Table 1.
AFDM (mg) versus crab width (mm), ash mass (mg) versus crab width (mm), DM (mg) versus crab width (mm) and handling time versus crab width (mm) for both crab species.
For Portunus we also determined the carapax AFDM (mg), ash mass (mg) and DM (mg) versus crab width (mm).
Table 2.
Crab characteristic of the crabs offered in the dichotomous prey choice experiment.
The number of crabs offered (n) as well as the average crab size (± SD) is shown. Average (± SD) AFDM (mg), handling time (s) and ash (mg) was calculated based on the crab sizes of each individual crab using the formulas in Table 1. Average (± SD) profitability (ei/hi) was calculated by dividing AFDM (mg) by handling time (s) for each individual crab. Average (± SD) digestive quality (ei/ki) was calculated by dividing AFDM (mg) by ash (mg) for each individual crab.
Fig 1.
Crab plover facing two different prey items in two separated trays.
The left tray contains a small Portunus and the right tray a Macrophthalmus.
Table 3.
Crab characteristic of the crabs offered in the cafetaria experiment (2 trials).
The number of crabs offered (n) as well as the average crab size (± SD) is shown. Average (± SD) AFDM (mg), handling time (s) and ash (mg) was calculated based on the crab sizes of each individual crab using the formulas in Table 1. Average (± SD) profitability (ei/hi) was calculated by dividing AFDM (mg) by handling time (s) for each individual crab. Average (± SD) digestive quality (ei/ki) was calculated by dividing AFDM (mg) by ash (mg) for each individual crab.
Fig 2.
(a) AFDM (ei) plotted versus crab width (mm) (note the logarithmic axes). Grey triangles represent Macrophthalmus and blue dots represents Portunus. Non-filled rhombs represent the amount of AFDM (ei) that was left in depredated carapaxes of Portunus. The amount left in the carapaxes in terms of AFDM (ei) was subtracted from the AFDM (ei) of intact Portunus, yielding the amount of AFDM (ei) obtained by crab plovers, superimposed with a red line. For formulas see Table 1. (b) Handling time (hi) plotted versus crab width (mm) (note the logarithmic axes). Handling time does not increase significantly with size for Macrophthalmus (superimposed with a grey line), while for Portunus handling time significantly increases with size (superimposed with a blue line). For formulas see Table 1. (c) Profitability (AFDM (ei) / handling time (hi)) plotted versus crab width (mm) (note the logarithmic axes). Grey triangles represent Macrophthalmus and blue dots represent Portunus. Profitability significantly increases with size for Macrophthalmus (y = 0.22x1.85; superimposed with a grey line), while for Portunus profitability does not significantly increase with size (y = 5.46x0.03; superimposed with a blue line).
Fig 3.
Intake rate (prey/s) plotted versus ash content of that prey (g/prey) in the ad libitum experiment.
The line represents the relation: 10log(IR) = -3.80 – 10log(ash).
Fig 4.
Energy intake rate (ei/hi) (mg/s) was plotted versus ballast (ash) intake rate (ki/hi) (mg/s) for both crab species. The grey triangle represents Macrophthalmus (n = 28), the light blue square represents small Portunus (n = 23) and the blue dot represents large Portunus (n = 43). Arrows represent the standard error of the mean. The inset gives a more detailed view of the Portunus size classes. The long solid black line represents the slope in terms of energy per ballast (ei/ki) for Macrophthalmus, whereas the short solid black line on the left represents the slope for Portunus. The solid red line represents the digestive constraint (c) (0.16 mg/s ash). For both prey species ki/hi > c, which means that the highest long-term energy gain can be obtained by choosing the prey with the highest slope (in this case Portunus).
Fig 5.
Dichotomous prey choice experiment.
The cardinal preference rank is plotted against prey type. A higher cardinal preference rank (y-axis) indicates a higher preference over the other prey species. Arrows represent the standard error of the mean. Large Portunus is set to zero (no SE) as we compared Macrophthalmus and small Portunus to large Portunus. We found crab plovers to prefer Macrophthalmus over large Portunus (t-value = 3.480, p = 0.001). We also found crab plovers to prefer small Portunus over large Portunus (t-value = 3.135, p = 0.003). We found no difference in preference for Macrophthalmus versus small Portunus (t-value = -0.587, p = 0.560).
Fig 6.
Cumulative intake (# prey) has been plotted on the y-axis and time (min) on the x-axis. Each point represents a crab that has been eaten. For trial 1 we obtained the whole video. For trial 2 the camera failed after some time. The vertical line represents the moment of camera failure. We know the number of crabs that were eaten after camera failure based on the number of crabs that were left after the feeding trials. These crabs have been plotted on the right side of the vertical line. Note that we do not know when these crabs were eaten and in which order. For simplicity, we plotted them in constant intervals to the end of the feeding trial. The birds had an initial preference for Macrophthalmus. In both trials the crab plover switched its prey choice from Macrophthalmus to small Portunus to the end of the feeding trial. The trials were conducted on two different birds.