The Attack of the Killer Goldfinches

Ok.  I admit that's bit dramatic and over the top but, honestly, there is something up with goldfinches and bushtit nests this year.  Let me explain:

Goldfinches don't build quite the elaborate nests that bushtits do.  Goldfinch nests are basically a cup fashioned with sticks and grass stitched together with.... spider web.  Unfortunately bushtit nests are a handy source of spider web since it (along with lichen) is the primary building block of the bushtit nest, allowing it to be stretchy and strong at the same time.  Every year I do get goldfinches attempting to pilfer material from bushtit nests.   Often the bushtits fend them off.  But sometimes they just can't.  Usually this interest by goldfinches is short-lived and peaks around the time bushtits are building their first nests so they can repair the damage.  After that, goldfinches have usually lost interest and are minding their own business.  Usually.

I already knew that goldfinches can have quite a dramatic and negative effect on a bushtit nest.   Once, in Arizona, I found a bushtit nest (after a laborious and exhausting search of several hours) in the building stage that was being constructed by a happy little pair.  The next day I came back to find a goldfinch ripping off material faster than the poor bushtits could bring it in.  And they didn't even seem to notice.  By the following day nothing remained of the nest and the pair had disappeared -- presumably to find a safer neighborhood.

Here in Portland I have seen goldfinches removing nest material from bushtit nests, but it rarely seems to effect the integrity of the nest.  Sometimes the nest looks a bit "fluffy" afterwards, but it survives.

Not so this year.  This year the goldfinches have gone mad.  I don't know if the unseasonably cold and rainy May has forced them to be more aggressive about stealing spider web from bushtits or what.  Perhaps they are rebuilding lost nests in record numbers? Regardless, they have caused devastating consequences to some of the nests I've been following.

One nest fledged a first brood successfully and was incubating a second when the goldfinches removed enough of the neck of the nest to cause it to be abandoned and then fall to the ground.  Another nest with only 10 day old nestlings (they fledge at 18 days) is just hanging by thread and a hole has been torn into the neck such that the poor nestlings are essentially in a hanging cup nest -- open to the elements.  I expect it to fall and the nestlings to disappear any day now.

Still another nest was close to fledging their first brood and the goldfinches removed enough material to cause it to fall, leaving the nestlings on the ground and easy pickings for the predators that doubtless devoured them that night.  Finally, I have far more bushtit nest starts that get to the "hanging sack" stage and then are abandoned as threadbare and forlorn wisps.  They have been stripped of their material by.....goldfinches.  

I thought nest predators were the bushtits' worst enemy.  Who would have guessed that this year it would be goldfinches?



 

The carnage begins

Nests are hatching already in spite of the cold and wet spring.  Interestingly, by far the greatest percentage of nest with nestlings is in Westmoreland Park.  Next is Reed campus.

But that's where the carnage is happening.  Alas, we may be following the same pattern I observed last year when the percentage of nests lost at Reed due to predation was about 80%.  In comparison, Oaks Bottom and Westmoreland nests were relatively safe at 12% losses. In 2018, Reed also lost few nests at 11%.

So what's up with Reed?  Well, my working hypothesis is that crows, being exceptionally smart and opportunistic, figured out that bushtit nests are lovely little parcels of protein and cue in on them just as they hatch and activity increases as the parents go to and fro with food for the kids.  In 2018, they were relatively clueless. It was last year that some individual crows figured it out.  I was hoping they would have forgotten.  But, so far, it appears they may have long memories.

Of course crows are not the only predators on bushtit nests.  All the corvids (jays, crows, and ravens) as well as squirrels and other mammals that can climb trees are happy to enjoy a meal of baby bushtits.  Owls may also pick off nests at night.

So it may be that Reed has a habitat that encourages populations of other predators besides crows.  But that doesn't explain the difference between 2018 and 2019, does it?   Only a predator that has learned about the delectability of bushtit nests would explain that.

My money is on the crows.

Regardless of the perpetrators, Reed campus is a frustration as nests that were lovely and easy to watch and reach eventually to band babies at are found torn and useless.  In every other way, Reed campus is the ideal study area this year.  Because of the current pandemic, there are few students and visitors on campus.  Campus security is out in force.  I can walk around, for the most part, without a mask.  It's really an ideal research site in these unusual times.  The rest of my study areas are crawling with people on nice days.  And, since nice days are unpredictable, I can't depend on being able to be out safely.  This is particularly true of Oak Bluff trail where joggers and walkers are impossible to distance from.  Consequently, those nests (and there are plenty!) have become very low priority.  This is extremely sad because that's the most "natural" area I have and I have always enjoyed my interactions with the people I meet there.

Oh well. This too shall pass.  In the meantime, we are doing the best with what we have and are keeping our fingers crossed that a few Reed nests survive the season.