of the nervous s ys tem, but s oon it became evident that ‘in s pite of the as s urances of the
ins ecticide people that their s prays were “harmles s to birds ” the robins were really dying of
ins ecticidal pois oning; they exhibited the well-known s ymptoms of los s of balance, followed by
tre mors , convuls ions , and death.’
Several facts s ugges ted that the robins were being pois oned, not s o much by di rect contact
with the ins ecticides as indirectly, by eating earthworms. Campus earthworms had been fed
inadvertently to crayfish in a research project and all the crayfis h had promptly died. A s nake
kept in a laboratory cage had gone into violent tre mors after being fed s uch w or ms. And
earthworms are the principal food of robins in the s pring. A key piece in the jigs aw puzzle of the
doomed robins was s oon to be s upplied by Dr. Roy Barker of the Illinois Natural History Survey
at Urbana. D r. Barke r’s work, publis hed in 1958, traced the intricate cycle of events by which
the robins’ fate is linked to the elm trees by way of the earthworms. The trees are s prayed in
the s pring (us ually at the rate of 2 to 5 pounds of DDT per 50-foot tree, which may be the
equivalent of as much as 23 pounds per ac re where el ms are nume rous ) and often again in July,
at about half this concentration. Powe rful s prayers direct a s tream of pois on to all parts of the
tallest trees, killing directly not only the target organism, the bark beetle, but other insects,
including pollinating s pecies and predatory s piders and bee tles. The pois on forms a tenacious
film over the leaves and bark. Rains do not was h it away. In the autumn the leaves fall to the
ground, accumulate in s odden layers , and begin the s low proces s of becoming one with the s oil.
In this they are aided by the toil of the earthworms, who feed in the leaf litter, for elm leaves
are among their favorite foods. In feeding on the leaves the worms also swallow the insecticide,
accumulating and concentrating it in their bodies. Dr. Barke r found depos its of DDT throughout
the diges tive tracts of the worms , their blood ves s els , nerves , and body wall. Undoubtedly
s ome of the earthworms thems elves s uccumb, but othe rs s urvive to become ‘biological
magnifiers ’ of the pois on. In the s pring the robins return to provide anothe r link in the cycle. As
few as 11 large earthworms can transfer a lethal dos e of DDT to a robin. And 11 worms form a
small part of a day’s rations to a bird that eats 10 to 12 earthworms in as many minutes.
Not all robins receive a lethal dos e, but anothe r cons eque nce may lead to the extinc tion of
their kind as surely as fatal pois oning. The s hadow of s terility lies over all the bird s tudies and
indeed lengthens to include all living things within its potential range. There are now only two
or three doze n robins to be found each s pring on the e ntire 185-acre campus of Michigan State
University, compared with a conservatively estimated 370 adults in this area before spraying. In
1954 every robi n nes t under obs ervation by Mehne r produced young. Toward the end of June,
1957, whe n at least 370 young birds (the normal re placement of the adult population) would
have been foraging over the campus in the years before s praying began, Mehner could find only
one young robin. A year later Dr. Wallace was to report: ‘At no time during the s pring or
s umme r [of 1958] did I s ee a fledgling robin any whe re on the main campus , and s o far I have
failed to find anyone else who has seen one there.’
Part of this failure to produce young is due, of cours e, to the fact that one or more of a pair of
robi ns dies before the nesting cycle is completed. But Wallace has significant records which
point to s ome thing more s inis ter—the actual des truction of the birds ’ capacity to reproduce.
He has , for example, ‘records of robins and other birds building nes ts but laying no eggs , and
othe rs laying eggs and incubating them but not hatching them. We have one record of a robin
that s at on its eggs faithfully for 21 days and they did not hatch. The normal incubation pe riod
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