Abstract. The average daily distance flown by Swedish Reed Buntings in autumn is estimated at 42 kms/day. In Swedish recovery lists there are no day-to-day recoveries illustrating even moderate stages in this species. By all likelihood, migrating Reed Buntings tie a sequence of rather short stages together when they do not meet with geographical barriers. The main transport must take place at night; in the daytime migrating Reed Buntings increase their weight at a rate of 0.2 g/hour, and this food intake has been converted to at least 0.4 - 0.5 g of fat at dusk. Males outnumber females in the morning catch; many of them should move some distance in the early hours of the day as well. Reed Buntings at Ånnsjön, Jämtland showed clear premigratory fattening in late September/early October; in contrast birds from Kvismaren, Närke had little or no fat up till the end of September, and migrants at Oskarshamn, Småland showed no increase in fat reserves from September to early October. We argue, that the fat that fuels migration is essentially converted during the day following a stage, and that this conversion capacity determines the average distance flown in a day. On the other hand historical, already existing fat depots are probably used up during periods of high air pressure, when Reed Buntings migrate at some altitude and should be capable of covering order of magnitude 200 - 300 km in a single stage.
An embryo to this paper, "Sävsparvens höstflyttning", was published by Lundwall & Persson (1988) in Fågelstudier 4:1. We had no computers back then, everything was calculated manually, and it was difficult to obtain the relevant references. We always intended to have a second try on this material; here it is, based on powerful hardware and software, and maybe some extra knowledge, gained over time. [CP]
The Reed Bunting Emberiza schoeniclus breeds throughout Europe (Voous 1960), the generally accepted subspecific division dates back to Vaurie 1959. North and Central European populations migrate to the Mediterranean area, South and West European birds are sedentary or partial migrants. A few birds winter even in South Sweden, but they do not belong to the breeding population; local breeders are migrants. Reed Buntings migrate both night and day (Hansen 1954); in October they can always be heard calling from some altitude in the hours before dawn, and this late night migration is continued with an apparent culmination around 07h - 08h (Ulfstrand et al. 1974). The figures from diurnal counts are a surprising nullity (Falsterbo average 1973 - 99: 1163 inds., Roos 1999, and still, not even this level was reached between 1949 and 1960, Ulfstrand et al. 1974). There is something peculiar, nonsensical about the annual totals from daily counts; with the use of a tape lure one hundred Reed Buntings can easily be caught on an October morning at Ljunghusen, Kämpinge or Foteviken, some ten kms east of Falsterbo - and ringers never imagined that they were close to catching even every tenth bird passing these sites! Reed Bunting annual totals do not match the numbers involved in migration, the way they do in e.g. Chaffinches Fringilla coelebs. By all likelihood Reed Buntings are essentially nocturnal migrants, under certain conditions flying considerable distances during dark hours, but moving an additional distance in the morning hours as well. Diurnal counts therefore must be extremely vulnerable to weather-induced changes in the relative ratio between nocturnal and diurnal transport, we would not trust them to reveal much about population levels in this species.
Following our recent analysis of the migration of Yellow Wagtails Motacilla flava from Sweden (Lundwall & Persson 2003), we want to pursue the straightforward and reductive approach of that study on exactly the Reed Bunting, an elusive species not only as a fattening migrant in reed-beds, but even more so in matters of strategy: the timing of premigratory fattening, the length of stages and the relative importance of nocturnal and diurnal migration. The progress of migrating Reed Buntings is not on a par with the progress of neighbouring reed species like Acrocephalus warblers (Bensch & Nielsen 1999, Bibby & Green 1982), nor with the swift and overwhelming mass evacuation of e.g. Fringilla species. The Dunnock Prunella modularis may be the closest simile, not very swift in its progress, capable of migrating both night and day, skulking in behaviour. In the following, biometry and recoveries are questioned in order to sketch at least an outline of the migration strategy of Reed Buntings.
