Week 62 – 24 September 2017:
Eggs For Supper? Again?
If you are not familiar with the foraging behaviour of Black Skimmers, Rynchops niger, I would like to suggest that you search for videos online. I will not spoil the surprise by describing that behaviour, but sincerely believe that you will not be disappointed.
Black Skimmers range from the southern United States and Mexico to Venezuela and Argentina, with a global population that is considered to be of least-concern, but in decline. Seabirds, these skimmers nest in modest-sized colonies in open situations, such as gravel and sandy beaches. While many seabirds nest on remote islands, presumably in order to avoid mammalian predators, Black Skimmers sometimes breed in less-isolated situations.
Elizabeth Forys of Eckert College in St. Petersburg and her colleagues observed the nesting of skimmers in Pinellas County on the west coast of Florida. Here the birds chose beaches that were lined with condominiums and hotels, and each day the birds shared their breeding habitat with hundreds of sun-seekers. Unfortunately, these Black Skimmers also had to share their world with Fish Crows, Corvus ossifragus.
Between 2005 and 2011, a colony of Black Skimmers had a degree of breeding success, fledging between 22 and 72 chicks each year. Things started going wrong for the skimmers in 2012, when they experienced complete breeding failure as a result of egg-predation by Fish Crows. Forys and her colleagues considered using aversion conditioning on the crows. In this scenario, some eggs would be treated with a chemical that would make crows ill, but would not kill them, training the predators to avoid eating skimmer eggs in the future. This sort of conditioning has been used with success in the past.
However, the sorts of chemical that will cause a crow to vomit require permits. While waiting for those permits to be processed, Forys and her crew were advised to try a different approach. Within a colony of 72 pairs of Black Skimmers, the research crew placed a number of models of dead crows. Some of the effigies were hung above the ground, and others were laid directly on the beach. Would these gruesome scarecrows deter hunting by Fish Crows, protecting the Black Skimmers’ eggs?
In a word, no. Forys et al. wrote: “After we placed the effigies, several crows flew to where they (the models) were placed and examined them, but quickly resumed their previous activities.” An egg was taken by a crow less than ten minutes after the crow models were installed, and it was not long before all of the eggs had been preyed upon. On the final day of the study, June 3, the skimmers abandoned the colony, and they did not attempt to breed again that year.
Fish Crows are not evil. Instead, they are clever birds that take advantage of foraging opportunities. The activities of a small number of crows on a single urbanized beach in Florida will not imperil the long-term survival of Black Skimmers. However, the work of Forys and her colleagues shows that some conservation questions do not have simple answers. The more we know about the natural history of all the players, the greater the likelihood that we can create solutions.
Forys, E. A., D. Hopkins, P. Ingham, M. Miller and L. Gluckman. 2015. Do effigies deter Fish Crows hunting in a Black Skimmer colony mid-season? Southeastern Naturalist 14:635-640.
Photo credits: Black Skimmers painting © Julianne Felton – fineartamerica.com