A treated lawn looks exactly the same the next morning.
The grass is untouched, since the product was chosen to leave it alone, and whatever was growing in it goes yellow over the following week.
Everything about the visible result says nothing else changed.
Underneath, the soil has just received a pulse of chemistry and, a few days later, a large delivery of dying root material.
Both of those are events for the animals living in it, and neither shows on the surface.
The ground is where the change lands.
Two worms in the same lawn live completely different lives
Lumping earthworms together is the mistake that makes this result confusing, because the word covers animals with almost nothing in common.
One kind digs a permanent vertical burrow going down several feet, stays in it, and comes to the surface at night to pull dead leaves down into the mouth of the hole.
That is the animal that leaves the little coiled heaps on a lawn, and it is the bird’s worm, because it is the only one that surfaces.
The other kind lives in the top few inches, burrows horizontally and continuously, eats soil rather than litter, and essentially never comes up.
A bird could stand on a lawn full of the second type and starve, and a lawn full of the first type is a feeding ground.
The two respond to different things.
What the pots actually contained
The work was done in a greenhouse rather than on anybody’s grass, in plastic pots holding about 12 gallons of soil each, roughly 16 inches across and 15 inches deep.
Field soil was mixed with quartz sand, planted with a mixture that was allowed to establish for eight weeks, and then stocked with both worm types.
The dose was deliberately conservative. Two commercial formulations were applied across three days at a total 53 percent below the rate printed on the label.
After treatment the pots were watched for four weeks for surface casting and for fifteen weeks for whether the cocoons produced any young.
Nothing in the setup involved a bird, a lawn mower, rain, or a sprinkler.
It is a controlled model.
Forty six percent and fifty six percent are different worms
The experiment ran from March to July of 2013 and produced two headline numbers that get quoted as though they described the same animal.
The deep burrowing surface feeder almost stopped casting. Its output fell 46 percent over four weeks, down to little more than half of what the untreated pots produced, and surface activity had nearly ceased within three.
Its reproduction looked bad too, with hatching dropping from 43 percent to 17, but that rests on 25 cocoons in total and no statistical test was run on it.
The shallow soil dweller kept casting normally and took the hit in reproduction instead, down 56 percent within three months, with hatching falling from 71 percent to 32 and a solid result behind it.
The soil chemistry moved as well. Nitrate rose by 1,592 percent and phosphate by 127.
The strong number belongs to the hidden worm.
Nobody in this study looked at a bird
That split is the whole reading, and the honest version of it is uncomfortable for both sides of the argument.
The statistically firm reproduction figure applies to a species a robin never encounters, and the effect that would matter to a robin is the collapse in surfacing, which was measured as casting rather than as availability.
These were also pots in a greenhouse, run once, over a single season, in the way that a mesocosm result or a preservative leaching quietly becomes a headline anyway.
No bird was counted, no foraging rate was recorded and no lawn was surveyed before and after, which the authors do not claim otherwise in the published paper.
The chain to the robin is inference.
A bird cannot read a lawn
What makes the question worth asking anyway is that the animal at the top of it has no way to tell.
A thrush working a lawn is reading cues that have nothing to do with chemistry. Wet grass, soft ground, cast heaps, vibration underfoot.
A treated lawn still offers every one of those signals for a while, and the burrows are still there even when nothing is coming out of them.
So the cost, if there is one, does not show up as avoidance. It shows up as a bird spending the same effort on ground that has stopped paying, which is the kind of effect that never produces a dead animal to find and never reaches the literature that gets cited.
Earthworm reproduction is the number that got measured.
It is not the one a robin feels.
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