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free forced control trial type

Fig. 7.3. The percentage of the first four visits to the correct flowers by hummingbirds in the single visit to the array in Phase 2 of each of the three trial types - free, forced, and control (see text for details); mean values ± SE. Chance performance is 50%. (Redrawn with permission from Healy & Hurly 1995.)

eight flowers, the four it had emptied earlier (not refilled) and four containing sucrose. If birds could remember which of the flowers they had depleted, they should avoid these when they return to the maze. Instead, they should feed from the four previously unvisited flowers. We assessed the birds' performances by comparing their ability to visit the four flowers not previously visited in Phase 1 with chance performance (50%). In both forced and free trials, birds were significantly better than expected by chance at visiting the non-depleted flowers (Fig. 7.3). In control trials, used to determine whether birds were using cues emanating from the sucrose reward to choose which flowers to visit, performance did not differ from chance. Because birds could choose which flowers to visit in Phase 1 of the free trials, performance in these trials may have been due to the birds choosing a systematic route around the maze. Although the data were not sufficient to make a thorough examination of the way in which the birds moved around the array, we did look to see whether the birds simply moved from one flower to its nearest neighbor. They did not, so some more complex behavior must explain the good performance by birds on forced trials.

Birds may have made the task easier by returning to the maze soon after Phase 1. We were not able to control a bird's return, and on some trials the retention interval was as short as a few minutes; on other trials it was longer than one hour. Performance decreased slightly with increasing delay between phases, but the effect of increasing time between the two phases of a trial was not significant. In spite of the lack of control over the timing of a bird's return to the maze, we showed that these hummingbirds are able to remember the locations of at least three flowers within such a maze for intervals of at least one hour. When the hummingbirds' performance is compared with that of other species tested under laboratory conditions, the hummingbirds do less well. However, it must be remembered that, unlike laboratory-tested animals, these birds fill the retention interval with activities that may interfere with their memories for our experimental flowers. They were away from the maze for an average of 11 min, and in that time may have fed at least once from natural flowers. Alternatively, the apparent decline in performance might not be based on a failure of memory, but on faulty assumptions on our part. For example, if flowers refill sooner than we have assumed or if there is a high probability of foraging competition from other animals, then the birds might be more likely to return to previously rewarded sites even over the time-scales observed in this experiment. None the less, our experiment was clearly not quantitatively comparable to natural foraging, and there are no data to show that these birds can remember several hundred flowers. These data do show, however, that rufous hummingbirds can and will remember point locations of flowers in the field.

Hurly (1996) carried out a one-trial learning experiment in order to determine whether rufous hummingbirds use the information about a flower's contents, in addition to its location, to make subsequent visit choices. He presented an array of four flowers, one of which contained 600 ^l sucrose, too much for a bird to deplete on a single visit. The other three flowers contained equal volumes of water, which these hummingbirds prefer to avoid. The birds' performances were assessed by observing the number of flowers visited until the rewarded flower was relocated. Not only were birds very accurate at returning to the correct flower, the few errors they did make were to flowers they had not previously visited. These birds appear to remember both the contents and the locations of visited flowers, at least over time periods of less than one hour (mean time to return was 12.6 min). Given the changing nature of nectar supplies in the flowers in a bird's territory, it would be unlikely that the bird need remember all of this information for longer than a few hours.

Having demonstrated that male rufous hummingbirds are able to remember single flower locations, we then investigated how close together in space flowers can be before the birds find it difficult to discriminate between them. A bird was presented with a flower containing more sucrose than it could empty in a single visit. On its return, there were two flowers to choose from: the original, containing the remaining sucrose, and an alternative, containing water. On half of the occasions, the alternative flower was the same color as the original and on the others it was another color. Birds returned to the original flower significantly more often than expected by chance. Furthermore, birds most often chose the original flower when the distracter was of a different color, whereas performance was poor when the distracter resembled the original flower (see Fig. 7.4). However, there was a significant interaction between distracter color and distance: when the flowers were of different colors, birds chose the original flower more accurately the further it was from the distracter. When the flowers were the same color, the birds were, if anything, more likely to visit the distracter the further it was from the original flower. This result suggests that the hummingbirds were able to discriminate between the original and distracter flowers under both conditions, but that they employed different foraging tactics according to the color of the distracter in the choice phase. It seems sensible for a bird to sample new flowers that look like flowers that have been rewarding previously, but to learn very quickly to avoid flowers of a color that has never been rewarding. It also seems that these birds may be able to remember and to discriminate between flowers separated by only 3 cm. This may mean that plants could reduce or increase revisiting "errors" by producing flowers spaced apart or by clumping them very close together. Although the results from the cue-dissociation experiment show that rufous hummingbirds seem to prefer to attend to, or to remember, the global position of flowers rather than their color/patterns, it seems as if the birds also discriminate and generalize the local visual features of flowers and alter their foraging tactics accordingly. Two different learning rules, then, may be used depending on the flower cue: the color/pattern can be used to generalize across flowers or to discriminate among them (within and between species) and the likely average profitability can be assessed. The location of a flower, on the other hand, is quite specific and its expected profitability has a temporal aspect (Hurly & Healy 1996).

We have demonstrated that hummingbirds attend more to spatial

3 40 80

Distance from focal flower (cm)

Fig. 7.4. The number of choices (out of 10) made by six rufous hummingbirds in favor of the focal flower when the distracter flower was either the same or a different color and was placed 3, 40, or 80 cm from the focal flower. Values are means and SE. (Figure reproduced with permission from Hurly & Healy 1996.)

3 40 80

Distance from focal flower (cm)

Fig. 7.4. The number of choices (out of 10) made by six rufous hummingbirds in favor of the focal flower when the distracter flower was either the same or a different color and was placed 3, 40, or 80 cm from the focal flower. Values are means and SE. (Figure reproduced with permission from Hurly & Healy 1996.)

cues than to color/pattern cues when making choices about revisiting flowers. However, the visual cues appear to enhance the rate of learning which locations are rewarded. In a laboratory task, Brown & Gass (1993) found that rufous hummingbirds learned the location of a rewarded feeder in an array faster when prominent visual cues were added. Visual conspicuousness, then, seems to work both to attract hummingbirds to flowers in the first place and, additionally, to increase the speed of learning a location (see also Healy & Hurly 1998, experiment 2). Enhancing the visual conspicuousness of its flowers may then have a dual benefit to a plant as the birds typically make very short visits to flowers (on the order of a few seconds).

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