[cobirds] Mountain irruption in my north-central Boulder yard

The irruption of mountain species to the plains has shown up in my yard in north-central Boulder. Over the past few weeks, I’ve had a trickle of species that are unusual for my yard: Steller’s Jays, Pygmy Nuthatches, and Cassin’s Finches.

But then today!!!

This single afternoon I’ve had in the yard:

• FOUR Steller’s Jays at once (only every seen 19 times in my 30 years in the house, and never more than one per day)
• NINE Cassin’s Finches at once, growing in numbers as the day progresses from two seen this morning to the mob on the feeder this afternoon (only seen 10 times in the yard outside of the spring of 2023 when they were all over Boulder)
• TWO Pygmy Nuthatches (only seen five times in the yard before this year)
• ONE Mountain Chickadee (only see in 10 of the 30 years since I’ve lived in the house)

At one point, all four of these species were in the yard at the same time.

And this morning, on my usual morning dog walk around the Boulder Community Gardens a block from my house, there was a vocalizing Evening Grosbeak — only the second time I’ve seen an EVGR at the Gardens ever. (Too bad it wasn’t in my yard: I’ve only ever had one EVGR in the years, in 2001.)

For context, my backyard is an ordinary urbanish backyard about 3/4 of a mile east of Mt Sanitas. Even though these mountain species are relatively common on Mt Sanitas, they rarely venture that short distance down from the pine forest to my yard, despite the attraction of my feeders and bird bath.

So what’s going on in the mountains that is driving these species east?  Little water, few insects, poor cone crops?

Richard Trinkner

Boulder