Half of the biometric material used was collected in SW Scania between 1987 and 1996; 810 birds weighed, measured and to some extent fat-ranked by the authors, Kerstin Norrman and Peter Olsson. The second half, 1099 birds (855 juveniles and 244 adults) was collected 1987 - 88 at four other Swedish sites: Oskarshamn (Tommy Larsson), Kvismaren Bird Observatory, Idö, Söderfjärden (Lars Broberg) and Ånnsjön Bird Observatory (courtesy by Thomas Holmberg); for locations see Fig. 4. In Scania birds were weighed and fat-ranked both at migration sites in the morning and at roosts in the evening, this full coverage makes multiple regression analysis of weight and rank developments particularly rewarding. Fat ranking was performed according to the "Operation Baltic" scale (e.g. Busse 2000) in Scania, according to the "Ottenby" scale (e.g. Pettersson 1982) at the other sites. These scales are not compatible, and the Ottenby scale gives poor separation between the two lowest rankings; there is some pedagogical gain in demonstrating this, and it will be done here. The reason is, that ranking "0" in the Operation Baltic case has no or little fat in the furculum, while "1" has fat-covered furculum; according to the Ottenby scale rankings "0" and "1" should have fat-free furculum. As a result there will be little or no separation between rankings "0" and "1" in many species, and it has been common practice among ringers to manipulate their rankings (change of ranking after weighing) in order to obtain consistent results. It is characteristic of the intellectual atmosphere of Swedish amateur and academic ornithology, that this problem has never been addressed. There is another problem; mean weights of morning and evening rankings are not compatible if feeding goes on throughout the day, this difficulty has been considered in the present analysis.
Basic references to energetics remain much the same as in the Yellow Wagtail paper (Lundwall & Persson 2003), in addition the following specific references to the Reed Bunting should be mentioned: Blümel 1982, Dornberger 1979, Haukioja 1966, Scott 1961. Recoveries have been extracted from "Annual report for 1960 of the Swedish Bird-Ringing Office" to "Report on Swedish Bird Ringing for 2001", the selection is listed in an Appendix. Early Swedish recoveries were discussed by Rendahl 1959.
3. Results
| Category | n | Mean value ± 1 s.e.; 1 s.d. | |
| 1y F, wing (mm) | 327 | 75.6 ± 0.1; 1.7 | |
| 1y F, weight (g) | 324 | 18.4 ± 0.1; 1.6 | |
| 1y F, fat ranking | 184 | 2.5 ± 0.1; 1.6 | |
| 1y M, wing (mm) | 326 | 80.7 ± 0.1; 2.0 | |
| 1y M, weight (g) | 321 | 20.4 ± 0.1; 1.8 | |
| 1y M, fat ranking | 191 | 2.4 ± 0.1; 1.7 | |
| 2y+ F, wing (mm) | 43 | 75.9 ± 0.3; 1.8 | |
| 2y+ F, weight (g) | 42 | 18.4 ± 0.2; 1.4 | |
| 2y+ F, fat ranking | 32 | 2.3 ± 0.3; 1.5 | |
| 2y+ M, wing (mm) | 81 | 82.5 ± 0.2; 2.0 | |
| 2y+ M, weight (g) | 82 | 21.0 ± 0.2; 1.6 | |
| 2y+ M, fat ranking | 53 | 2.7 ± 0.2; 1.4 | |
In order to investigate the weight development during the day further, the polynomial regression WEIGHT versus HOUR from dawn to dusk was calculated for males and females (all ages) separately, the results are shown in Figs. 1 and 2. The average daily rate of weight increase from 06h to 19h was 2.65 g in males, 2.78 g in females, some 15 % of the dawn weight. Both materials are highly representative, covering the whole period of daylight and a little more. (Linear regressions give highly significant increases 0.21 g/hour in both sexes). As a reflection of this weight increase the average fat ranking increases from "2" at dawn to "3" (females) or a little more than "3" (males) at dusk, establishing the self-evident fact: fat ranking is not a (time-) independent variable. Using our conservative estimates from Table II, this means, that 0.4 - 0.5 g of fat have been made available for migration flight during the course of the day. According to the polynomial regression, male weights increase very little until c10h (but a 2-jet may understate this rate in males and overstate it in females), indicating that they are more active during morning hours than females; the material contains 101 males up till and including 09h but only 64 females from the same interval, the whole difference in material size depends on these morning hours. On the other hand the male weight increase just before dusk seems to be more pronounced than that of females; whatever weight they have lost due to extra morning activity is repaired in the evening. If we resort to linear regression it indicates a possible weight increase amounting to 0.1 g/hour between 05h and 11h (the interval of diurnal migration), as can be seen from the following regressions:
Next, all parameters of this material were standardized by subtracting mean values and dividing with standard deviations, and standardized weight was regressed against standardized wing, time and fat ranking. The procedure is slightly irregular, since ranking as noted above depends on time, but it was done only to have some preliminary guidance. The standard partial regression coefficients express the rate of change in standard deviation units of WEIGHT per one standard deviation unit change of a particular parameter, while the others are kept constant. The two equations are given below (note the low contribution from body size as measured by wing-length):
Between 1987 and 1995 252 males and 228 females were fat-ranked according to the Operation Baltic scale (e.g. Busse 2000) at four sites in the SW corner of Scania: Ljunghusen on the Falsterbo peninsula (Christer Persson), Foteviken (Kerstin Norrman, Peter Olsson, Christer Persson) and Klagshamn and Vellinge (Peter Olsson). The earliest birds were caught at 05h in Ljunghusen, the latest at 20h at the other sites, i.e. both before sunrise and after sunset. By 1987 we already knew: pooled fat rankings of birds ranked by different workers, at different sites and at different times of day are problematic, in many cases they are useless in a "raw" state. So, when we first calculated the Scanian material from 1987 together with materials sent to us from other parts of Sweden in 1987, the revelation that the materials were incompatible meant no surprise to us. At best one can use small samples, collected by one single person, with the utmost care.
