When an Egg Won’t Hatch

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Three San Clemente loggerhead shrike eggs are ready to be tested.

Three San Clemente loggerhead shrike eggs are ready to be tested.

When an egg doesn’t hatch, what went wrong? Captive breeding of critically endangered birds, where every egg counts, can be tricky. Here in the Reproductive Physiology Division of the San Diego Zoo Institute for Conservation Research, we are working hard to assist avian conservation programs by developing new fertility testing methods. One new method, which I am helping to develop, is in ovo sperm detection.

Barren eggs are not uncommon, and every year we receive hundreds of potentially infertile eggs from the
San Diego Zoo, San Diego Zoo Safari Park, and offsite breeding facilities. Numerous undeveloped eggs from a given breeding pair may indicate the pair’s inability to produce offspring. This can occur for a variety of reasons, including sterility, genetic incompatibility, or the absence of mating. Although discovering the absence of sperm within an egg does not provide all the answers, it is a key piece of the puzzle that can help narrow down the potential cause of a reproductive problem.

When eggs arrive in our lab, the yolk membranes are removed and stained for sperm. There is nothing more exciting than locating a sperm under the microscope! Not only does this indicate that the male is producing viable motile sperm, but their presence provides hope that offspring from that pair may be possible in the future. Most of the eggs received by our lab are partially incubated, and we know that sperm slowly degrade during the incubation process. The determination of a sperm-degradation timeline is especially helpful for kiwi conservation. The kiwi has an unusually long incubation period of almost three months, so their eggs spend a significant amount of time incubating before they are declared potentially infertile and sent our way for sperm detection. Although one of the female kiwis at the Zoo is producing eggs, none of them have hatched. But because we have been able to identify sperm in a few of her eggs, now we know that she is actively breeding!

Last year, I also analyzed eggs from the San Clemente loggerhead shrike, a subspecies for which we have been managing a breeding facility funded by the U.S. Navy since the early 1990s. One of the captive pairs had laid a clutch and then abandoned it. The male was one of the program’s oldest birds, and Susan Farabaugh, Ph.D., the conservation program manager, suspected that he might be ready for retirement. She brought several fresh eggs to the lab from the fruitless shrike pair, as well as a presumed fertile egg from a prolific pair that had accidentally broken during incubation. Upon inspection, I was unable to locate a single sperm on the membranes from the unsuccessful pair, while the cracked egg contained numerous glowing sperm. This shed light on the root of the older male’s reproductive problem, and Susan immediately re-paired the female with a younger male. Her decision paid off and resulted in three additional chicks in 2012!

While sperm detection is not the only method for assessing breeding potential, it is an important management approach for our avian conservation programs. This technique is yet another tool to more confidently assess breeding pairs, allowing us to hatch the greatest number of chicks before the end of the breeding season. For birds on the verge of extinction, every chick brings the species one step closer to sustainability.

Kaitlin Croyle is an research assistant with the San Diego Zoo Institute for Conservation Research.