Bird Study Aspects of the breeding ecology of ...

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Aspects of the breeding ecology of Welsh Grey Wagtails Motacilla cinerea a

S. J. Ormerod & Stephanie J. Tyler

b

a

Department of Applied Biology, University of Wales Institute of Science and Technology, King Edward VII Avenue, Cardiff, CF1 3NU, UK b

RSPB, Wales Office, Frolic Street, Newtown, Powys, SY16 1AP, UK Published online: 24 Jun 2009.

To cite this article: S. J. Ormerod & Stephanie J. Tyler (1987) Aspects of the breeding ecology of Welsh Grey Wagtails Motacilla cinerea , Bird Study, 34:1, 43-51, DOI: 10.1080/00063658709476935 To link to this article: http://dx.doi.org/10.1080/00063658709476935

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Bird Study (198

4, 43-51

Aspects of the breeding ecology of Welsh Grey Wagtails Motacilla cinerea


S.J. ORMEROD Department of Applied Biology, University of Wales Institute of Science and Technology, King Edward VII Avenue, Cardiff CF1 3NU, UK STEPHANIE J. TYLER RSPB, Wales Office, Frolic Street, Newtown, Powys 5Y16 1AP, UK We studied the abundance, reproductive performance and diet of breeding Grey Wagtails on rivers in mid and south Wales between 1978 and 1985. Breeding abundances ranged from 0 to 15 pairs per 10 km. Birds were commonest on hard water streams with abundant aquatic invertebrates and they appeared to favour streams lined by deciduous trees. Most females probably started to lay in April but replacement and subsequent clutches extended the breeding season into July. Median laying dates varied annually and were correlated with the mean air temperature in March. Pairs at altitudes over 300 m bred significantly later than those at lower altitudes. Fifty-two per cent of nesting attempts produced fledged young. The mean size of 147 clutches was 5.07. Clutches started in May were significantly larger than those started in April or June. We suggest that the increased reproductive effort coincides with an increase in breeding success. Adults and nestlings had similar diets in terms of insect orders: Trichoptera, Ephemeroptera and Plecoptera each provided 10-20% of the items taken and adult Diptera about 55%; aquatic insects formed 25% of the diet.

P

because of the concern expressed 1 over influences by man on upland British rivers, the ecology of riverine birds has recently received considerable attention (Marchant & Hyde 1980a; Holland, Robson & Yalden 1982a,b; Round & Moss 1984; Ormerod 1985a; Tyler & Ormerod 1985; Ormerod & Tyler 1986; Ormerod, Allinson, Hudson & Tyler 1986; Yalden 1986). Such ornithological information is clearly desirable if the effects of pollution, changes in land-use, acidification, impoundment, and land drainage are to be detected and ameliorated (e.g. Scullion & Edwards 1980; Inverarity, Rosehill & Brooker 1983; Stoner, Gee & Wade 1984; Gregory, Hockin, Brookes & Brooker 1985; Orrnerod et al. 1986). Whilst Dippers C. cinclus and Common Sandpipers Actitis hypoleucos have figured

prominently in this work, there is relatively little detailed infoimation on another characteristic breeding bird of upland rivers, the Grey Wagtail Motacilla cinerea. In Britain, studies of breeding Grey Wagtails have been confined to local accounts of reproductive performance and distribution (Tyler 1970; Merritt, Greenhalf & Bonham 1970; Tyler & Tyler 1972; Nicoll 1979), the analysis of a relatively small sample of nest record cards (Tyler 1972) and monitoring during the Waterways Bird Survey (Marchant & Hyde 1980b). Similarly, on mainland Europe, there have been only a few studies of either breeding performance (Herroelen 1955; Schifferli 1972; Jorgenson 1977a) or feeding ecology (Schifferli 1961; Schifferli 1972). In this paper, we report on the abundance of Grey Wagtails on some Welsh rivers, de-

44 S.J. Ormerod and S.J. Tyler scribe their breeding biology, and provide data on the diet of nestlings and breeding adults. A study of the ecology of Grey Wagtails in relation to stream acidity is in progress and will be reported separately.

abundances of some aquatic invertebrates (nymphal Plecoptera and Epherneroptera, larval Trichoptera: see Ormerod et al. 1985 for methods). Altitudes and slopes for each plot were derived from 1:50000 Ordnance Survey maps.