Still, these initial results were something of a disappointment. The worst thing is, that we did not realize the main reason for incompatibility: the substantial food intake of migrant Reed Buntings during the course of the day (Figs. 1 and 2). Many migrants obviously reach rather low fat levels in the early morning hours, and refueling becomes imperative. Such birds may present ranking problems; their weights may be rather high in spite of the absence of visible fat. When first confronted with the material, we believed that some subjective factor - differences between ringers when applying the scales - was the exclusive cause of inconclusive results. However, if a material contains 50 % birds ranked "0" at dawn and 50 % birds ranked "0" at dusk (a hypothetical, although somewhat unlikely case) their average weight will not differ from a cohort of birds ranked "2" in the middle of the day. When we realized this in the process of writing the present paper, we turned to investigating the fat rankings of old materials anew and worked out the following (preliminary) thumb-rule: the mean weight of ranking "0" should be calculated from material collected between 05h and 09h, no later, ranking "1" may reach into the afternoon, ranking "2" may begin at noon, no sooner, and rankings "3" to "5" should preferably be restricted to evening hours: from 16h onwards. The suggested procedure excluded 89 birds out of 480 from our original SW Scanian material. With such precautions any material will most likely give regular and useful results, as is seen from the practically linear development of Fig. 3. The excluded material: ranking "0" from late hours etc. must be treated separately. If such exclusions are not made, even the best material will be of little use. On the other hand: many species are not feeding at a seemingly constant rate from dawn to dusk, and caught from dawn to dusk, as Reed Buntings were in our case, so the problem is not brought to the fore in all species.
One problem remains, it was indicated above: the average 5h - 9h weight of ranking "0" may be some 2 - 2.5 g in excess of the fat-free or fuel-free weight; in the first morning hours this extra weight must consist of metabolic water to a great extent. There is no way of telling the ratio between metabolic water, remaining fat and newly ingested food, and it most certainly varies from specimen to specimen. These birds probably contain everything from 0 to 2 - 4 % (barely visible) fat. From fat-class "1" onwards we have assumed the ratio fat : [ingested food + water] at 50 : 50 (Helms & Drury 1960), a very rough estimate; when it comes to it this ratio probably changes progressively from dawn to dusk.
| Fat class | n | Mean weight ± 1 s.e.; 1 s.d. (g) | Mean wing ±1 s.e.; 1 s.d. (mm) | Extra weight/fat (fat %) |
| Males; n=208 | ||||
| 0 | 36 | 19.3 ± 0.2; 1.1 | 81.1 ± 0.2; 1.6 | 2.3/? (?) |
| 1 | 32 | 19.8 ± 0.3; 1.6 | 81.1 ± 0.4; 2.1 | 2.8/1.4 (7 %) |
| 2 | 40 | 20.4 ± 0.2; 1.3 | 81.8 ± 0.3; 2.2 | 3.4/1.7 (8.5 %) |
| 3 | 34 | 21.3 ± 0.3; 1.6 | 81.4 ± 0.3; 2.2 | 4.3/2.15 (10 %) |
| 4 | 38 | 21.9 ± 0.3; 1.8 | 82.0 ± 0.3; 1.9 | 4.9/2.45 (11 %) |
| 5 | 28 | 22.3 ± 0.3; 1.7 | 80.9 ± 0.3; 1.8 | 5.3/2.65 (12 %) |
| Females; n=183 | ||||
| 0 | 25 | 16.5 ± 0.2; 1.0 | 75.7 ± 0.3; 1.9 | 1.5/? (?) |
| 1 | 36 | 17.3 ± 0.2; 1.4 | 75.3 ± 0.3; 1.6 | 2.3/1.15 (6.5 %) |
| 2 | 21 | 18.0 ± 0.2; 1.1 | 76.0 ± 0.3; 1.7 | 3.0/1.5 (8.5 %) |
| 3 | 55 | 18.8 ± 0.2; 1.3 | 76.2 ± 0.2; 1.6 | 3.8/1.9 (10 %) |
| 4 | 28 | 19.3 ± 0.3; 1.4 | 76.1 ± 0.3; 1.9 | 4.3/2.15 (11 %) |
| 5 | 18 | 20.1 ± 0.5; 2.1 | 75.6 ± 0.4; 1.8 | 5.1/2.55 (12.5 %) |
