**Natural causes:**
* **Disease:** Avian influenza, avian malaria, and West Nile virus are some common diseases that can affect grebes and other birds.
* **Predation:** Hawks, owls, and other predators can prey on grebes, especially during vulnerable times like nesting or migration.
* **Habitat loss or degradation:** Changes in water quality, invasive species, and destruction of nesting sites can threaten grebe populations.
* **Starvation:** Lack of food due to environmental changes or competition with other species can lead to malnutrition and death.
**Other factors:**
* **Accidents:** Collisions with boats, power lines, or other structures can injure or kill birds.
* **Lead poisoning:** Ingestion of lead fishing tackle or other lead-contaminated objects can be fatal for birds.
* **Pollution:** Contamination of water or food with pesticides, herbicides, or other pollutants can harm birds.
**dichlorodiphenyldichloroethane (DDT)**, may have played a role in the deaths of grebes in Clear Lake. Historically, DDT was used extensively as an insecticide, including in agricultural and aquatic environments. Its use was eventually banned due to its harmful effects on wildlife and the environment.
Here's how DDD might have impacted grebes in Clear Lake:
**Bioaccumulation:** DDT is a persistent organic pollutant, meaning it doesn't readily break down in the environment and can accumulate in the tissues of organisms over time. Fish, which are a primary food source for grebes, can accumulate high levels of DDT from contaminated water. As grebes consume contaminated fish, they take in the DDT, leading to increased concentrations in their own bodies.
**Eggshell thinning:** One of the most well-known consequences of DDT exposure in birds is eggshell thinning. DDT disrupts calcium metabolism, making it difficult for birds to produce strong eggshells. Grebes with thin eggshells are more susceptible to breakage during incubation, leading to decreased hatching success.
**Reproductive problems:** Beyond eggshell thinning, DDT can also disrupt other aspects of bird reproduction, including hormone levels, behavior, and immune function. These disruptions can further contribute to population declines.
While DDD wasn't directly applied to Clear Lake, it may have entered the lake through runoff from surrounding agricultural areas or from atmospheric deposition. Studies conducted in the 1960s revealed high levels of DDD in the lake's fish and grebe eggs, suggesting it was a significant factor in the decline of the grebe population at that time.
However, it's important to note that other factors may have also contributed to the deaths of grebes in Clear Lake, making it difficult to definitively attribute the decline solely to DDD. Habitat loss, pollution, and other environmental factors could have played a role as well.
Overall, while DDD was likely a significant contributor to the historical decline of grebes in Clear Lake, it's crucial to
2. INTRODUCTION:-
At Clear Lake, Western grebe is a breeding
bird & winter visitant, attracted by abundant fish
of lake.
In December, 1954, 100 Grebes were reported
dead on the shores of Clear Lake in California.
By the toxicological examination, DDD was
found in the fats of dead grebes in very high
concentration (1600 ppm ).
3. Related to grebe’s death by DDD, different
questions arises as they are fish eater. It means
that they feed on contaminated fishes, poisoned
by DDD. Now the most possible questions are;
What is the source of DDD in the fish?
Do grebes eat all the types of fish examined , or
do they eat only certain ones?
Why is the DDD concentration in the visceral fats
so much higher than in the flesh of most of fish?
SOURCE OF DDD IN FISH:-
when fish of the clear lake were analyzed,
DDD was found in their body.
4. As the fish feed on gnats killed by DDD
sprays, so the fish became contaminated
by DDD which enter into their body by
eating gnats.
FOOD OF GREBES:-
Extensive studies conducted at clear lake
in 1950 showed that the primary food
source for western grebes is Blue gill &
minnow while few large mouth bass were
also found in their stomach. Beside these,
they also feed on carp & herring.
6. DDD Concentration in fatty tissues of grebes:-
DDD is a conservative pollutant which can not be
metabolized & therefore when an organism
containing a pollutant is eaten, the pollutants are
simply passed on to the predators. So, by
consuming many preys, Grebes may build up very
high concentration of DDD in their tissues. Since
DDD is readily soluble in fats. Consequently, they
will often accumulate in fatty tissues of grebes. In
the time of food shortage, fat reserves may be
metabolized, released DDD into the blood of grebes
& will cause their death.
7. CONCLUSION:-
From the above studies it can be
concluded that death of Grebes will
occur if the DDD released into their
blood from the fat reserves, as the DDD
readily concentrate in their fatty tissues.
