Rachel the first toxic effects were detected

Carson’s formative Silent Spring was the first popular
attempt to warn the world pesticides were contributing to the “sudden silencing
of the song of birds.” Back in the mid-1900s, Carson battled against an
entirely different kind of chemicals—organophosphates like dichlorodiphenyltrichloroethane
(DDT)—the effects still exist in the fields today. In 1874, synthesis of
DDT pesticide began, and the discovery of its insecticidal potential occurred
in 1939. Intensive spraying occurred during the Second World War to fight the
outbreak of more than 30 infectious diseases (Wheeler 1946; Jukes 1963). In
addition to treating the widely known malaria illness, the use of DDT against
typhus transmitted by lice and plague carried by rats was groundbreaking. An
additional use for this biocide was to treat Dutch elm disease in agricultural
areas (Wurster et al. 1965). After extensive spraying of the DDT pesticide, the
first toxic effects were detected in birds in the 1940s, which died “with their
feet in the air” (Carson 1962; Ratcliffe 1970). These chemical “biocides”, as Carson called them, were
inimical to all life. Annually, approximately 72 million birds die in the
United States alone because of unintentional poisoning by agricultural
pesticides used (Horowitz et al. 2016).  Plainly stated, chemical pesticides of all
kind are harming biodiversity. For that reason and because of the extensive
accolade of research, it is essential to be knowledgeable to the effects of DDT
on birds. This paper aims to analyze the use of pesticides for
human purposes such as eradication of disease, implementation as a warfare
tactic, and regulation of agricultural pests, and how they have unintentionally
affected birds.

            As environmental contaminants, pesticides
are a concern as they are prevalent with intentional distribution and are
designed to exterminate targeted pests. The effects are extended to not just
the pests but mammals, arthropods, and birds causing health defects, mortality,
and reproductive effects (Etterson et al. 2017). The presence of manmade chemicals
becomes an environmental concern when they are persistent in the environment,
bioaccumulate in organisms, and are associated with destructive effects (Hellou
et al. 2013). It took a decade for the United States to ban DDT, just one of
the products that were causing catastrophic declines in emblematic species such
as the bald eagle in North America and the peregrine falcon worldwide.

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            While the decline of the birds was
evident, the source of the mortality was unseen. In the tissues of the
predators, the organochlorines were accumulating. The results were issues of
infertility and disappearance of multiple species of birds. Overall, there is an irrefutable
association between the spraying of technical DDT and endocrine disruption in
birds. The demonstrated connection motivated the ban on the production and use
of DDT in Canada, US, and many European countries since 1970.

            Within the pest control world, a new
breed of insecticides known as the neonicotinoids has been introduced recently.
Neonicotinoids – often used as herbicides and pesticides to exterminate pests
in agriculture are prevalent. The products work by being absorbed into the
system of plants. Several European countries are discussing to ban the
pesticide, but parts of industry are opposed and ignorant to the detrimental
effects, a parallel with and echo of Silent Spring. With another
insecticide making its way into the world, it will bring along hidden
ecological impacts. With
this, scientists are increasingly worried about the fate of our birds, some of
which may be found in or near suburban settlements. The dwindling numbers of
migrant species…

            Over the last century, the amount of
conventional pesticide mass applied nationwide in the United States has
generally increased to current use levels of around 1.2 billion pounds active
ingredient per year by the 1990s. In 2006–2007, estimated annual use of
pesticide in the U.S. was 1.1 billion pounds; with approximately 90–100 millions
of those pounds represented by insecticides. The set of specific pesticide used
at any given time is dynamic and capricious – changing in response to many
factors such as crop rotations, pest pressure, changing economics of pest
control, development of new pestticides, resistance evolution, and increased
understanding of risks associated with the products (Etterson et al. 2017).



