Kentucky Warbler food habits

[DAVID A LEATHERMAN]

I will try to limit the bulk of my comments about the Kentucky Warbler in Fort Collins to its food habits and not say what I really want to say about what almost seems to be the inevitable unraveling of such situations by over-zealous chasers.   "Gettable" is one thing, "gettable and photogenic" is the kiss of death nowadays.

When I had the privilege of laying eyes on this beautiful bird, two things occurred to me.  One, it stayed in one small area and, two, it often remained motionless in one spot for 10 minutes straight.  Usually such behavior means abundant food, and the presence of danger, respectively.  A key food was adults of the Elm Leafminer (Kaliofenusa ulmi).  More about this insect to follow.  The danger, which I thought might be the local Cooper's Hawks, was in fact another offender walking the ditch for a closer look - an outdoor cat.  Not sure which is worse, a frothing birder with a big camera or a cat, but I imagine the answer is the cat.  Most birders, even inconsiderate ones, don't kill an average of 33 birds a year.  Once the cat was encouraged to leave by one of us, the warbler resumed what appears to be its routine of actively foraging in or under a small set of sapling Siberian Elms growing on the ditch bank.

The Elm Leafminer is a non-stinging wasp in the group referred to as "sawflies" because the females have a sawlike ovipositor used to insert eggs into plant tissue.  This particular species is introduced from Europe, as is the tree.  They overwinter as pupae in the soil or leaf litter and the blue-black adults emerge at this time of year to lay eggs on developing leaves shortly after the buds open.  The adults are sought by birds, many of them neotropical migrant passerines such as this KY Warbler. 

 

An adult of the Elm Leafminer (actual length about 1/4 inch).  This is what the KY Warbler was snatching when up in the branches of the elm saplings and, I suspect, what it was getting when on the ground as the adults emerged from the soil.  Apparently, while on the ground, it also found an annelid worm ("earthworm"), which was also fair game.

 

Note the Elm Leafminer adult (dark dot) on the leaf directly in the line of sight of the warbler.   One second after this photo was taken, the "dot" disappeared.

The sawfly eggs hatch and the larvae tunnel and feed between the upper and lower layers of fully-expanded leaves.  This feeding results in brown areas of "mined" tissue.

 

Fully mined leaf.  A single larva is responsible for each of the linear mines defined between two main veins going diagonally off the big midvein (ten larval mines left of the midvein, eleven right of the midvein).  Larvae are still within the mines, as evidenced by the lack of an exit hole at the leaf margin end of each mine.

 

After feeding is complete, usually in June, the larvae bail from the leaves, drop to the ground, burrow into the understory substrate and remain there until they emerge for another go around the following May. 

 

Elm Leafminer larvae (actual length about 3/16s of an inch) collected from the sidewalk a few years ago in June after they dropped out of mined leaves en route to overwintering in the soil.  One robin was getting these by the dozens.

This exotic insect in an exotic tree is, nonetheless, ecologically valuable to our native avifauna.  Birds have figured them out.  They eat them at three times: as adults (early to mid May) per this situation, somewhat while the larvae are mining leaves (late May-early June), and when they drop out and are briefly vulnerable on the sidewalk or bare ground until tunneling into the soil (June).  

Elm Leafminer, along with another insect that mines elm leaves, the European Elm Flea Weevil,  has been covered in "The Hungry Bird" column in "Colorado Birds" multiple times but primarily in the July 2016 issue (Volume 50, #3).  These articles are archived on the Colorado Field Ornithologists' website.

Dave Leatherman

Fort Collins