Why do frog eggs have proportionately less yolk than bird eggs?

* Developmental Stage: Frog eggs are laid in water, and the tadpole stage is a free-swimming, filter-feeding organism. This means the tadpole doesn't rely on yolk for long-term nourishment like a bird chick does. The tadpole's yolk provides enough energy to develop into a larva capable of finding its own food.

* External Development: Frog embryos develop externally, unlike bird embryos which develop inside an egg. This means the developing frog embryo has access to the surrounding water for gas exchange and nutrient uptake. The yolk provides essential nutrients for the initial stages of development, but the tadpole will soon rely on external food sources.

* Parental Care: While birds provide extensive parental care, most frogs do not. The tadpole is largely left to fend for itself, so it needs to be able to develop quickly and start feeding.

In contrast, bird eggs require a larger yolk:

* Internal Development: Bird embryos develop internally, relying solely on the yolk for their energy and nutrient needs throughout the entire incubation period.

* Limited Parental Care: Birds provide varying levels of parental care, but chicks are generally born helpless and rely on the yolk for their initial food and energy.

Therefore, the difference in yolk size reflects the different developmental strategies and environmental conditions faced by frogs and birds.