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Measuring time budgets with instantaneous sampling: a cautionary note
Anim. Behav., 1991, 42, 317-318
Measuring time budgets with instantaneous sampling: a cautionary note HANNU
POYS)k
Finnish Game and Fisheries Research Institute, Evo Game Research Station, SF-16970 Evo, Finland (Received29 September 1990; initial acceptance 2 November 1990; .final acceptance 26 November 1990; MS. number: sc-596)
Instantaneous sampling (Altmann 1974) has frequently been used as a method of sampling data for time budget studies of animal behaviour, e.g. individual vigilance (Caraco et al. 1980; Caro 1987; Lima 1987). In instantaneous sampling the observer records an individual's current activity at preselected intervals, e.g. every 15 s. While the main concern of the methodological analyses by Dunbar (1976) and Tyler (1979) was to compare different time sampling methods, these authors also briefly considered the problem whether the choice of sampling interval in instantaneous sampling has any biasing effect on the time budget result. Both authors concluded that, independently of sampling interval, instantaneous sampling gives reliable estimates of true time use. The analyses and conclusions made by Dunbar (1976) and Tyler (1979) are correct but two additional examinations are needed. First, the sampling intervals examined by both authors were short (i.e. 5, t0, 15, 30 and 60 s) relative to the mean bout length of the behavioural variable under consideration, i.e. 14.0-124.6 s in Dunbar (1976) and 40-50 s in Tyler (1979). However, in many avian species, for instance, the mean length of a behavioural sequence may be very short, e.g. scanning bouts of less than 1 s in the house sparrow, Passer domesticus (Elcavage & Caraco t983) and in the yellow-eyed junco, Junco phaeonotus (Pulliam et al. 1982). Here I extend the comparisons to include short behavioural sequences as well. Second, although averages calculated from several individuals may give reliable estimates of true time use, individual instantaneous values may differ drastically from real ones (see Table 5 in Dunbar 1976). As I show here, this may be a serious source of inaccuracy if one tries to examine whether variation in time use is caused by changes in an independent variable. I used the feeding-vigilance time budgeting of the teal, Anas creeca, as an example since data of real time budgets are easily available and the durations of behavioural sequences of an actively foraging teal are very short, averaging between 0003-3472/91/080317 + 02 $03.00/0
0.6 and 1.5 s (P6ysfi 1987). I use the data pooled over the feeding methods of head submerged and neck submerged. The behavioural sequences of an actively feeding teal consist of lowering the head to feed, followed by raising the head to an upright position. In the feeding posture the head is beneath the water surface and in the vigilance posture above the surface (see P6ys/i 1987). I arbitrarily chose each individual and recorded all head lowerings and raisings into a continuously running cassette tape recorder using short code words. Recordings were later transcribed on a F-J Electronics 363 intensity meter coupled with a Siemens Oscillomink L running at 50 mm per s. The duration of each feeding and vigilant sequence was measured from the peaks produced by the code words on the oscillograms to the nearest 0.1 s; the use of this accuracy was warranted because the changes of the behaviour of foraging teals were easy to code in the field (i.e. only two behavioural states) and because a running speed fast enough was used with the plotter when transcribing the field recordings in the laboratory. This procedure gives the real proportions of time each individual spent on feeding and vigilance. From the same oscillograms, instantaneous samples of the feeding-vigilance activity of the individuals were taken at 5-, 10- and 15-s intervals. This procedure gives the instantaneous proportions of the time each individual spent on each activity. I was thus able to compare instantaneous and real feeding-vigilance budgets using exactly the same primary recordings ( N = 43; only individuals for which the number of instantaneous sampling events was five or more for all sampling intervals were included). As feeding and vigilance are mutually exclusive and are the only behavioural sequences used by the teals, it is necessary to consider only one of them, vigilance. The mean proportion of the time spent vigilant did not differ between instantaneous and true time budgets in any of the sampling intervals (Table I, Wilcoxon's matched-pairs signed-ranks test, P > 0 . 3 t in all cases). However, the absolute
9 1991 The Association for the Study of Animal Behaviour 317
318
Animal Behaviour, 42, 2
TaMe I. The percentage of time spent vigilant in real time budgets and in time budgets derived using the instantaneous sampling method with different sampling intervals and the proportion (r2) of the variation between individuals of real time spent vigilant explained by the variation in instantaneous estimates of vigilance time % of time spent vigilant Mean Real budget Instantaneous budget 5-s interval 10-s interval 15-s interval
SD
33.2
8.7
31.8 32.0 31.6
12.2 17.2 19.8
r2
0.327 P< 0.001 0.428 P<0.001 0.035 NS
values of the deviations between the true and the instantaneous proportion varied considerably in each sampling interval, such that the correlations between the instantaneous estimates of time budgets and the true time budget were poor (Table I). Note that the sample sizes used in deriving the instantaneous proportions had little effect on the accuracy of the estimated time budget, since the absolute value of the deviation between the estimated and actual time budget did not correlate with the number of sampling events per individual in the 5-s sampling interval when all individuals regardless of sample size were included (range of the number of sampling events per individual 6-32, range of the absolute value of the deviation 0.126.9; r = - 0 . 1 4 , df=82, P>0.20). Most importantly, when the instantaneous sampling estimates of the time spent vigilant were regressed on an independent variable (here hypothetical data on forager group size) that explained a significant amount of the variation in true vigilance time (r2=0.356, df=41, P < 0-001), no significant regressions were found in any of the sampling intervals (5-s r2= 0"007, 10-s r2=0.012, 15-s r2=0"010; df=41 and P > 0.45 in all cases).
