Determining which bird lays the smallest egg between the robin and the hummingbird requires looking at the average size of eggs for each species. The size of eggs, like most traits, exhibits variation within a species, so looking at average egg masses gives a general sense of which species tends to lay smaller eggs. Research has quantified the average egg sizes of hundreds of bird species, so data is available to compare robins and hummingbirds.
Average Egg Sizes
According to extensive data on bird egg sizes compiled by the University of Texas at Austin, the average mass of a robin egg is about 3.4 grams. The average mass of a hummingbird egg, on the other hand, is only about 0.365 grams. This more than 10-fold difference in average egg mass is substantial and suggests hummingbirds lay much smaller eggs than robins.
However, keep in mind that these are averages across all robin and hummingbird species. There are several different species of robins and hundreds of different hummingbird species. Exploring the egg masses of individual species provides more detailed information.
Egg Sizes of Individual Species
For robins, we can look specifically at the American robin, the most common robin species found in North America. The American robin lays eggs that weight 2.7 – 3.9 grams, averaging around 3.3 grams. This is consistent with the overall average for robins.
For hummingbirds, we can zero in on a few of the most widespread species:
- Ruby-throated hummingbird eggs weigh 0.3 – 0.4 grams
- Black-chinned hummingbird eggs weigh 0.35 – 0.45 grams
- Anna’s hummingbird eggs weigh 0.5 – 0.6 grams
- Calliope hummingbird eggs weigh 0.35 – 0.4 grams
- Broad-tailed hummingbird eggs weigh 0.45 – 0.55 grams
The egg masses for these common hummingbird species are very small, ranging from 0.3 – 0.6 grams on average. This supports the overall average hummingbird egg mass of around 0.365 grams.
The Smallest Eggs
Clearly, hummingbirds lay much smaller eggs than robins. In fact, hummingbirds lay the smallest eggs of all birds! The bee hummingbird, found in Cuba, lays eggs that are only 10-12 mm long and weigh just 0.375 grams. This is the smallest bird egg in the world.
In comparison, the eggs of the largest living bird, the ostrich, are over 3,000 times heavier than a bee hummingbird egg. Ostrich eggs can weight 1.5 kg and are 15 cm long. The difference between the largest and smallest bird eggs is astounding!
While robin eggs are larger than hummingbird eggs, they are still quite small compared to most bird species. Robin eggs are about the size of a jelly bean, while hummingbird eggs are as small as a coffee bean. But the bee hummingbird takes the prize for the tiniest of all with eggs not much bigger than a pea.
Why Are Hummingbird Eggs So Small?
The extreme miniaturization of hummingbird eggs is related to the small body size of hummingbirds. Hummingbirds weigh only 2-20 grams, with bee hummingbirds being the smallest at around 2 grams. Their tiny size limits the absolute dimensions of their eggs.
However, hummingbird eggs are not just miniature versions of larger bird eggs. Relative to adult body mass, hummingbird eggs are still extremely small. The following table compares relative egg sizes across some bird species:
| Bird | Average Egg Mass | Average Body Mass | Egg/Body Mass Ratio |
|---|---|---|---|
| Ostrich | 1,500 g | 115 kg | 1.3% |
| Robin | 3.3 g | 77 g | 4.3% |
| Hummingbird | 0.365 g | 3 g | 12% |
The egg-to-body mass ratio for hummingbirds is nearly 3 times higher than that for robins. Hummingbirds produce incredibly tiny eggs even after accounting for their small size. So what drives the extreme miniaturization of hummingbird eggs?
High Metabolism
Hummingbirds have among the highest metabolic rates of any animal. To support their fast metabolism, they need to eat frequently. Mother hummingbirds lay 1-2 eggs at a time so they can continue to meet their own nutritional demands. Larger clutches would require longer incubation periods that would interfere with the mothers’ ability to feed.
Frequent Nesting
Hummingbirds build a new nest and lay eggs every 2-3 months during the breeding season. The small clutches and tiny eggs allow them to reproduce frequently and maximize nesting opportunities. Larger eggs require more nutrients and energy to produce, limiting how often a female could lay eggs.
Small Nest Size
Hummingbird nests are only 3-5 cm across. The petite nests can only accommodate small eggs and clutches. Hummingbirds have evolved tiny eggs to properly fit and incubate inside their nests.
Rapid Development
Despite their small size, hummingbird eggs contain enough nutrients for the embryos to develop rapidly. The chicks hatch after only 2 weeks of incubation and reach adult size in 2-4 weeks after hatching. Producing smaller eggs allows hummingbirds to complete this rapid growth.
How Do Small Eggs Impact Hummingbirds?
The tiny size of hummingbird eggs enables key evolutionary adaptations but also comes with risks and challenges:
Advantages
- Higher reproduction rate by laying more frequent, smaller clutches
- Easier to incubate smaller eggs in tiny nests
- Females can continue feeding regularly
- Eggs and nest take less energy and nutrients to produce
Disadvantages
- Greater risk of eggs drying out
- Increased predation and nest disruption
- Higher survival demands on hatchlings
- Less room for errors in embryonic development
The extra challenges imposed by tiny eggs are balanced by the reproductive benefits for hummingbirds. Their high metabolism and energetic lifestyle favor producing many small eggs.
Robin Egg Development
While robin eggs are much larger than hummingbird eggs, they are still quite small relative to the robin’s body size. How do robin eggs compare in their incubation requirements?
Incubation Period
Robin eggs hatch after 12-14 days of incubation. This is slightly shorter than the 2-week incubation time for hummingbirds.
Clutch Size
Robins lay clutches of 3-5 eggs. This moderate clutch size balances the nutritional demands on the parents versus needing to frequently lay new clutches.
Hatchling Growth
Robin chicks fledge from the nest at 14-16 days old. This is a slower growth rate than hummingbirds, but robins are larger birds. Robin nestlings initially weigh about 5 grams at hatching versus 2 grams for hummingbirds.
Egg Energy Content
The larger robin eggs contain more fat and protein to sustain embryonic development. This gives the hatchlings a boost in growth and survival compared to the relatively nutrient-poor hummingbird eggs.
Comparison Summary
In summary:
- Hummingbird eggs weigh 0.3 – 0.6 grams on average
- Robin eggs weigh around 3.3 grams on average
- The smallest hummingbird egg is 0.375 grams (bee hummingbird)
- Hummingbirds have a higher egg/body mass ratio than robins
- Tiny egg size allows hummingbirds to breed frequently
- Robin eggs are larger with more nutrients to help chick growth
Based on extensive research and data on egg sizes, hummingbirds definitively lay the smallest eggs of any bird. While robin eggs are smaller than average, they are much larger than the tiny eggs produced by hummingbirds.
Conclusion
Hummingbirds lay the smallest eggs of any birds due to their incredibly high metabolism, small body size, and frequent breeding strategy. The average hummingbird egg only weighs 0.365 grams, while the bee hummingbird lays eggs weighing just 0.375 grams. In comparison, robin eggs weigh 2.7 – 3.9 grams on average. The 10-fold difference between hummingbird and robin egg sizes results from evolutionary adaptations related to differences in the birds’ size, metabolism, nesting behaviors, and reproductive strategies. Research shows definitively that hummingbirds produce the smallest eggs in the avian world, enabling their unique hovering lifestyle.
