Knowing how and where birds migrate and breed is an important part of understanding how and why their numbers increase or decrease over time. However, we don't know much about the exact migratory patterns of most birds. After all, they are one of the most itinerant animals on earth, coming and going from one place to another as regularly as the seasons change. Where are they going? How will they get there? Why do they go? Based on what we currently know about migration, we can assume that they head toward areas where the weather is more conducive to survival and breeding. Now, a new technique to track birds is helping researchers understand an important concept called migratory connectivity.
Migratory connectivity is the degree to which breeding and non-breeding populations of birds are linked to one another; it is the relationship that helps us understand how these interactions contribute to the natural ecology of the animals' habitats. Until recently, the mark-recapture method has been the only technique used to do this. This method includes tagging individual birds and recording where they were recaptured, if they were recaptured at all. Unfortunately, this method was not always successful if the bird was not caught again.
Researchers recently tried a new method to track gray catbirds (Dumetella carolinensis), in addition to the older mark-recapture technique. The scientists fit 13 male and nine female birds with geolocators, which are special devices that resemble tags on the birds' legs and that can record the estimated latitude and longitude of the wearer based on sunlight levels every 10 minutes.
Birds wore these during the breeding and non-breeding seasons from July 2009 through May/June of 2010, when researchers recovered the devices. Only three males and three females were recaptured, so the data from the six geolocators was the only new information that the researchers had to work with. They were able to successfully download this data and use special software to correct and calibrate any errors in the information. They were also aware that the readings were sometimes slightly skewed if the bird had been perching in a shady area rather than in direct sunlight, but despite this, the team was able to get a good impression as to where the birds had been.
Next, the data was compared to previously documented mark-recapture records from 1914 to 2009 in order to provide a wide-range view of migratory connectivity. The data from both were similar, indicating a strong connectivity. It also showed that the gray catbirds that bred in Washington D.C. migrated to Florida and the Caribbean during the winters. In addition, long-term mark recapture data from the U.S. Geological Services Bird Banding Lab indicated that gray catbirds from the Midwest migrate down to Central America during the winters.
Although the data was consistent from both sources, there are limitations to both. The geolocators require proper light so that data is not misrepresented. The birds wearing these devices may also have trouble getting around, as weight and drag are increased, and a piece protrudes unnaturally. This can ultimately affect their survival. The authors did state however, that statistically the recapture and return rates of birds from both the geolocator study and the historical records were about the same. Data collected via mark-recapture techniques only seems to have meaning when the data is collected over a long period of time.
This article summarizes the information in this publication:
Ryder, Thomas B., Fox, James W., and Peter P. Marra. 2011. Estimating Migratory Connectivity of Gray Catbirds (Dumetella carolinensis) Using Geolocator and Mark-Recapture Data. The Auk 128(3):448-453.
Understanding the connectivity between breeding and nonbreeding populations of migratory birds is fundamental to our knowledge of biological phenomena such as population dynamics and dispersal. Moreover, our ability to quantify migratory connectivity has inevitable consequences for both conservation and management of species that utilize distinct geographic locations. Technology is rapidly advancing our ability to track birds throughout the annual cycle and to collect data on the degree of connectivity among breeding and nonbreeding populations. We combined two direct methods, mark–recapture (n = 17) and geolocation (n = 6), to estimate the migratory connectivity of breeding and nonbreeding populations of Gray Catbirds (Dumetella carolinensis). Data from geolocators show that birds breeding in the Mid-Atlantic overwinter in both Cuba and southern Florida. Mark–recapture data supported our geolocator results but also provided a broader spatial perspective by documenting that Mid-Atlantic and Midwestern populations occupy distinct geographic localities during the nonbreeding period. This research underscores the importance of geolocators, as well as other tools, to advance our understanding of migratory connectivity. Finally, our results highlight the potential value of U.S. Geological Survey (USGS) Bird Banding Laboratory mark–recapture data, which are often underutilized in ornithological research.
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