Emlen funnel experiments revisited: methods update for studying compass orientation in songbirds

Abstract

Migratory songbirds carry an inherited capacity to migrate several thousand kilometers each year crossing continental landmasses and barriers between distant breeding sites and wintering areas. How individual songbirds manage with extreme precision to find their way is still largely unknown. The functional characteristics of biological compasses used by songbird migrants has mainly been investigated by recording the birds directed migratory activity in circular cages, so-called Emlen funnels. This method is 50 years old and has not received major updates over the past decades. The aim of this work was to compare the results from newly developed digital methods with the established manual methods to evaluate songbird migratory activity and orientation in circular cages.We performed orientation experiments using the European robin (Erithacus rubecula) using modified Emlen funnels equipped with thermal paper and simultaneously recorded the songbird movements from above. We evaluated and compared the results obtained with five different methods. Two methods have been commonly used in songbirds' orientation experiments; the other three methods were developed for this study and were based either on evaluation of the thermal paper using automated image analysis, or on the analysis of videos recorded during the experiment.The methods used to evaluate scratches produced by the claws of birds on the thermal papers presented some differences compared with the video analyses. These differences were caused mainly by differences in scatter, as any movement of the bird along the sloping walls of the funnel was recorded on the thermal paper, whereas video evaluations allowed us to detect single takeoff attempts by the birds and to consider only this behavior in the orientation analyses. Using computer vision, we were also able to identify and separately evaluate different behaviors that were impossible to record by the thermal paper.The traditional Emlen funnel is still the most used method to investigate compass orientation in songbirds under controlled conditions. However, new numerical image analysis techniques provide a much higher level of detail of songbirds' migratory behavior and will provide an increasing number of possibilities to evaluate and quantify specific behaviors as new algorithms will be developed.

Keywords: Computer vision; image analysis; magnetic alignment; navigation.

Figure 1

European robin (Erithacus rubecula) was the species used in this study to compare different methods to evaluate orientation assays performed in the Emlen funnel. Photograph by Magnus Hellström.

Figure 2

Procedure for the automatic evaluation of claw marks left on thermal paper. (A) The thermal paper is first digitalized in 8‐bit gray scale file and successively manually selected in the scanned image (shaded yellow area). (B) The selection is automatically straightened to align the claw marks along the vertical axis. (C) The image intensity profile (see text for definition) is plotted and successively smoothed with a 6° moving average filter before being used for orientation evaluation.

Figure 3

Screenshot of manual annotation procedure of the videos recorded during the orientation experiments performed in modified Emlen funnels (300 mm top diameter). The bird's orientation is annotated by manual clicking on the position of the beak of the bird during the attempt to take off (example shown for the top right cage).

Figure 4

Four consecutive frames of a crop of a single cage of the video recorded during the orientation experiments showing features automatically extracted with the computer vision method. (A) Before taking off, the position of the bird is annotated as stationary mode (red cross) and the bird's body alignment is determined as the main axis of the ellipse fitted around its body. (B) The bird quickly moves during takeoff, and its position is tracked as flying mode (green cross). (C) The bird is still in flying mode while reaching the sloped wall of the funnel. (D) The bird hits the wall, and its position is now back to stationary mode (red cross). The takeoff direction is determined as the bird position before and after a mode transition that is from (A) to (D), and it is reported (purple line) on all four panels. Notice how the direction of takeoff is taken from the bird's position before taking off in (A) and not from the center of the cage as for the manual annotation procedure (Fig. 3). (see also the example in Movie S1).

Figure 5

Mean orientation of European robins recorded in modified Emlen funnels obtained with five different methods. Each dot outside the unit circle indicates the mean orientation of a single bird for all three assays performed. The blue lines show group mean angle (α) drawn in the unit circle relatively to the mean vector length (r). The dashed circles indicate the minimum length of the mean vector needed for 5% significance according to the Rayleigh test (Batschelet 1981). The 95% confidence interval (gray area) is reported for significantly oriented distributions. Comparison between methods is presented as correlation table in Figure S8.

Figure 6

(A) Comparison of different methods to estimate the activity expressed by European robins in circular cages. Results are reported as birds’ mean ± SE (n = 4) of three consecutive 1‐h assays performed at sunset and after 1.5 h and 3 h. Activity values are normalized within each method for comparison purpose. Differences between methods are discussed in the text. (B) Relationship between number of scratches manually counted on thermal paper and number of jumps detected by computer vision algorithm for all birds and assays. Linear regression (line and equation) and 95% confidence interval (shaded area) are also reported.

Figure 7

Example of temporal information obtained for one assay (Sunset, bird ring 2KG36805) using computer vision data. The mean orientation vector length (r) and the bird activity (expressed as jumps min⁻¹) are calculated using a moving window of 30 jumps (gray points) and presented as continuous line using a local regression smoothing. In the upper panel, when r is above the critical level of the Rayleigh test (1% level reported as black horizontal line according to Batschelet 1981), the bird shows directionality (i.e., the mean direction is not random). The shaded areas indicated with α ₁ and α ₂ represents the time intervals when the bird shows orientation significantly different from random. In the lower panel, the mean jumps frequency is also reported (as a black horizontal line) to show higher and lower levels of activity.