[cobirds] Re: Nesting Cordies, But... - Cenntennial (Arapahoe)

It's tragic but it's also exciting to see with your own eyes what one normally just reads about in books!  

Here's a study (2004) showing cowbirds don't push out host eggs because the begging noise of the larger group of nestlings elicits more feeding by the parent:

https://www.berkeley.edu/news/media/releases/2004/08/05_cowbird.shtml

Dr. Alex Cruz (who recently passed away as noted in these forums) was an expert on brood parasitism - apparently studied cowbird parasitism in Boulder County a while back - I found an old manuscript here: https://www-static.bouldercolorado.gov/docs/4260_Cruz_Alexander_Long-1-201307091313.pdf

Apparently hosts sometimes do successfully raise young in addition to the cowbird chick, though obviously would be better off without the cowbird, which outcompetes the host chicks as was mentioned.

On Saturday, July 7, 2018 at 11:56:35 AM UTC-6, Jared Del Rosso wrote:

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 along.

 

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: cowbird.

 

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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