Defining Biological Phases with T-Plots: Woodstorks

The data used in this section is from four woodstorks Mycteria americana monitored by the Woodstork Project in the area of Georgia and Florida in the US (http://allison.clark.net/pub/wcsweb/stork/). The data were collected from 27.8.1996 to 20.5.1997. An overview of the data is given in figure 6.1.

Figure 6.1: Overview of the Woodstork data from Florida and Georgia.

In the T-x plot illustrated in figure 6.2b there seem to be no major changes within the observation period. When examining the figure in greater detail, a difference can be seen that in the first half of the observation period there is a higher tendency for excursions in the x-direction compared to the second part of the observation period. The picture is quite different when looking at the T-y plot shown in figure 6.2b. Here we can distinguish several phases in the data. In the first phase, the birds stay relatively constant at the same location (latitude in this case). After that, quite an abrupt and simultaneous change can be observed when all four birds leave the area. Now we can distinguish two types of movements. The first type is shown by the woodstork represented with yellow markers in figure 6.2b. It moves directly without any longer resting periods to the southernmost area where it will stay. The same is true for the woodstork with the green color in figure 6.2b, with the difference that it only migrates about half the distance compared to the former bird. The second type of movements can be seen in the two birds marked with red and blue colors. They also leave the area at about the same time as the other ones, but they make a longer stay at the same latitude as the 'green' bird. After that they go on migrating further south until they reach approximately the same latitude as the 'yellow' woodstork which directly migrated down to that area.

Figure 6.2: a) T-x and b) T-y plots for the woodstork data from 27.8.1996 to 20.5.1997.

a
b

In the third period all birds^6.1seem to have stayed at the same locations without larger excursions. Interestingly there is a change in location in two birds (red and blue) moving further north, and when examining the plot for the bird indicated in green, the same is true yet to a much smaller amount. This may well indicate that this should be considered as a fourth period in the data. The fifth period of time starts with the spring migration back to the north. It is interesting that the woodstork which already made a longer stay during the migration southwards again stays for some time at a latitude in between the winter residence and its breeding site. Another thing that is noteworthy is that it looks as if the spring migration takes a longer time than the migration in fall. In European regions the inverse is normally observed for birds. The spring migration is shorter than the fall migration.

These two T-plots illustrated are easy to create, but as became evident above, they provide a splendid overview of the spatial movements of these birds.