STUDY AREA AND METHODS Breeding biology


Breeding abundance Our data on the breeding abundance of Grey Wagtails came entirely from 20 tributaries of the River Wye and 13 of the River Teifi. These rise in central Wales before flowing south-eastwards (Wye) or westwards (Teifi). Their catchments are predominantly rural and free from polluting discharges; they show a range of chemical conditions (e.g. total hardness 10-220 mg CaCO 3 1 -1 in Wye tributaries, 16-90 mg CaCO 3 1 -1 in Teifi tributaries), a corresponding range of invertebrate abundances (Jenkins, Wade & Pugh 1984; Ormerod 1985b), and a range of relief (river slopes: Wye tributaries 1-60 m km - '; Teifi tributaries 1-50 m km - '). Each river system has been more fully described elsewhere (Edwards & Brooker 1982; Jenkins, Wade & Pugh 1984; Ormerod, Boilstone & Tyler 1985; Tyler & Ormerod 1985). We divided the 33 tributaries into survey plots of 5-10 km. Breeding abundances (standardized as pairs per 10 km) were estimated using methods similar to the British Trust for Ornithology Waterways Bird Survey (Merchant & Hyde 1979; Ormerod et al. 1985). Because Grey Wagtails sometimes nest or forage away from rivers (Schifferli 1972) and are difficult to census, our minimum criterion to indicate the occupation of a territory was either the presence of a pair on one occasion or the presence of a single bird on two. In practice most territories were confirmed on stronger evidence. Plots on the Teifi were visited 5 times between 6 May and 9 July 1981, whilst those on the Wye were mostly surveyed 7 times between 9 March and 7 July 1982. Some plots on the Monnow sub-catchment of the Wye were visited on fewer occasions, although these plots generally produced most birds. In all, data were available from 55 survey plots, of which 39 also had chemical data (provided by Welsh Water Authority) and 20 (all in the Wye) had compatible data on the

We searched for nests predominantly during April-June in 1978-85, mostly in the catchment of the River Wye. The majority were found on the tributaries around Monmouth (51°41'N 2°43'W) although we also found nests on the headwaters of the Usk and Taff to the west of the Wye and on the upper Severn, Ystwyth, Teifi and Tywi. Information on the contents of each nest was collected from several visits, according to criteria indicated by the BTO Nest Record Scheme. To obtain first-egg dates, we backcalculated from the estimated age of nestlings at ringing, allowing 12 days for incubation and 1 day for each egg laid. Because some pairs laid subsequent clutches in different nests from the first, we could not be certain whether any nesting attempt was a first, second, third or a relay. In 1985, we deliberately included more nests at high elevations to investigate the relationship between first-egg date and altitude. To maintain consistency with our existing data, these additional nests were excluded from all analyses except that presented in Fig. 5. Diets We analysed faeces from 16 sites in the catchments of the Wye (7 sites), Tywi (5), Taff (2) and upper Severn (2), choosing locations with a wide range of altitudes and chemical conditions. We visited each site between 5 May and 10 July 1985 and collected 5 faecal pellets from nestlings (aged 5-12 days) and 10 from adults, the latter being picked fresh from stones in the vicinity of nests. Samples were deflocculated for at least 4 hours before examination at magnifications of x12 to X40. Nymphs and larvae were identified and quantified by counting mouthparts (Ormerod 1985a; Ormerod & Tyler 1986) whilst adult insects were quantified from recognizable parts of their wings or elytra. Because

Breeding Grey Wagtails 45 insect wings were often fragmented, we could not identify some items beyond order so our analyses have been performed at this level.

•

15

• 12

RESULTS

•


Breeding abundances Seventy-five per cent of our survey plots were occupied by Grey Wagtails and, where birds were present, abundances ranged from 1 to 15 pairs per 10 km (unweighted mean 3.8, s.d. 3.35; 2.18 and 3.15 including plots without birds) (Fig. 1). Pairs occurred throughout the altitudinal range of the plots (0-320 m o.d.) and on river slopes between 3 and 60 m km -1 : there were no significant correlations between breeding abundance and these factors. Grey Wagtails also occurred across a wide range of water chemistry, although harder waters supported significantly more birds (Fig. 2a). In the Wye system, breeding abundance was significantly correlated with the numbers of aquatic invertebrates (Fig. 2b).

• as • • •• •• • •• •• S • • • 80 mg CaCO 3 1

160

9 6 *

• • 0 1 100

200

I 400

*

•

• •• • • 1000

1 3000

Relative abundance

Figure 2. The breeding abundance of Grey Wagtails in relation to: (a) total hardness (r, = 0.41, P < 0.01); (b) combined abundance of aquatic Plecoptera, Ephemeroptera and Trichoptera in April (Wye sites only, r, = 0.51, P < 0.05). There was no significant correlation between breeding abundance and either altitude (r, = —0.30) or slope (r, = 0.09).