3.1. World and U.S. Pesticide Amounts of Active Ingredient at Producer Level by
Pesticide Type, 2012 Estimates

            The process of neonicotinoids and
other pesticides that enter the body of birds are also complex. Birds are
exposed to the pesticides through breathing, dermal contact, and by consumption.
The first step is ingestion. Studies have shown that while neonicotinoids are
commonly considered to be safer for mammals and birds than for insects, they are still lethal in high enough doses (especially
through seeds coated in them) (citation). A study by the U.S. Environmental Protection
Agency discovered that the Passerines sparrows are more aerially-challenged after
consuming a tiny amount of imidacloprid, an organic pesticide, and become immobile at
higher dosage (citation). Piscivorous
birds (in addition to marine mammals) are at the top of the aquatic food web
and have been studied for the trophic transfer of contaminants leading to
biomagnification. In regard to the predaceous birds – size, physiology
and biotransformation capacity are distinct. The body size of a prey being
consumed by a predator begins with a contaminant arriving in the stomach. The
lipophilic chemicals will be transported to other tissues through the blood and
with time and continuous exposure, they will be retained in higher proportion
in lipid-rich organs and tissues. The preen “uropygial” gland of birds are the
most lipid-rich organs. The oil-secreting gland is located near the tail of a
bird and contains oil used for preening feathers (Yamashita et al. 2007).

            In more detail, neonicotinoids bind
the nicotinic acetylcholine receptors in the central nervous system of its
target – invertebrates, and they have less affinity with those receptors in
vertebrates such as birds (Hallman et al. 2014). They have lasting lives in
soil and are water soluble, accumulating in soils and present in surface water
and ground water (Hallman et al. 2014). Their ability to spread throughout the
tissues of the plants treated with neonicotinoids in addition to the widespread
use, indicates the exposure to many organisms. For example, organophosphates
and carbamates inhibit acetylcholinesterase leading to the overstimulation of
the nervous system and eventually leading to death. Additionally, these
pesticides can be absorbed through the respiratory system, gastrointestinal
tract, and skin, and they act by inhibiting the activity of the enzyme
cholinesterase (Horowitz et al. 2016). Clinical signs and symptoms go
undetected for birds in the wild as it can resemble symptoms of other illnesses
or migration exhaustion (Horowitz et al. 2016).

            Barron (2002) reviewed the behavioral
effects associated with the central nervous system of birds and linked to DDT
exposure. The effects observed included reduced aggression, impaired avoidance,
as well as reduced defense and attentiveness at a nest. The behavioral effects
were observed at levels 10 to 100 times lower than those associated with
lethality (Peakall 1985). The lowest body burden was described for ring doves (Streptopelia
risoria), where 3000–8000 ng/g of DDE were related to changes in courtship
behavior, relative to 300 000–400 000 ng/g associated with lethality. In
comparison, Hellou (2011) reviewed the literature linking contaminants’ body size
and behavioral toxicology involving aquatic species. The author discussed the
sensitivity of this multifaceted response and recommended behavioral toxicology
as an “early warning signal” to investigate the effects of contaminants. This
organism level effect was up to 1000 times more sensitive than the lethality
endpoint. The size of the brain and forebrain of the birds was negatively
correlated to the levels of DDE in males where a 13% and 15% reduction in
volume was associated with increased concentration from nearly 40,000 to 150,000
ng/g in male eggs. The forebrain is the site of the brain related to behavioral
effects associated with mating and singing. Changes in singing rate and in the
song repertoire of the robins were expected in the birds, although they were
not reported.

            In Silent Spring, Carson describes the eggshell thinning discovered in
birds when DDT was beginning to be widely used. Bird studies varied from
sampling eggs to whole animals or specific tissues (Kunisue et al. 2002; Minh
et al. 2002; Goutner et al. 2011). For research, it has been reported that eggs
are the stage at which maximum concentration is evident in birds (detailed in Ridgway
et al. 2000; Vos et al. 2000; Norstrom et al. 2007). Eggshell thinning effects
reflect DDT concentrations and hatching success (Ratcliffe 1967). For in depth
analysis of what types of birds, in terms of feeding preferences, are most
affected by pesticides, variables such as migratory or resident and selective
or nonselective feeding habits are taken into consideration (Minh et al. 2002; Jimenez
et al. 2007; Volta et al. 2009). When examining birds with different feeding
preferences, carnivorous birds have higher levels of DDE concentrations than omnivorous
birds and they have higher levels than insectivorous birds. Within the
carnivorous group, raptors, which feed on other birds, tend to have higher bioaccumulation
levels than piscivorous birds (Naso et al. 2003). On the other hand, non-migratory
insectivorous birds have the advantage of being bio-indicators of local
contamination as they remain in one location and consume on the insects that
have been directly targeted by pesticides (Dauwe et al. 2006). For example, it was shown in two
dipper species that changes in the mothers’ diet from eating insects to eating
fish resulted in a higher contaminants’ load de- posited in eggs (Morrissey et
al. 2010). In another case illustrated by Gabrielsen et al. (1995), the
diversity of food sources dictated cau- tion in interpreting the origin of the
pesticide in glaucous gulls (Larus hyperboreus) because food items
included not only aquatic-derived food such as cod and crab, but also carrion
and garbage.

