Spotlight: An Abundance of Shorebirds in Panama

A stretch of mangrove and mudflat extends 200 km east of Panama City in the Republic of Panama. In some places, at low tide, the mudflat extends out an astonishing 4 km. In other places, where the wetland sits right next to a metropolitan area of over 1.5 million people, the mangroves are literally shadowed by condominium towers.

This is the Bay of Panama.

It is said that the word Panama is derived from an old indigenous word meaning abundance. The Bay of Panama lives up to its name. This wetland supports over a million shorebirds annually¹. It is one of the most important sites for migratory shorebirds in the Western Hemisphere and it is a designated a site of Hemispheric Importance in the Western Hemisphere Shorebird Reserve Network (WHSRN) and a Ramsar Wetland of International Importance.

In this issue of Wader Study, Kaufmann and colleagues from the Panama Audubon Society describe 5 years of monitoring work on this incredible wetland². They recorded 23 species of birds and estimated winter peak counts totaling more than 300,000.

That is a lot of birds to count over long stretches of coastline. How did the researchers manage to census (count and identify) all those birds over such a large area? First, they divided the larger area into sites and subsites that could be accessed from the ground by foot or car. This yielded three main sites: Panama-Juan Diaz, adjacent to Panama City, Pacora-West just to the west of the Río Pacora, and Pacora-Chico between the Río Pacora and the Río Chico. They then divided these sites into 26 subsites covering a total of 21.2 km between Panama City and the Río Bayano. These sites are also part of the Migratory Shorebird Project, composed of partners from 12 countries along the western flyway.

By breaking the sites into subsites and with the help of a few volunteers the Panama Audubon staff could census all three sites with teams of two to four people during 5 to 6  neap tides in January and early February from 2013 to 2017. Surveys began when the rising tide pushed birds off their feeding grounds 500 to 4000 m offshore to within 300m of shore where they could be counted. Neap tides were used to provide the maximum amount of time for birds to be within 300m of the shore and so that the exposed mud in front of the mangroves would be dry enough to walk on.

To help with counting shorebirds of different types, the researchers also classified species into groups by size: small, medium and large. The details can be found in Table 2, but examples of small shorebirds include Western and Semipalmated Sandpiper, of medium shorebirds include the Black-bellied Plover and Short-billed Dowitcher and of large shorebirds include Willet and Whimbrel.

Most birds were generally counted one-by-one, but to count the huge numbers of small birds, up to 200,000 in a flock, the researchers counted them in groups of 100 usually with a scope and using a hand-held tally counter. During one survey at the Pacora-Chico site the birds sat still long enough for the researchers to take a series of 51 overlapping telephoto pictures (which they estimated to cover about two thirds of the birds present at that site). They digitally stitched the photos together, superimposed a grid, and counted the birds that way. They took this to be their best estimate of numbers at that site.

It is notable that Kaufmann and colleagues chose to census birds during the northern winter. Shorebirds are most abundant in the Bay of Panama during fall migration and most bird monitoring has taken place then^1,3,4. Yet Kaufmann and colleagues point out that fall numbers are constantly changing as birds pass through, whereas winter monitoring provides data on a relatively stable population for the season and a good estimate of the local wintering population. It can also serve as an index of habitat quality for all seasons.

The authors report that small shorebirds were the most abundant shorebird type in the bay comprising 95.5% of all shorebirds counted over the five years. This group of small sized birds was about 68.7% Western Sandpipers, 14.1% Semipalmated Sandpipers and 13.5% Semipalmated Plovers. The remaining birds were all medium or large shorebirds composed mainly of Whimbrel, Marbled Godwit, Willet, Black-bellied Plover, and Short-billed Dowitcher.

An interesting finding of the study is how the birds distributed themselves over the shoreline west to east and urban to rural. Four of the five common medium and large species, Marbled Godwit, Willet, Black-bellied Plover, and Short-billed Dowitcher were concentrated closer to the city at Panama-Juan Diaz in the west. In contrast, small shorebirds were almost 15 times denser (linear density) at Pacora-Chico about 20 km east of Panama-Juan Diaz. This uneven distribution might mean that different species have different high tide habitat preferences.

Panama-Juan Diaz, which large and medium sized shorebirds seemed to prefer, is now an urban site. Over the past 30 years the proportion of coastal mangroves has increased, going from mangroves occupying 51% of the coast in 1987 to 89% in 2016. The biggest changes happened between 2003 and 2013, when low lying wetlands inland from the coast were filled in and developed. During this period, the entire shoreline advanced seaward by 150 to 300 m, including part of the waterfront of the oldest part of the city, Panama Viejo, founded in 1519. This filling and development, coupled with sand mining up to and including the channel of the Río Pacora likely provided the source of sediment that fueled the seaward advance of the coastline and the growth of mangroves. The new mangroves may provide attractive roosting areas for Willets and Whimbrels at high tide and paradoxically insulate them from human disturbance. The mudflats seaward of the mangroves are often inaccessible compared to the beach at Pacora-West, which still has an active sand extraction operation, or the beach at Pacora-Chico which fishermen commonly drive along. While this urban habitat may attract large and medium sized shorebirds, it may also expose them to higher levels of pollution than they would experience farther from the city.

At high neap tides the small shorebirds prefer the mudflats adjacent to the beaches without mangroves at the Pacora-Chico site further east. This site has been relatively unchanged in terms of mangrove coverage which was 14% in 2000 and 12% in 2016. Large numbers of small shorebirds once roosted in the low-lying, but mangrove free, wetlands closer to the city, but these have been developed into condominiums and subdivisions. The tall buildings now adjacent to Pacora-Juan Diaz, but absent at the other two sites, provide roosting areas for overwintering Peregrine Falcons which are known to drive shorebirds away from otherwise acceptable sites.

Western and Semipalmated Sandpipers and Semipalmated Plovers at the Pacora-Chico site in the foreground and Panama City in the background about 20 km away. (photo: Karl Kaufmann).

A final question is whether the total number of birds using the monitored area changed over time. In the five-year study, Kaufmann and colleagues found no change in the populations of any species except for Marbled Godwit, which decreased. Over the longer term, they found more shorebirds than expected for the winter period, compared to numbers found over larger areas in a previous aerial survey in the fall of 1993³. However, it is difficult to determine whether this is due to differences in methodology or to actual population changes. Regardless, this research shows that the Bay of Panama hosts more winter birds than previously understood and that it remains an important area for shorebirds.

1. Watts, B.D. 1998. An Investigation of Waterbirds within the Panama Canal Zone and the Upper Bay of Panama (lay summary). 68 pp. Center for Conservation Biology, College of William and Mary, USA.
2. Kaufmann, K., R. Miró, Y. Díaz, M. Caballero, S. Carty & Panama Audubon Society. 2018. Monitoring winter shorebird populations in the Bay of Panama – 2013-2017. Wader Study 125(2): 97-106.
3. Morrison, R.I.G., R.W. Butler, F.S. Delgado & R.K. Ross. 1998. Atlas of Nearctic shorebirds and other waterbirds on the coast of Panama. Canadian Wildlife Service, Ottawa.
4. Buehler, D.M., A. I. Castillo & G. R. Angehr. 2004. Shorebird counts in the Upper Bay of Panama highlight the importance of this key site and the need to improve its protection. Wader Study Group Bulletin 105:56–64.

PDF is available for download here: https://www.waderstudygroup.org/article/11591/