Descriptive statistics from the four other Swedish sites (location: see Fig. 4) are given in Tables III - VI, seasonal developments from the same sites in Figs. Figs. 5, 6, 7 and 8. Birds were weighed to the nearest 0.5 g at Oskarshamn, to the nearest 0.1 g at the other sites; there are only adult measurements from Söderfjärden (data collected in a moult study). Birds at Oskarshamn were mainly migrants caught in the morning, the three other sites have good breeding populations, birds are present throughout the day and feeding should be no problem. Still practically all biometrical data are from morning hours. Wing length may have been used as sex criterion in some cases; this can no longer be checked. Adult measurements were pooled in order to get some average, all-the-year-round value from each site; no moulting birds with growing outer primaries were included, but breeding birds with some wear. Juvenile mean values were calculated from birds that could be sexed after having performed a full body moult, this is the case from mid-August to late August. (Still, retraps from Germany, Belgium and France reveal that surprisingly many Swedish juveniles may have been wrongly sexed in late September and early October. But where does the majority error lie: with Swedish ringers not looking close enough, or with Continent ringers mistaking females with collar for males?) The tables present mean values and variances for comparison between sites, the figures illustrate the rate of weight increase from July to September/October.
| Category | n | Mean value ± 1 s.e.; 1 s.d. | Category | n | Mean value ± 1 s.e.; 1 s.d. |
| 1y F, wing (mm) | 80 | 75.4 ± 0.1; 1.3 | 1y M, wing (mm) | 84 | 80.6 ± 0.2; 2.2 |
| 1y F, weight (g) | 80 | 18.3 ± 0.1; 0.9 | 1y M, weight (g) | 84 | 20.1 ± 0.1; 1.1 |
| 1y F, fat ranking | 80 | 1.6 ± 0.1; 0.9 | 1y M, fat ranking | 84 | 1.8 ± 0.1; 1.2 |
| Category | n | Mean value ± 1 s.e.; 1 s.d. | Category | n | Mean value ± 1 s.e.; 1 s.d. |
| 1y F, wing (mm) | 135 | 76.1 ± 0.1; 1.6 | 1y M, wing (mm) | 165 | 81.4 ± 0.1; 1.7 |
| 1y F, weight (g) | 135 | 17.1 ± 0.1; 0.9 | 1y M, weight (g) | 164 | 19.3 ± 0.1; 1.0 |
| 1y F, fat ranking | 135 | 0.8 ± 0.1; 1.1 | 1y M, fat ranking | 164 | 0.5 ± 0.1; 0.9 |
| 2y+ F, wing (mm) | 45 | 76.3 ± 0.4; 2.4 | 2y+ M, wing (mm) | 37 | 81.4 ± 0.4; 2.7 |
| 2y+ F, weight (g) | 45 | 17.4 ± 0.2; 1.0 | 2y+ M, weight (g) | 37 | 19.5 ± 0.2; 1.4 |
| 2y+ F, fat ranking | 47 | 1.4 ± 0.2; 1.6 | 2y+ M, fat ranking | 36 | 0.6 ±0.2; 1.1 |
| Category | n | Mean value ± 1 s.e.; 1 s.d. | Category | n | Mean value ± 1 s.e.; 1 s.d. |
| 2y+ F, wing (mm) | 42 | 77.5 ± 0.3; 1.9 | 2y+ M, wing (mm) | 67 | 84.0 ± 0.2; 2.0 |
| 2y+ F, weight (g) | 39 | 18.0 ± 0.2; 1.3 | 2y+ M, weight (g) | 66 | 19.9 ± 0.1; 1.1 |
| Category | n | Mean value ± 1 s.e.; 1 s.d. | Category | n | Mean value ± 1 s.e.; 1 s.d. |
| 1y F, wing (mm) | 28 | 76.7 ± 0.4; 2.3 | 1y M, wing (mm) | 30 | 80.7 ± 0.2; 1.2 |
| 1y F, weight (g) | 28 | 17.0 ± 0.3; 1.4 | 1y M, weight (g) | 30 | 18.5 ± 0.2; 1.2 |
| 1y F, fat ranking | 12 | 3.5 ± 0.5; 1.6 | 1y M, fat ranking | 8 | 3.5 ± 0.7; 1.9 |
| 2y+ F, wing (mm) | 15 | 74.9 ± 0.6; 2.2 | 2y+ M, wing (mm) | 13 | 80.6 ± 0.9; 3.2 |
| 2y+ F, weight (g) | 21 | 15.9 ± 0.2; 1.1 | 2y+ M, weight (g) | 15 | 18.8 ± 0.3; 1.2 |
Turning to the weight development during the day, Oskarshamn differs from other sites in having a negative regression between 05h and 11h (no evening catch), while there are weak positive tendencies at both Kvismaren and Ånnsjön (but negative tendencies in single days at Kvismaren). In general, time of day is not a good predictor for weight at these latitudes or under the particular circumstances at these sites, but the evening catch has been weak everywhere. Regressions might change with more evening material:
Finally the standardized regressions are given for all three sites. The Oskarshamn and Kvismaren materials remain the same as above, while the full Ånnsjön material was used in order to have some significance. In all three cases half of the weight variation is explained by body size (wing-length) and visible fat, the low importance of time of day reflects the fact that there are no afternoon or evening weights from Oskarshamn and Kvismaren and only three from Ånnsjön/Rödön:
