The difference in the reproductive strategies of fish and birds, such as hens, is primarily due to their respective environments and evolutionary adaptations.

1. Aquatic vs. Terrestrial Environments: Fish lay eggs in water, where the environment provides relatively fewer protections for developing embryos compared to land. As a result, fish typically produce a large number of eggs to increase the chances that some will survive predation, environmental hazards, and other risks. By spreading their reproductive efforts across a large number of eggs, fish increase the likelihood that at least some offspring will reach maturity.

2. Fertilization Mechanisms: Many fish species practice external fertilization, where eggs and sperm are released into the water simultaneously. This method requires a large number of eggs to maximize the chances of successful fertilization. Additionally, because fish do not provide parental care to their offspring in the same way as birds, they rely on sheer numbers to compensate for the lack of protection and nurturing.

3. Terrestrial Reproduction: Birds, including hens, lay eggs on land and typically engage in internal fertilization, where sperm fertilize the eggs within the female's body. This method allows for more precise control over fertilization and reduces the need for producing a large number of eggs. Additionally, birds invest more parental care into their offspring compared to many fish species. This includes incubating the eggs, protecting the nest, and providing food and guidance to the chicks after hatching. As a result, birds can afford to invest more resources into each individual egg, leading to the production of fewer eggs compared to fish.

4. Life History Strategies: Fish often have shorter lifespans, higher mortality rates, and faster reproductive rates compared to birds. Producing a large number of eggs maximizes reproductive output and ensures the survival of at least some offspring in environments where predation and environmental variability are high. In contrast, birds tend to have longer lifespans, lower mortality rates, and invest more resources into each offspring, leading to a lower number of eggs produced per reproductive event.

In summary, the difference in egg production between fish and birds, such as hens, is primarily a result of their respective environments, reproductive mechanisms, and life history strategies. Fish produce a large number of eggs to compensate for environmental risks and lack of parental care, while birds invest more resources into fewer eggs to ensure the survival of their offspring in terrestrial environments with greater parental involvement.