Assertion: There is a decrease in the bird population in an area where DDT is used.
Reason: DDT increases calcium absorption, causing calcification in birds.

Explanation:
DDT (dichlorodiphenyltrichloroethane) is a synthetic pesticide that was widely used in the past to control pests, especially mosquitoes. It was highly effective in killing insects, but its use has been largely restricted or banned in many countries due to its harmful effects on the environment and wildlife.

DDT and Calcium Absorption:
One of the reasons for the decline in bird populations in areas where DDT is used is its impact on calcium absorption. DDT has been found to interfere with calcium metabolism in birds. Calcium is an essential mineral required for various physiological processes, including the formation of eggshells and proper bone development.

Calcification in Birds:
When birds are exposed to DDT, it can lead to an increased absorption of calcium from their diet. This excessive calcium absorption can result in the deposition of calcium in various tissues and organs, a condition known as calcification. Calcification can have severe consequences for birds, including the formation of abnormal eggshells and skeletal deformities.

Impact on Bird Reproduction:
The formation of abnormal eggshells due to DDT-induced calcification can have a detrimental effect on bird reproduction. Thin and fragile eggshells are more likely to break during incubation, leading to reduced hatching success. This can result in a decline in the overall bird population as fewer chicks are successfully hatched and survive to adulthood.

Direct and Indirect Effects on Bird Population:
The decrease in bird populations observed in areas where DDT is used can be attributed to both direct and indirect effects. The direct effect is caused by the detrimental impact of DDT on bird reproduction, as discussed above. The indirect effect is related to the biomagnification of DDT in the food chain.

Biomagnification of DDT:
DDT is a persistent organic pollutant that does not readily break down in the environment. When DDT is used, it enters the food chain through the ingestion of contaminated prey by birds. As birds consume organisms that have accumulated DDT, the pesticide accumulates in their bodies at higher concentrations. This process is known as biomagnification.

Consequences of Biomagnification:
As DDT accumulates in the bodies of birds through biomagnification, it can have systemic effects on their health and reproduction. The high concentrations of DDT can impair the immune system of birds, making them more susceptible to diseases. It can also disrupt their hormonal balance and reproductive functions, further contributing to the decline in bird populations.

Conclusion:
In conclusion, the decrease in bird populations in areas where DDT is used can be attributed to the harmful effects of DDT on calcium absorption, resulting in calcification and abnormal eggshell formation. Additionally, the biomagnification of DDT in the food chain further compounds the negative impact on bird health and reproduction. Due to these reasons, the use of DDT has been restricted or banned in many countries to protect bird populations and prevent further environmental damage.

A is true, but R is false.