In conclusion, first, when averages of a great number of individuals are used instantaneous sampiing gives reliable estimates of true time budgets even for behavioural sequences of very short duration. However, if the duration of a particular action is very short, compared with other actions under study, rate measures should be considered instead of instantaneous sampling (see Altmann 1974). Second, owing to the large and unsystematic deviation of estimates of individual time budgets produced by individual instantaneous samples from true time budgets, instantaneous samples may lead to erroneous conclusions if one tries to find a biological explanation for the variation between individuals in time budgeting. To minimize this source of inaccuracy, one should make sure that sample sizes per individual are large enough. I thank P. Inkinen, A. J/intti and T. Kyl/itasku for measuring the instantaneous samples from the oscillograms and R. Dunbar, N. Metcalfe and J. Sorjonen for useful comments on the manuscript.
REFERENCES
Altmann, J. 1974. Observational study of behavior: sampling methods. Behaviour, 49, 227 267. Caraco, T., Martindale, S. & Pulliam, H. R. 1980. Avian time budgets and distance to cover. Auk, 97, 872-875. Caro, T. M. 1987. Cheetah mothers' vigilance: looking out for prey or for predators? Behav. Ecol. SociobioL, 20, 351-361. Dunbar, R. I. M. 1976. Some aspects of research design and their implications in the observational study of behaviour. Behaviour, 58, 78-98. Elcavage, P. & Caraco, T. 1983. Vigilance behaviour in house sparrow flocks. Anita. Behav., 31,303-312. Lima, S. L. 1987. Distance to cover, visual obstructions, and vigilance in house sparrows. Behaviour, 102, 231-238, Pulliam, H. R., Pyke, G. H. & Caraco, T. 1982. The scanning behaviour of juncos: a game-theoretical approach. J. theor. Biol., 95, 8~103. P6ysfi, H. 1987. Feedin~vigilance trade-off in the teal (Anas crecca): effects of feeding method and predation risk. Behaviour, 103, 108 122. Tyler, S. 1979. Time-sampling: a matter of convention. Anim. Behav., 27, 801-810.
Measuring time budgets with instantaneous sampling: a cautionary note HANNU
POYS)k
Finnish Game and Fisheries Research Institute, Evo Game Research Station, SF-16970 Evo, Finland (Received29 September 1990; initial acceptance 2 November 1990; .final acceptance 26 November 1990; MS. number: sc-596)
Instantaneous sampling (Altmann 1974) has frequently been used as a method of sampling data for time budget studies of animal behaviour, e.g. individual vigilance (Caraco et al. 1980; Caro 1987; Lima 1987). In instantaneous sampling the observer records an individual's current activity at preselected intervals, e.g. every 15 s. While the main concern of the methodological analyses by Dunbar (1976) and Tyler (1979) was to compare different time sampling methods, these authors also briefly considered the problem whether the choice of sampling interval in instantaneous sampling has any biasing effect on the time budget result. Both authors concluded that, independently of sampling interval, instantaneous sampling gives reliable estimates of true time use. The analyses and conclusions made by Dunbar (1976) and Tyler (1979) are correct but two additional examinations are needed. First, the sampling intervals examined by both authors were short (i.e. 5, t0, 15, 30 and 60 s) relative to the mean bout length of the behavioural variable under consideration, i.e. 14.0-124.6 s in Dunbar (1976) and 40-50 s in Tyler (1979). However, in many avian species, for instance, the mean length of a behavioural sequence may be very short, e.g. scanning bouts of less than 1 s in the house sparrow, Passer domesticus (Elcavage & Caraco t983) and in the yellow-eyed junco, Junco phaeonotus (Pulliam et al. 1982). Here I extend the comparisons to include short behavioural sequences as well. Second, although averages calculated from several individuals may give reliable estimates of true time use, individual instantaneous values may differ drastically from real ones (see Table 5 in Dunbar 1976). As I show here, this may be a serious source of inaccuracy if one tries to examine whether variation in time use is caused by changes in an independent variable. I used the feeding-vigilance time budgeting of the teal, Anas creeca, as an example since data of real time budgets are easily available and the durations of behavioural sequences of an actively foraging teal are very short, averaging between 0003-3472/91/080317 + 02 $03.00/0
