cuckoo ID

[DAVID A LEATHERMAN]

I am back from my long-seemed-short visit to Lamar.  Grand total for the Lamar area ended up being 103 species.

On 17September2017 Janeal and I visited Melody Tempel's Grove.  Among the few things we saw was a young Yellow-billed Cuckoo.  This individual stayed in one spot within the crown of a mulberry for over an hour and was fed at least 4 large grasshoppers (Melanoplus sp. (either bivittatus or differentialis) by a parent cuckoo.  It's shortish, mostly straight bill was essentially all black or dark gray.  It had an inconspicuous gray eyering, subtle rufous coloring in the wing and a few hard to glimpse, large white spots on the underside of the tail.  It would have been very easy to call this bird a Black-billed Cuckoo had we only seen the head and not the other features, and not the feeding by an obvious Yellow-billed parent.

                                                                                     

                                                                       Juvenile Yellow-billed Cuckoo still dependent on parent, Tempel's Grove on 17September2017.

                                                                                     

                                                                       Two-striped Grasshopper (Melanoplus bivittatus), probable species being fed to young cuckoo.

                           

                                                  Individual Fall Webworm caterpillar at left, large Fall Webworm "tent" as seen the other day at Tempel's Grove at right.

Dave Leatherman

Fort Collins
 

                                                                                    

                                                                                    

I am NOT saying the recent reports of Black-billed Cuckoos by John Drummond and Steve Mlodinow are in error.  Their detailed reports/photos leave no doubt as to the veracity of their sightings.  All I am saying is that at this time of year, cuckoo ID can be tricky.  Bill color is not good enough.  Zeal for the rarer species is not good enough. 

My other comment would be this seems to be late for a dependent young cuckoo to still be in the nesting locale.  John Drummond suggested perhaps the presence/lingering of both Yellow-billed and Black-billed Cuckoos at Tempel's Grove is enhanced by the fairly extensive infestation of caterpillars at this site.  I would agree.  The prominent caterpillar present is the Fall Webworm (Hyphantria cunea).  The BNA account for Yellow-billed Cuckoo mentions this caterpillar as being seasonally prominent in their diet, along with grasshoppers.  As an aside, we also saw a Cassin's Vireo eating this caterpillar.  I suspect other local breeders and fall migrants eat these as well.

Fall Webworms are distinct from the various caterpillars in the genus Malacosoma, the true "tent caterpillars".  Tent caterpillars (we have 5 species in CO (in order of abundance from most to least: western, forest, southwestern, Sonoran, and eastern) make small white tents, mostly in late spring-early summer, and they use these tents as home bases from which they venture to feed on leaves during the day, returning to the tents at night.  Fall Webworms, on the other hand, are mostly late summer-autumn creatures, living mostly within their webs which are continually enlarged to include the majority of leaves they feed upon).  Thus, webworm "tents" can be huge and the larvae are normally not available to birds, being protected by the mandible-gumming web surrounding them.  However, after John made his comment on COBIRDS, I paid more attention to these caterpillars and discovered quite a number just cruising outside the webs on branches and on the ground.  I suspect these were individuals that had completed feeding and were on their way to the ground for pupation and overwintering.  Regardless of their mission outside the webs, they were definitely vulnerable to bird predation, despite their hairy exterior (another evolved "trick" of caterpillars attempting to avoid predation).  

[DAVID A LEATHERMAN]

Somehow, the bulk of text composed for my post about cuckoo ID did not appear in what got send out. 

Basically what I meant to send out with the four photos was that ID of cuckoos at this time of year can be compounded by the presence of young "black-billed" yellow-billed cuckoos.

I also wanted to say I am NOT questioning here the ID of black-billed cuckoos recently reported by both John Drummond and Steve Mlodinow.  Their reports/photos are solid.

Fall Webworms (Hyphantria cunea) are present at Tempel's Grove and are known to be prominent in the late summer diet of cuckoos.  They live within webs enlarged to include the leaves on which they feed.  At this time of year the webs can be multiple feet across.  They leave the webs, however, after feeding for a period of several weeks to pupate and overwinter in the leaf litter on the ground.  This is when they are most vulnerable to bird predation.  The true tent caterpillars often mentioned in the diet of cuckoos are in the genus Malacosoma (we have five species in CO) are late spring-early summer caterpillars which build smaller, whiter webs they use as home bases at night, venturing out to feed "unprotected" during the day.

Grasshoppers, such as the two-striped shown on the other email, were being fed to the young dependent cuckoo by an adult yellow-billed.  They are also mentioned in the BNA account of cuckoos as being prominent in cuckoo diets.

Dave Leatherman

Fort Collins