Thomas Holmberg ranked 25 birds from Rödön between 10.9 and 2.10; six were ranked 0 - 2, nineteen 3 - 5. There is a substantial fattening at this latitude before departure, but the weights were by no means extreme; only six males with rankings 4 - 6 weighed in excess of 20 g. During two summer months birds at these latitudes seem to carry little or no fat.
Fat-ranked and sexed birds from Kvismaren/Ässön, late August and September, are presented in Table VII. The main purpose is to demonstrate the difficulties connected with the ranking-scale, so the material is not pressed for further secrets. The two "4"-ranked females were caught 26 and 29.9, the "4" and "5" ranked males 26 and 29.9; fattening may proceed very swiftly just prior to migration, and the absence of higher ranking might be caused by the fact that there is no October material from the year in question:
| Fat class | n | Mean weight ± 1 s.e.; 1 s.d. (g) |
| Males; n=162 | ||
| 0 | 115 | 19.3 ± 0.1; 1.0 |
| 1 | 26 | 18.9 ± 0.2; 0.9 |
| 2 | 16 | 19.8 ± 0.2; 0.7 |
| 3 | 3 | 19.4 ± 0.6; 1.0 |
| 4 | 1 | 20.7 |
| 5 | 1 | 20.1 |
| Females; n=134 | ||
| 0 | 76 | 17.0 ± 0.1; 0.9 |
| 1 | 28 | 17.1 ± 0.1; 0.8 |
| 2 | 15 | 17.0 ± 0.2; 0.9 |
| 3 | 13 | 17.6 ± 0.2; 0.7 |
| 4 | 2 | 18.2 ± 0.6; 0.8 |
At Oskarshamn, not a single bird was ranked "0", here rankings "1" and "2" weighed the same. We will never know if some weakness of the Ottenby scale is involved here; at least the absence of ranking "0" at a site like Oskarshamn seems perfectly credible. Full dates are given in Table VIII, the mean weight of 74 males ranked 1 - 3 was 19.98 ± 0.13 g, s.d. 1.12 g, of 79 females ranked 1 - 3 18.26 ± 0.10 g, s.d. 0.92 g:
| Fat class | n | Mean weight±1 s.e.; 1 s.d. (g) |
| Males; n=84 | ||
| 1 | 47 | 19.9 ± 0.1; 1.0 |
| 2 | 19 | 19.9 ± 0.3; 1.2 |
| 3 | 8 | 20.6 ± 0.5; 1.5 |
| 4 | 5 | 20.7 ± 0.5; 1.1 |
| 5 | 5 | 21.0 ± 0.2; 0.5 |
| Females; n=79 | ||
| 1 | 44 | 18.1 ± 0.1; 0.8 |
| 2 | 24 | 18.2 ± 0.2; 0.9 |
| 3 | 11 | 19.0 ± 0.2; 1.1 |
Direct autumn recoveries of Reed Buntings from Sweden (to some extent migrating by way of SW Scania) are listed in the Appendix, distances were calculated as orthodromes. The material contains 21 birds sexed as males, 48 as females; if the bird was ringed as belonging to either sex and controlled as belonging to the other, the sexing has been disregarded. The five fastest movements are marked with red colour, the times elapsed were 6, 9, 13, 13 and 16 days, the transport 80 - 100 kms/day. Birds ringed before 15 September in S. Sweden were not considered, but a few birds from August were included from the northernmost counties. Still the date 15 September need not approximate the departure date of a south Swedish bird if it was recovered 45 days later; it may have stayed at the ringing-place for another week or two (cf. the low level of fattening at Kvismaren in late September noted under 3.2.2.) But the material, depending on a remarkable Swedish effort and interest in Reed Buntings from the mid-eighties to the late nineties, is as good as it will ever be. On the Continent late birds from Belgium (November, December) and France (after mid-November) were not considered, although some arrival seems to be taking place at e.g. Villeton in late November. Strangely enough this impressive material does not contain one single recovery between two consecutive days of a Swedish Reed Bunting, a fact that must reflect some particular mode of migration. Distances plotted versus time elapsed and the regression line are given in Fig. 9; the average progress of migrating Swedish Reed Buntings is some 42 kms/day, differences between speed within Sweden and on the Continent small and not significant.
