fledgling near bridge, fed by Cordy pair (definitely by
female, possibly by male).

Not the outcome I was hoping for, but so it goes.

- Jared Del Rosso

Centennial, CO

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