Breed early, breed late?

Investigating the mechanisms regulating bird reproduction timing

A dark-eyed junco. Since 2000, IU researchers have been experimenting with breeding juncos in captivity to aid research.

The Problem

As climate change causes rapid shifts in seasonal changes, native animal species face new reproductive challenges.

In previous seasons, animals that reproduced during the spring or summer when food is most plentiful were most successful in rearing healthy offspring. However, that period of abundance is coming earlier and earlier due to climate change. This has placed new stressors on animals’ reproductive timing in Indiana and around the world.

The Project

With funding from the National Science Foundation, IU researchers Ellen Ketterson and Adam Fudickar and North Dakota State University professor Tim Greives are collaborating on a three-pronged study to learn more about the potential effects daylight length has on reproductive timing.

Animals that live in a seasonal environment, like birds, typically have reproductive systems that respond to daylight length. When daylight grows shorter in the winter, their reproductive systems shut down, but as days start to get longer, their brains release hormones that prepare their reproductive systems for the spring and summer.

Past research suggested this adaptation was genetic, and therefore could not be shifted by anything other than natural selection. However, recent studies have suggested that this behavior may have a basis in the animal’s development as well as its genes.

Greives is seeking to determine whether wild birds whose reproductive systems are ready earlier truly reproduce earlier. He and his team travel to the Mountain Lake Biological Station in Pembroke, Virginia each spring and summer to capture wild birds and measure their hormone levels to determine reproductive preparedness. Later in the year, the team then returns to observe whether those birds with more prepared reproductive systems reproduced earlier.

Fudickar is performing an experiment to determine if the timing of reproductive cycles in Dark-eyed Juncos is truly flexible based on daylight exposure during development. In summer 2021, he and his team captured newborn Juncos from the Mountain Lake Biological Station and raised them in two different conditions—one simulating lighting conditions in Alaska and the other simulating lighting conditions in Virginia. When the birds were old enough, they were moved to Kent Farm at IU, where their hormone levels and other physiological factors were observed weekly throughout their first breeding season, from February to April 2022. The birds were then euthanized so that their DNA could be studied to determine any differences in gene expression between the two populations.

Ketterson, alongside collaborator Devraj Singh, is examining seasonal transition of gene expression by sampling wild birds at different times of year—the middle of winter, the start of breeding season, and in the summer. By comparing different populations that reproduce at different times, the team is hoping to build a framework to understand the genetic, endocrine, and neuroendocrine mechanisms that determine when birds reproduce.

The Path Forward

Analysis of the physiological data on the captive Juncos is ongoing and expected to be completed in summer 2023. Research on the tissue and brain cells of these specimens will begin spring 2023.

Insights gained from this study could point to possible interventions on behalf of vulnerable species. If the Juncos are found to have flexible reproductive systems, other animal species might be similarly flexible and could be raised to quickly adapt to seasonal changes in their environment. Or, the research could highlight the inflexibility of animals’ reproductive timing, which may necessitate special wildlife protections.

Updated April 19, 2023