4.2. Range estimates with different models.
There are basically two kinds of models on which to base calculations: one aerodynamic, one energetic. From the basic aerodynamic model of Pennycuick 1975 ranges were estimated with initial weights 20.3 g (male) and 18 g (female) and different consumptions based on caloric equivalents 9.5 kcal/g (pure fat) and 6 kcal/g. (This is the average value calculated for chaffinches at Rybachi, Kurland Spit by Dolnik & Gavrilov 1971a, b, a "black box" hiding possible mixed use of fat, carbohydrates and protein as fuel, metabolization of ingested food, water loss or gain during flight). Consumption of 3.3 (males) or 3 (females) g brings the birds down to assumed fat-free or fuel-free weights; 17 and 15 g. The wing span of males was estimated at 225 mm, of females at 215 mm. Values are given in Table IX:
| Weight loss (g) | Calc. range (km) CE= 9.5 kcal/g | Calc. range (km) CE= 6 kcal/g |
| MALE, initial weight 20.3 g, est. lift : drag ratio 4.58 - 4.63. | ||
| 1.1 | 240 | 152 |
| 2.2 | 490 | 309 |
| 3.3 | 770 | 486 |
| FEMALE, initial weight 18 g, est. lift : drag ratio 4.70 - 4.75. | ||
| 1 | 254 | 162 |
| 2 | 521 | 332 |
| 3 | 813 | 518 |
a. Biometrical differences between sexes, ages, populations
The recent, generally accepted subspecific division of Reed Buntings dates back to Vaurie 1959, it contains three groups and fifteen subspecies. There are older subdivisions, however, one of them, based on the views of Steinbacher 1930, envisaged a subspecies 'steinbacheri' north of c60° 30' (Dalälven) in Sweden. Svensson 1992 gives bill measurements (depth at feathering) from the same area and concludes, that there is cause for rejection of this particular "subspecies". One of the authors (CP) has measured breeding birds between 67° and 68°30' N in July in Swedish Lapland; 10 males hade wing mean 80.7 mm, s.d. 2.5 mm, mean weight 19.7 g, s.d. 1.0 g, 7 females mean wing 75.7 mm, s.d. 1.5 mm, mean weight 18.0 g, s.d. 1.0 g; these values lie between Ånnsjön and Kvismaren values and do not mark Lapland birds as particularly small of stature. (Bill depths will be saved for another paper to come). Strong wear of breeding birds in scrubby northern habitats is a more likely cause for short wings; in particular first-time breeders use to have broken wing-tips, and a high proportion of such birds in a particular habitat will result in low mean values. Wear and age structure may have influenced e.g. the mean value of wing-lengths in adult birds from Ånnsjön.
At the other end of the scale there is some degree of enlargement in males after the first moult, this effect is met with in most finches; among e.g. Chaffinches Fringilla coelebs and Bramblings Fringilla montifringilla 3y+ males always have longer wings and weigh more than 2y males (our own material; we know of no references). This effect should lie behind the high mean values from Söderfjärden, where Reed Buntings are caught in a principal habitat, that probably attracts and to some extent is defended by older birds. 29 of the birds measured at this site were ringed in earlier years. We had expected Kvismaren to be such a prime habitat as well, but all mean values from this site remain low or average throughout the season.
