Franklin's gull is a bird that migrates long distances and requires precise timing. It winters along the west coast of South America until returning to the prairie wetlands of North America, where it nests in large colonies come springtime. The dark hood, gray wings and pink-tinted breast are a harbinger of spring to the people of the Northern Great Plains, who affectionately call it the prairie rose gull. Soon after large wetlands thaw, Franklin's gulls arrive to build floating nests from wetland vegetation to hold three green-and-black speckled eggs.
Inside these dark eggs, the developing chicks also sense spring days. "The growing embryos integrate signals from the nutrients provided to eggs by mothers with the amount of daylight," said Dr. Clark. "The signals let the chick know whether the egg was laid at the beginning, or at the end of the nesting period."
Clark and Reed note that chicks from eggs produced at the beginning of nesting take longer to hatch, but are larger than chicks from eggs laid at the end of nesting. Contrast that with eggs laid at the end of the nesting period, which hatch in less time, but at a smaller size.
"Chicks hatching later in the season have less time to grow, less time to become independent, and less time for flying lessons before they must migrate to South America in the fall," said Dr. Reed.
According to Dr. Clark, data indicate embryos in late season eggs appear to be sensing external environments and adjusting their development. These changes in development time and size may be important for chicks to successfully migrate.
Many birds, including Franklin's gulls, are arriving earlier on their breeding grounds. "This research suggests that the impacts of changing seasonal signals have far reaching effects on bird biology, including chick development," said Dr. Clark.
Researchers evaluated the ability of avian embryos to integrate cues of season from photoperiod and maternal environments present in eggs to produce season variation among phenotypes among Franklin's gull (Leucophaeus pipixcan) hatchlings.
Field research was conducted at the J. Clark Salyer National Wildlife Refuge and Lake Alice National Wildlife Refuge in north-central North Dakota along the Souris River. Researchers collected early and late season eggs, separating some into component parts and incubating others for short or long photoperiods. Upon hatching, chicks were evaluated for size and yolk sac reserves.
Results of the study show that hatchling size is sensitive both to egg contents provided by mothers and to photoperiod, and development time increases across the season. When cues of season from eggs are mismatched with cues from photoperiod, alternate phenotypes are created.
Clark and Reed also found that seasonal variation in egg size, yolk, albumen or shell content of the eggs does not account for the seasonal maternal egg effect on hatchling size. "We expect our results to initiate new studies on how vertebrate embryos integrate environmental cues with maternal effects and offspring responses to optimize the expression of offspring phenotype," said Clark.
Previous NDSU graduate students who participated in the research include Shawn Weissenfluh and Emily Davenport-Berg. Other NDSU students who assisted in the research include Nathaniel Cross, Peter Martin, Dan Larsen, Michelle Harviell and Andrew Nygaard, along with Petar Miljkovic from Grinnell College.
Research funding was provided by the National Science Foundation (IOS-0445848), the North Dakota Game and Fish Department and the U.S. Fish and Wildlife Service.