Gambling at a high-elevations: the risks of enlarged eggs for Mountain Bluebirds

Most studies that have looked at why female birds lay the number of eggs they do, and no more, have focused on the consequences of having too many mouths to feed. Few studies have focused on potential problems with having too many eggs to heat. One rarely tested hypothesis suggests that females lay as many eggs as they can effectively incubate. If they laid more eggs, then some or perhaps all eggs would fail to hatch.

I tested this hypothesis in Mountain Bluebirds breeding at an elevation of ~2500 m/8100 ft in Wyoming’s Bighorn Mountains. Most females in this population lay 6 eggs; < 5% lay 7 eggs. With clutches of 6, females arrange eggs in two rows of three, i.e., in a long rectangle that roughly matches the shape of the brood patch. Interestingly, though, when there are 7 eggs they are arranged in a broad circle, potentially making it more difficult to heat all eggs adequately. My approach was simple: At the end of laying, I added eggs to a random set of nests to create a clutch of 7. The nests I left unmanipulated as controls. I later checked each nest to determine how many eggs hatched.

Experimental nest in which 6 eggs were laid, but an extra, marked extra egg was added.

Ornithologists typically pray to the gods of field research for good weather. I, however, hoped for weather that would challenge my incubating females. As it happened, we had an extended period of unusually cold and wet weather in mid-June when many females were incubating. This should have increased my chances of detecting any negative effects of incubating an enlarged clutch.
Contrary to my hypothesis, I found that, on average, 95% of the fertile eggs in 39 enlarged clutches hatched compared to 92% of eggs in 44 control nests. My results clearly show that females do not stop at 6 eggs because they can’t effectively heat 7 embryos. Something else limits clutch size in this population.

One observation suggests a possible factor: During the foul weather, 5 of the 11 females incubating 7 eggs abandoned their nests (or died), compared to only 1 of 11 females incubating a smaller clutch. While more data are needed to confirm this trend, we know that larger clutches take more energy to incubate. Thus, in bouts of bad weather, females incubating 7 eggs could more often face a situation where they risk starvation if they continue incubating and so abandon the clutch to save themselves for another day. Challenging weather may occur often enough in the mountains that females who lay 7 eggs would, on average, produce fewer offspring over their lifetimes than females who conservatively stick to 6 eggs.

Incubating female Mountain Bluebird.

With the support of a Bergstrom Award, not only was I able to conduct one of the most conclusive studies of the incubation capacity hypotheses to date, but the research led to an alternative hypothesis for what limits clutch size in birds. The award also provided me with my first real opportunity to do research, preparing me to do additional research in graduate school.

Awardee Mariah Maxwell repairing nest boxes.


Guest post by:

Mariah Maxwell
Towson University