This meeting and presentation took place entirely online via Zoom.
At 7:00 pm, President Debbie Mullins called the Society meeting to order.
President Mullins made the following announcements:
The Christmas Bird Count begins this weekend; the Brooklyn count is Saturday and the New York County count is Sunday. Registration is required; the form to register can be found on the New York City Bird Alliance website. For counties outside of New York City, information can be found on the New York State Ornithological Association website, nysoa.org.
Governor Hochul still has not signed the New York State Horseshoe Crab Protection Act, and there are only a few days left to do so. The bill puts forth measures to help protect horseshoe crabs, whose numbers have dropped to unhealthy levels. President Mullins noted that New Jersey and Connecticut already have legislation in place. She encouraged people to contact Governor Hochul tonight by email using this address: correspondence.office@exec.ny.gov.
Field trips will resume in January, and more volunteers are needed to serve as registrars to manage the trips. Please send an email to trips@linnaeannewyork.org if you would like to volunteer to become a trip registrar.
President Mullins welcomed the following new Linnaean Society members:
- Signe Hammer
- Stephen Kelly
- Mary Kentros
- Steven Kessler
- Anne Swaim
- Shaul Yahil
President Mullins invited non-member attendees to join the Society. The membership application is on the Society website.
At 7:04 pm, President Mullins turned to the lecture program and introduced the speaker, Dr. Christina P. Riehl, associate professor of ecology and evolutionary biology at Princeton University. Dr. Riehl is a fellow of the American Ornithological Society, and editor-in-chief of the journal Ornithology.
Lecture: “Cheaters and Collaborators: The Evolution of Cooperative Breeding in a Tropical Bird,” presented by Dr. Christina P. Riehl
The tropical bird referenced in Dr. Riehl’s lecture title is the Greater Ani, a Central and South American species in the cuckoo family that Dr. Riehl has been studying for nearly twenty years. She shared her fascination with these birds, giving an in-depth discussion of where and how they live and their behavior, and described her research on their unusual breeding system and its evolution.
Dr. Riehl does field work in central Panama in the middle of the Panama Canal, at Barro Colorado Island. It is a tropical habitat, and the Greater Ani live there year-round. During breeding season, to protect their nests from depredation, the birds nest in the emergent vegetation along the shoreline, or in tree branches that hang over the water’s edge. Access for observation, banding and DNA sampling is by motorboat, and remote monitoring by camera.
Dr. Riehl described the breeding strategies of the Greater Ani. They are communal nesters like the other species of their genus, but unlike them, the Greater Ani seem to be obligate cooperative breeders (they must nest with more than one pair): in all the years that Dr. Riehl has been observing the Greater Ani, she has never found a nest with a lone pair. And, echoing a strategy that cuckoos are well-known for, Greater Ani are also parasitic breeders.
Greater Ani Communal and Cooperative Nesting
Both males and females from at least two breeding pairs contribute to building a single nest. The females lay their eggs in the same nest, and all parents share the parental care. Females cannot recognize which eggs are theirs, nor which chicks. Some nesting groups also include a non-breeding helper that assists in raising the young (often an offspring from the previous year of one of the breeding pairs); this behavior is referred to as cooperative breeding rather than communal breeding. Cooperative breeders are rare in North America, but one example is the Florida Scrub Jay.
Why did this system of communal/cooperative breeding evolve? Why not breed as monogamous pairs with biparental care as most birds do? Dr. Riehl identified some of the costs of cooperative breeding. One has to do with the female competitive behavior shown by females during the initial part of the egg laying phase: until a female has laid her own egg, she will eject any egg that she finds in the nest. Once she starts laying eggs, she stops pushing any eggs out of the nest, likely because she would risk destroying her own. This behavior also likely limits the upper number of breeding pairs sharing a nest: when more females are present, more eggs get ejected; in that case, what normally happens is that either the nest is abandoned, or some of the pairs leave the group. Dr. Riehl looked at data to see if there was something that would predict which female would lay first, and which would lay last, but found no clue. Eventually, however, in the course of several breeding cycles, each female got to be the first one to lay its egg; thus the cost of losing eggs was equally shared across the breeding group.
One of the benefits to nesting in groups is that it provides protection of the eggs from predators. For the Greater Ani, predators are monkeys and snakes, and the anis will mob them to keep them away from the nest. Furthermore, the loss of eggs due to nest predation is substantially greater than the losses due to competitive egg-tossing out of the nest by the females. It would seem that larger numbers of nesting pairs would be more effective against predators, and Dr. Riehl shared graphical data which seemed to support that assumption. But further evaluation led her to a different conclusion: it turns out that weather patterns also have an impact on the success rate of reproduction that can cancel out the benefits of large nesting group size. During years with wet conditions during the breeding season, the nests with larger numbers of pair groups do indeed do better than smaller groups; however, Dr. Riehl noticed that during cycles of the El Nino-Southern Oscillation, which result in a dry period in Panama and hence not much food, the anis do poorly, and the groups with many nesting pairs do far worse than two-pair groups. Hence the costs and benefits of the size of the group vary over time depending on the weather patterns, and the reproductive difference between two-pair and three-pair groups averages out over time.
Conspecific Brood Parasitism
Until recently not recognized, Greater Anis also breed by conspecific brood parasitism, meaning a female will lay her egg in the nest of another of her own species. At first, when Dr. Riehl came upon an egg with a genotype that didn’t match any of the females in the group, she assumed it was a genotyping error; but at some point, her team began to observe that females who were not part of the social group would approach a nest, and be chased off. This seemed to suggest the possibility of parasitic eggs (eggs laid by a Greater Ani outsider, not of the same social group), which they ultimately concluded was the case.
Dr. Riehl noted that the prevailing hypothesis for how this communal breeding system evolved was through conspecific brood parasitism behavior, and she is interested in understanding the costs and benefits of parasitism. By observing the Greater Ani nests, Dr. Riehl’s team followed the changes in the appearance of the eggs during incubation, and watched to see if the nesting birds could recognize a parasitic egg, particularly when it was laid at the wrong time (because in that situation the egg would likely look different than those of the host clutch). A female would often would eject the parasitic egg. Parasitic eggs that do manage to be incubated with the host eggs still have a low hatching success. Why, then, do females act as parasitic layers? Because Dr. Riehl has been collecting data for so many years, she was able to analyze around 2,000 eggs collected over a ten year period, and identified 65 parasitic eggs whose mothers she could track. Of those, 55 were laid by females who were part of social groups. She concluded that they had begun the breeding season as cooperators, but if their nest was depredated while they were still laying, a minority of them would parasitically place their eggs in another Greater Ani social group’s nest.
Dr. Riehl concluded her talk by encouraging those in the audience who might go birding in the tropics to look out for anis, identify which are Smooth-billed, Grooved-billed and Greater, watch their very interesting communal behavior, and look under their nests to see if there are piles of rejected eggs.
At 7:51 pm, Vice President Doug Futuyma told Dr. Riehl how much he enjoyed her fascinating talk. He then hosted the Q&A session, which covered more details about the Greater Ani, including their lineage and connections to other cuckoos.
At the conclusion of the Q&A, Vice President Futuyma thanked Dr. Riehl for a marvelous talk about a fascinating species and a fascinating life history, and at 8:15 pm the meeting was adjourned.
A recording of this meeting in its entirety can be found on the Linnaean Society of New York website.
Respectfully submitted by Lisa Kroop, Recording Secretary