            Blood is one bio-fluid that can be
sampled in birds in a nondestructive manner and where concentrations reflect the
current exposure of the individual to pesticides. The ratio of concentrations
in blood relative to whole bird expressed on a lipid basis reported by Norstrom
et al. (2007) for adult female herring gulls (L. argentatus) was 1, the
standard for a stable bird in terms of pesticide exposure. Another tissue
specific to birds, feathers have been used for metal analyses but there has
been minimal studies regarding this tissue. Other organs such as the liver, kidney,
muscle, brain, and even feces can also be assessed (Gabrielsen et al. 1995; Dykstra
et al. 2010). However, the analysis of eggs solely reveals the reproductive
risk associated with DDE, providing a reliable connection between contaminant
exposure and health. An example is a study by Ratcliffe (1970) in which the
connection between eggshell thinning and the spraying of DDT was established, comparing
the pesticide concentrations in affected and unaffected eggs. As outlined by Vasseur and
Cossu-Leguille (2006), The critical level of eggshell thinning linked to
the presence of DDT in bird eggs is established at 18%–25%, where there is a destruction
of eggs during the incubation time leading to decreased hatchability and
population level effects.

            Herbicides are also used as Warfare
Tactic, as seen in the Vietnam War with Agent Orange. An herbicide manufactured
for the U.S. Department of Defense by Monsanto Corporation and Dow Chemical. The
name was derived from the color of the orange-striped 55-gallon barrels in
which it was shipped to Asia. It was part of the United States herbicidal
warfare program during the Vietnam War between 1962 and 1971. In the course of
10 years, American forces sprayed nearly 20 million gallons of the destructive
chemical in Vietnam, Laos and parts of Cambodia with the goal to rid guerrilla
fighters of cover by destroying plants and trees where they could potentially seek
refuge. The 2,4,5-T used to produce Agent Orange was contaminated with 2,3,7,8
– Tetrachlorodibenzodiocin (TCDD) an extremely toxic dioxin compound. In some
areas, TCDD concentrations in soil and water were hundreds of times greater
than the levels considered safe by the United States Environmental Protection
Agency. About 3.1 million hectares
of the total forested area of Vietnam was sprayed during the war, which in
turn, disrupted the ecological equilibrium. The persistent nature of dioxins, erosion from
lack of trees and loose soil, and loss of growing forest meant that
reforestation was nearly impossible in many areas. Moreover, many deforested
areas were invaded by aggressive species (Bamboo and Cogon grass) making even
harder for reforestation. As expected, bird-species diversity was greatly impacted.  

