Re: [cobirds] Denver City Park Birds

This change since last year won't account for all the change but Lake Mary at the Arsenal has had 50-75 DCCO's the last few weeks.  I can see at least 3 nests on the big tree in the middle of the lake but I am sure there are more.  I do not recall these numbers from the prior years.  Also 3 GBHEs my last visits and 10 or more Snowy Egrets during last two visits.  I don't remember if that is above normal.  

I am not going to draw any conclusions but would lean towards food source (or maybe elbow room)😂😂😂

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From: cobirds@googlegroups.com <cobirds@googlegroups.com> on behalf of Patrick O'Driscoll <patodrisk@gmail.com>
Sent: Thursday, June 23, 2022 11:56 AM
To: ROBERT G DEBOER <BOBOVETADB@msn.com>
Cc: cobirds@googlegroups.com <cobirds@googlegroups.com>
Subject: Re: [cobirds] Denver City Park Birds

 

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Hi Oveta -- 

This will take some explaining . . . 

The disappearances of these species all have taken place in June -- but 3 years apart.

The egret and heron colonies at Ferril Lake emptied out in 2019 and have never re-established.

There's a theory about that, which I'll get to below.

What you've seen this month at Duck Lake, with the cormorants, is something else.

If birds are dying, they're not dying in City Park. No signs of (or reason for) hazing, either.

The Double-crested Cormorants did begin to dwindle rather suddenly a couple of weeks back.

I have no reason why -- and I've been tracking all three colonial waterbird species at City Park for the past 6 years.

This year's cormorants returned suddenly, too, but rather late.

After fewer than 10 forerunners showed up at Duck Lake on and off in mid-February, it was quiet until nearly 100 birds materialized at the rookery island the first couple of days in March.

This year's cormorant population at Duck Lake appeared to have peaked in May at roughly 500 adult birds, with at least one bird on each of 240+ nests.

A little early and a little below average, but not alarming -- or so I thought.

But after the first 20 or so broods of young were seen begging and being fed that month, things seemed to pause.

Lots of birds still hung around. Even a few still performed mating displays on nests.

Adults were still going and coming with nesting material and food -- and scores were sitting on nests, presumably incubating clutches of eggs.

But the total counts were slipping below 400 cormorants when they should've been rising to 600, even 700 total birds by mid-summer peak.

Fewer new broods of young were coming on, too.

The usually loud chorus of chicks screeching for food seemed thin and muted. (Hatching normally runs through June and into July.)

By late May, the first 50-75 fledglings were perching around and swimming in Duck Lake.

And that turned out to be this year's peak. Many of the nests never produced young.

Total counts drifted below 400, then down to about 300.

On June 8, I counted 301 cormorants, adults and young.

Eight days later, June 16, there were only 65.

On June 19, just 44.

It was baffling. My mid-June average count for the years 2017-21 was 515 cormorants -- adult and juvenile, nestlings and fledglings. As high as 600+ and as low as 450.

Why the sudden desertion? I have no idea.

Dwindling food resources?

Some cormorants nominally fish Duck and Ferril Lakes, but most fly off to other more reliable sources like the South Platte and larger reservoirs.

Disease? Again, no bodies lying around, and no unusual behavior, lethargy, etc.

Meanwhile . . . about the Black-crowned Night-Herons and Snowy Egrets:

We had only a handful of herons in the park this spring, all at Duck Lake, typically no more than 4-5 birds.

But there was zero mating or nesting, unlike in the past 3 years, when a few nests in the lower branches of the Duck Lake cormorant rookery did produce a few young herons.

Snowy Egret?

Only the usual 1 or 2 hanging around fishing at Duck Lake (and sometimes Ferril Lake), but no nesting.

Snowys haven't nested in City Park since 2019.

The Ferril Lake vanishing in 2019 began sometime the first week of June.

On June 3, 2019, I counted about 60 herons (occupying 25 + nests) and 20-25 egrets (at about 10 nests) on the Ferril island.

No young were yet present.  The herons and egrets arrived and nested later than the cormorants.

Then, I was away for 2 weeks -- and when I returned on June 17, I did a double take.

There were only 12 herons and 6 egrets.

A month later, I counted 6 herons (with single chicks in 2 nests) and 2 egrets.

By August, I found 4 fledged herons (2 adults still present) and a single fledged egret (the last of a handful).

Why the desertion?

Some immediately thought it had something to do with that summer's controversial Denver Parks & Rec "roundup" of year-round Canada Geese in several Denver parks, including City Park.

But they didn't round up geese in City Park until early the morning of July 1, after all but a handful of herons and egrets had left.

Any human commotion nearby shouldn't have caused the colonies never to return.

And a handful of birds HAD stuck it out and produced a few young.

I puzzled about this a long time before having a conversation with Garth Spellman, head of ornithology at the Denver Museum of Nature & Science.

He thinks the Ferril Lake clearing out may have had something to do with the city's practice of oiling goose eggs in nests on the island (another way to keep the year-round goose population down; the oil deprives the eggs of oxygen needed for fetal geese to grow and hatch).

He suggested that perhaps the herons and egrets were repelled by the oiling -- either by the oil itself, or by the stench of rotting goose eggs on the island ground beneath their nests.

Whatever the case, 2019 was the last time Snowy Egrets nested in City Park.

This year, no Black-crowned Night-Herons nested at Duck Lake, either.

And now, the Duck Lake cormorants have all but cleared out, and it's not even midsummer yet.

Patrick O'Driscoll

Denver

On Wed, Jun 22, 2022 at 6:53 PM ROBERT G DEBOER <BOBOVETADB@msn.com> wrote:

Does anyone know what has happened to the D.C. Cormorant colony as well as the nesting Snowy Egrets and Black Crowned Night Herons?  Yesterday there were only about a dozen D.C. Cormorants and we did not observe any Snowy Egrets or Black Crowned Night Herons.  Were they affected by the Avian Flu or were they hazed?

Oveta DeBoer

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