Finally the prospects of separating sexes in unmoulted juveniles based on wing-length. Everything below the female mean should be female with high probability, everything above the male mean males with high probability. One s.d. above the female mean there is already some mixture, the same applies for one s.d. below the male mean. Sexing based on wing-length is defendable below the mean value and 1 mm above it in the case of females, 1 mm below the mean and above it in the case of males. Knowledge of the overall biometry of the investigated population is an advantage!
b. Upper and lower weight limits of Reed Buntings
In Figs. 1 and 2 the lowest dawn weight of males is 16 - 17 g, of females 14.5 - 15 g. Table II under 3.1.2. assumes a fat-free weight of c17 g in males, c15 g in females. There will always be question marks to these estimates; at what stage is fat no longer the sole energy source, when is protein metabolized, and how often does this occur? In addition there must be differences between populations in this respect; the Söderfjärden breeding birds are large and well-nourished in summer, females weigh on average 2 g more than Ånnsjön females, the latter subsisting at levels that would be regarded as dangerously close to fat-free under other circumstances. So what do we have in our hands: a tiny North Swedish female with a fat- or fuel-free weight of maybe only 14 g, or a large South Swedish female with fat- or fuel-free weight 15.5 g? We are quite satisfied to establish the fact, that migrating Reed Buntings in SW Scania have on average at least 3.5 (mid-day average) to 4 - 4.5 (dusk value) g of body weight to fuel nocturnal flight. The regression coefficients 0.2 g/hour reveal that at least 2.5 g of this overall weight are ingested between 06h and 19h already on the first day following a migration stage. The literature contains little all-day-round weight material for comparison; in some species the weight acceleration of the last hour (e.g. in the Robin Erithacus rubecula) is due to the fact that all birds drink just prior to the departure. Dalberg-Petersen 1972 gives data for twelve hours of weight increase in insectivorous spring migrants from Hesselö/Kattegatt; the increase is ca. 1 g till noon, and the regression is zero already by that time. In the case of Reed Buntings the unlimited access to Phragmites seeds is likely to be the factor deciding the ratio between fat carried for fuel and food more or less directly (over the day or at night) combusted for fuel. At least males weighing 23 - 24 g at 20h also have large depots of pure fat, in their case any use of ingested food can be compensated for in calculations by assuming a lower caloric equivalent for body weight than that of pure fat, 9.5 kcal/g. Important is, that we know that fat estimates based on body weight with much ingested food may exaggerate the potential range of a given species, or just: some particular individual.
c. The problem of fat rankings
If the Scanian fat rankings must be constrained in order to give consistent results, shouldn't the materials from other parts of Sweden be given the same favour? Yes, maybe. But downright morning rankings from Scania give consistent results, mid-Swedish rankings from morning do not; here the particular scale must be held responsible. The problem with the Scanian material arose from the fact that we had biometry from dawn to dusk, the problems with other materials from the fact, that the scale used provided poor separation between the lower rankings. The tool should be sharpened, and this can be done only in a sort of trial-and-error process, where mean weights are calculated, criteria corrected and so on, till the scale works with Reed Buntings. This need is particularly emphasized by Busse 2001. There is no similar problem with the higher rankings; here visible fat is so overwhelming and the criteria so distinct, that there can be no doubt: the weight surplus is due primarily to fat, ingested food can not match it. There may even be some degree of dehydration. On the other hand, in a dawn bird with visible air-sack in the furculum ("0" according to the OB scale) but weight 2 - 3 g above the assumed "fat-free" weight doubt may be immediate: should it be ranked "0", "1" or even "2", and should it be used in calculations? This boils down to a plea for some sort of general discipline in ranking: an original ranking must not be changed after weighing, and maybe there should be some sort of additional "tagging" of rankings, an asterisk indicating: this is a clear case, no suspicion of excessive water or ingested food content, it can be used in calculations.