0.6 and 1.5 s (P6ysfi 1987). I use the data pooled over the feeding methods of head submerged and neck submerged. The behavioural sequences of an actively feeding teal consist of lowering the head to feed, followed by raising the head to an upright position. In the feeding posture the head is beneath the water surface and in the vigilance posture above the surface (see P6ys/i 1987). I arbitrarily chose each individual and recorded all head lowerings and raisings into a continuously running cassette tape recorder using short code words. Recordings were later transcribed on a F-J Electronics 363 intensity meter coupled with a Siemens Oscillomink L running at 50 mm per s. The duration of each feeding and vigilant sequence was measured from the peaks produced by the code words on the oscillograms to the nearest 0.1 s; the use of this accuracy was warranted because the changes of the behaviour of foraging teals were easy to code in the field (i.e. only two behavioural states) and because a running speed fast enough was used with the plotter when transcribing the field recordings in the laboratory. This procedure gives the real proportions of time each individual spent on feeding and vigilance. From the same oscillograms, instantaneous samples of the feeding-vigilance activity of the individuals were taken at 5-, 10- and 15-s intervals. This procedure gives the instantaneous proportions of the time each individual spent on each activity. I was thus able to compare instantaneous and real feeding-vigilance budgets using exactly the same primary recordings ( N = 43; only individuals for which the number of instantaneous sampling events was five or more for all sampling intervals were included). As feeding and vigilance are mutually exclusive and are the only behavioural sequences used by the teals, it is necessary to consider only one of them, vigilance. The mean proportion of the time spent vigilant did not differ between instantaneous and true time budgets in any of the sampling intervals (Table I, Wilcoxon's matched-pairs signed-ranks test, P > 0 . 3 t in all cases). However, the absolute
9 1991 The Association for the Study of Animal Behaviour 317
318
Animal Behaviour, 42, 2
TaMe I. The percentage of time spent vigilant in real time budgets and in time budgets derived using the instantaneous sampling method with different sampling intervals and the proportion (r2) of the variation between individuals of real time spent vigilant explained by the variation in instantaneous estimates of vigilance time % of time spent vigilant Mean Real budget Instantaneous budget 5-s interval 10-s interval 15-s interval
SD
33.2
8.7
31.8 32.0 31.6
12.2 17.2 19.8
r2
0.327 P< 0.001 0.428 P<0.001 0.035 NS
values of the deviations between the true and the instantaneous proportion varied considerably in each sampling interval, such that the correlations between the instantaneous estimates of time budgets and the true time budget were poor (Table I). Note that the sample sizes used in deriving the instantaneous proportions had little effect on the accuracy of the estimated time budget, since the absolute value of the deviation between the estimated and actual time budget did not correlate with the number of sampling events per individual in the 5-s sampling interval when all individuals regardless of sample size were included (range of the number of sampling events per individual 6-32, range of the absolute value of the deviation 0.126.9; r = - 0 . 1 4 , df=82, P>0.20). Most importantly, when the instantaneous sampling estimates of the time spent vigilant were regressed on an independent variable (here hypothetical data on forager group size) that explained a significant amount of the variation in true vigilance time (r2=0.356, df=41, P < 0-001), no significant regressions were found in any of the sampling intervals (5-s r2= 0"007, 10-s r2=0.012, 15-s r2=0"010; df=41 and P > 0.45 in all cases).
In conclusion, first, when averages of a great number of individuals are used instantaneous sampiing gives reliable estimates of true time budgets even for behavioural sequences of very short duration. However, if the duration of a particular action is very short, compared with other actions under study, rate measures should be considered instead of instantaneous sampling (see Altmann 1974). Second, owing to the large and unsystematic deviation of estimates of individual time budgets produced by individual instantaneous samples from true time budgets, instantaneous samples may lead to erroneous conclusions if one tries to find a biological explanation for the variation between individuals in time budgeting. To minimize this source of inaccuracy, one should make sure that sample sizes per individual are large enough. I thank P. Inkinen, A. J/intti and T. Kyl/itasku for measuring the instantaneous samples from the oscillograms and R. Dunbar, N. Metcalfe and J. Sorjonen for useful comments on the manuscript.
REFERENCES
Altmann, J. 1974. Observational study of behavior: sampling methods. Behaviour, 49, 227 267. Caraco, T., Martindale, S. & Pulliam, H. R. 1980. Avian time budgets and distance to cover. Auk, 97, 872-875. Caro, T. M. 1987. Cheetah mothers' vigilance: looking out for prey or for predators? Behav. Ecol. SociobioL, 20, 351-361. Dunbar, R. I. M. 1976. Some aspects of research design and their implications in the observational study of behaviour. Behaviour, 58, 78-98. Elcavage, P. & Caraco, T. 1983. Vigilance behaviour in house sparrow flocks. Anita. Behav., 31,303-312. Lima, S. L. 1987. Distance to cover, visual obstructions, and vigilance in house sparrows. Behaviour, 102, 231-238, Pulliam, H. R., Pyke, G. H. & Caraco, T. 1982. The scanning behaviour of juncos: a game-theoretical approach. J. theor. Biol., 95, 8~103. P6ysfi, H. 1987. Feedin~vigilance trade-off in the teal (Anas crecca): effects of feeding method and predation risk. Behaviour, 103, 108 122. Tyler, S. 1979. Time-sampling: a matter of convention. Anim. Behav., 27, 801-810.