            A Harvard biological study reported
that there were 24 species of birds in a sprayed forest, while in two adjacent
sections of unsprayed forest there were 170 species of birds. These dioxins from Agent Orange have since
settled in the soil and sediment, entering the food chain through animals and
fish which feed in the contaminated areas. This
movement of dioxins through the food web has resulted in bioaccumulation. Even though Agent Orange is no
longer used today, other similar pollutants such as fertilizers, pesticides and
other herbicides continue to be widely used today. Their impact may not be as
severe as that of Agent Orange but this incident strongly hints at the potential toxicity and harmfulness of
using such chemicals. However, such chemicals occupy very entrenched roles in the production of
food and thus their phasing out will not be easy.        Similarly, Minh et al. (2002) reported
the first results on Persistent Organic Pollutants (POPs) in avian
samples from North Vietnam where the use of DDT only ceased at the end of the
20th century. The team detected a large amount of the pesticide, between
140 and 3100 ng/g (wet weight) in multiple species of birds. DDT concentrations
were much higher than other targeted organochlorine pesticides (OCs). For
example, the study looked at the white-breasted water hen (Amaurornis
phoenicurus) in which the mean sum of DDT was 100 times higher than that of
polychlorinated biphenyls – which are man-made organic chemicals of carbon,
hydrogen, and chlorine. The DDT derivative dominated with significantly higher
DDT levels in residential birds. In 1991-1997, a comparison of the full body
concentrations reported for species collected in Lake Baikal, Russia; Chubu,
Japan; South India; and Australia showed that birds from N. Vietnam had the
highest DDT levels. The gathered data made the proximity of the pesticide input
undeniable. With specific regard to the use of herbicides, their usage will
continue to remain a disputable problem among environmentalists since
selectivity of these agents has never been completely achieved – hence, their
end does not appear to be coming anytime soon either.

of DDT and the more abundant DDE isomer greatly decreased in biota over the
decades, soon after the pesticide was banned in 1970. However, these trends
were not maintained over time or worldwide (e.g., Dittmann et al. 2012). There
are varying concentrations among geographical locations, species and/or tissues
presently reported in literature. Concentrations are of concern where
intentional or illegal use of the pesticide has occurred when levels are
affected through atmospheric transport or measured near contaminated sites. So
far, the efficiency of DDT in annihilating mosquitoes that transmit malaria has
been unequalled by any other synthetic competitor, including the pyrethroids
which are the next candidates for use. However, other chemical choices are not
powerful enough as an alternative where malaria is prevalent. The World Health
Organization proclaimed in 2006 that DDT could be used to combat malaria in
Asia and Africa (Cone 2009). Van den Berg (2009) addressed the environmental
and human health costs associated with using DDT for indoor spraying relative
to the benefits and other available means to control the spread of the disease.
Some discoveries are now appearing with ongoing commercialization studies for
the treatment of malaria without the use of DDT. For example, the application
of the natural product nepetalactone, which comes from the extract of the
catnip plant (Di Menna 2011) is being researched. The properties of this anticipated
insect repellent were patented for more than a decade before an opportunity
arose to start studies to grow the plant in Burundi. Research into the development
of this alternative treatment that could be produced in Africa with beneficial
outcomes is ongoing. There is also work in progress to eliminate the parasite
transmitted by mosquitoes that act as a vector for the disease. Research using
bacteria or fungi to attack insects aims to reduce the incidence of the disease
by using what can be called a bio-pesticide (Cirimotich et al. 2011; Fang et
al. 2011). All of these pursuits are to ensure that the biodiversity is
maintained, and unintentional harm to organisms exposed to the pesticides are

the number of birds are dwindling, extensive research has suggested that
agricultural intensification and, for some species, the indirect effects of
pesticides mediated through a loss of insect food resource is partly
responsible. Worldwide, the improper application of insecticides, herbicides,
and fungicides, combined with the difficulty of restricting avian access to
pesticides-treated landscapes, can result in toxic exposure via primary or
secondary consumption of animals containing the toxins (Horowitz et al. 2016).
The pesticides can adversely affect population dynamics and lead to significant
damage to entire ecosystems. The double-edged sword of using a toxic pesticide
to fight lethal diseases to humans and such as malaria, but in turn have a
deleterious ecological impact on wildlife such as birds is a topic of
discussion that needs attention. The precautionary
principle is discussed by scientists who care about the environment and try to
raise awareness of it. The principle states that, before taking any action, one
must prove it will not be harmful to the environment (Weaver 2011). This view should be adopted by present and future
generations to protect the environment. Likewise, Carson was not opposed to insecticide
use per se. Her message was emphasizing the need for moderation; to understand
the implications and act with care. She called for “humility in place of
arrogance” in the ways we apply science to the natural world, and in the
agricultural field that too often we neglect.