d. The stage of migrating Reed Buntings, body weight metabolized to achieve it
Newton 1972 emphasized the "extremely rapid conversion of food to fat" in Bullfinches Pyrrhula pyrrhula. In this particular species, under autumn conditions, the estimated that food stored in the gullet and gut contributed at most 1.6 kcal overnight, glycogen in liver and muscles some 0.8 kcal, and fat (metabolized at a rate of 0.04 g/hour) 3.8 kcal. At least 60 % of the nocturnal energy consumption was attributed to metabolization of stored fat. Scott 1961 remarked on the high daily food intake - at least 2.5 g - of British Reed Buntings in early spring, and our own Figs. 1 and 2 bear witness of a similar appetite among migrants in autumn. We think that an analogy with Bullfinches can be defended; the Reed Bunting is a seed-eating species, and the increase of average ranking from dawn "2" to dusk "3" (see 3.1.1.) gives evidence of a rapid food conversion in this species as well. Even if we resort to the conservative energetic model, this means that Reed Buntings are capable of acquiring the fat necessary to fuel a 40 - 45 km flight in a single day. According to Ellegren 1990 juvenile Bluethroats Luscinia svecica fly stages of the same order of magnitude within the Baltic area. This may be what the daily (nocturnal) range is about: the amount of food that can be converted into fat by the same bird during one single day of normal foraging. Theoretically Reed Buntings could migrate all the way to winter quarters in this way; we need not overexert birds or models or refer to already existing, historical fat depots. All short-time recoveries connected with Åhus (see the Appendix) on the east coast of Scania speak in favour of this hypothesis; the fact that there is no recovery within 12 - 24 hours indicates a "frog-leap" migration with very short stages rather than one single 100 kilometer transport from Åhus to the SW corner of the county.
On the other hand the Appendix also contains half a dozen recoveries of birds flying almost 100 km/day for a period of 6 - 16 days (red text). We believe that these birds didn't fly 100 km night after night; longer stages - 200 - 300 km - must have been involved. The southbound coast of Scania presents opportunities to observe such situations: when a rain front approaches from the south just before dawn, ending a high and interrupting a night of intense migration. Suddenly Reed Buntings start to descend almost vertically, choosing to interrupt their migration rather than risk a 2 1/2-hour to 3 1/2-hour-flight in headwind to Rügen/Germany, 100 km to the south. They may turn west after 50 kms and land on Mön/Denmark, too, but in most cases they probably intended (and were fueled) to perform the full journey, and they had already flown some unknown distance when reaching the Scanian coast at 04h or 05h. In our opinion, birds passing the Scanian south coast on such nights were fueled for stages order of magnitude 200 - 300 kms, and this readiness is linked with a geographical barrier, the Baltic.
After such an effort at least those birds who flew an appreciable stage insert a period of build-up, where an optimal water-balance is restored and fat deposited anew. In contrast to most other finches Reed Buntings do not forage in flocks; when feeding they disperse over reed-beds, along shorelines, on fallow fields. In reed-beds they are well spaced and hard to flush when starting to gain weight, the same applies to other habitats. This may be the reason why Reed Buntings seem to "disappear" after arriving and being ringed at a particular site: they disperse and go into cover (others may move some 40 km after one day, based on their daily food intake). The same probably applies to the wintering grounds; we suspect that there is some "lag" before many newly-arrived birds are caught at e.g. Villeton, France. In the morning hours migrants are easily caught at guiding-lines with the help of a tape lure, the same applies for birds at small, compact roosts in the evening, but fattening birds are immobile, hard to flush and difficult to recapture - so we never get the decisive last recapture, that would have shortened the time elapsed between two halts on the journey. We repeat what was said above on this point: the material is as good as it will ever be. Because of the dispersion of resting birds recoveries alone will probably never reveal the whole truth about resting times and length of stages in this species. But we think that we have pointed out a couple of interesting features: the moderate average transport, the substantial daily weight increase in migrants and the absence of recoveries between two consecutive days.
Summary:
1. It is argued, that effective migration in Reed Buntings takes place mainly during dark hours, but birds move some distance in the daytime as well. The daily average progress of Swedish Reed Bunting, calculated from a selection of direct recoveries, was 42 km/day.
2. With linear regression there is a weight increase of 0.2 g/hour from dawn to dusk in migrant Reed Buntings in SW Scania. In particular males move some distance during morning hours; their growth curve is better approximated with a polynomial curve.
3. During the course of the day the average fat ranking increases from "2" to "3", indicating that food has been converted to at least 0.4 - 0.5 g of fat.
4. Even from a flight metabolism model this amount of fat is enough to fuel the average daily progress. By all likelihood Reed Buntings fly one or several longer stages (2 - 300 kms) as well, using up fat stored for some time.
5. Birds from Ånnsjön/Jämtland showed distinct premigratory fattening in late September/early October, on the other hand birds from Kvismaren Närke had low weights and little visible fat till the end of September. Migrants at Oskarshamn, Småland had low weights and moderate fat depots during morning hours.
Thanks to Kvismare Bird Observatory, Ånnsjöns Bird Observatory (Thomas Holmberg), Tommy Larsson and Lars Broberg for generously sharing their material with us! An important part of the Scanian material rests on field work done by Kerstin Norrman and Peter Olsson.
APPENDIX:
Within Sweden, to Finland (n=18):