**Ectotherms vs Endotherms:**

Ectotherms are organisms that rely on external sources of heat to regulate their body temperature. They cannot generate their own body heat and their internal body temperature fluctuates with the temperature of their environment. Reptiles are ectotherms, meaning their body temperature is determined by the temperature of their surroundings.

On the other hand, endotherms are organisms that can generate their own body heat and have the ability to regulate their body temperature internally. Birds, along with mammals, are endotherms. They have the ability to maintain a relatively constant body temperature, regardless of the temperature of their surroundings.

**Feathers:**

Feathers are a defining characteristic of birds. They are specialized structures that provide insulation, enable flight, and display colorful patterns for courtship and display purposes. Reptiles, including snakes, lizards, and turtles, do not possess feathers.

**Water Tight Eggs:**

Reptiles, including reptiles like turtles, crocodiles, and snakes, have water-tight eggs. These eggs have a tough, leathery shell that prevents water loss and protects the developing embryo from desiccation. Birds, on the other hand, have hard-shelled eggs that are not water-tight.

**Explanation:**

Among the given options, the only characteristic that describes reptiles but not birds is that they are ectotherms. Reptiles rely on external sources of heat to regulate their body temperature, while birds are endotherms and can generate their own body heat.

Feathers and water-tight eggs are characteristics specific to birds and are not found in reptiles. Feathers are unique to birds and have various functions, such as insulation and flight. Water-tight eggs, with a hard shell, are a characteristic of reptiles, while birds have hard-shelled eggs that are not water-tight.

Therefore, option A, which states that reptiles are ectotherms, accurately describes reptiles but not birds.