C‘

01 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Pairs per 10 km.

Figure 1. The abundance of Grey Wagtails (pairs per 10 km) on 55 survey plots in the Wye and Teifi river

systems.

Altitudinal distribution of nests Our sample of nests was probably biased towards lower altitudes, with 50% being below 100 m, 75% below 200 m and 90% below 300 m. However, the patterns of clutch size, brood size and breeding performance were probably not affected as a result (see below).

Laying dates For all years combined, a peak of egg laying occurred in the last week of April, with a second peak around the third week of May (Fig. 3a). This pattern was apparent in most individual years, although median first-egg dates differed significantly (Kruskall-Wallis test, H = 26, P < 0.001), probably because of variable spring weather. When we examined median laying dates in relation to mean temperatures and rainfall for each month between December and June (as measured at Cardiff airport, 51°23'N 3°20'W, a station for which the most comprehensive data were available)

46 S.J. Ormerod and S.J. Tyler


we found that the mean air temperature in March explained 64% of the variance (Fig. 4). Laying dates above 300 m o.d. were generally 12-14 days later than at lower altitudes, with early clutches being particularly scarce (Fig. 3). A similar pattern was apparent in our 1985 'expanded' set of data, when first-egg dates were retarded by approximately 13 days for every 100 m increase in altitude (Fig. 5).

125 139 April

May

153 167 in June

Dam

Figure 3. The percentage distribution of first-egg dates at various altitudes for Welsh Grey Wagtails. 1 January = day 1. The arrows show medians. The differences between altitudes were significant (oneway anova, F2,239 = 3.4, P < 0.05), due mostly to significantly later laying above 300 m than at each of the lower altitudes (t = 2.04 and 2.67, P < 0.05). (Note: to remove the effects of differences in laying date between years before pooling at each altitude, we standardized the data by subtracting the relevant annual mean and dividing by the corresponding standard deviation.)

13

79

.

12

120

cl 112 •

82

104

• 81

4.5 5.5 66 7.5 86

Figure 4. The median laying date of Welsh Grey Wagtails in each year in relation to the mean air temperature in March. 1 January = day 1. The line was fitted by least squares regression (median date = 152-4.62 x mean temperature, r 2 = 0.64).

Figure 5. First-egg dates in relation to altitude in

1985. 1 January =day 1. The line was fitted by least squares regression (first-egg date = 0.13x altitude +103.5, r2 = 0.32, n = 51).

Clutch size Only clutches of 4 (12.2%), 5 (68%) and 6 (19.7%) eggs were found and for all data combined, the mean clutch size was 5.07 (95% CL = -± 0.09, n = 147). Clutch size did not vary significantly between years (H = 5.9, n.s.) or between altitudes (within months H = 0.32, 0.44, 0.79: n.s.). However, a distinct trend was apparent within the breeding season, with clutches in May being larger than those in either April or June (Table 1). The pattern was apparent within each altitudinal range.

Brood size Overall, the mean brood size at ringing age (5-8 days) was 4.40 young (95% CL = ±0.16, n = 171). No significant differences in brood size were apparent between years (H = 8.3, n.s.) or altitudes (H = 0.89-1.03, n.s.) and, -

Breeding Grey Wagtails 47 Table 1. The breeding performance of Welsh Grey Wagtails according to month of laying and altitude. Mean clutch and brood sizes are given (with standard deviations and sample sizes). Success is given as percentage of nests that produced at least one fledged young (with sample size), calculated only from those nests found at the egg stage


April (a) Clutches 0-149 m o.d. 150-299 m o.d. > 300 m o.d. All altitudes (b) Broods 0-149 mo.d. 150-299 m o.d. > 300 m o.d. All altitudes (c) Success 0-149 m o.d. 150-299 m o.d. > 300 m o.d. All altitudes

June

May

All months

4.91 (0.42,45) 4.95 (0.21,22) 5.00 (0.70,5) 4.93 (0.39,76)

5.41 (0.65,24) 5.26 (0.79,15) 5.33 (0.49,12) 5.38 (0.63,52)

5.00 (0.57,7) 4.75 (0.70,8) 4.75 (0.50,4) 4.90 (0.64,19)

5.08 (0.56,79) 5.04 (0.57,47) 5.13 (0.56,21) 5.07 (0.56,147)

4.32 (0.95,50) 4.46 (0.77,27) 4.80 (1.30,5) 4.35 (1.01,82)

4.62 (1.11,38) 4.41 (0.93,18) 4.88 (0.92,9) 4.55 (1.09,65)

4.20 (1.32,15) 4.25 (0.35,9) No data 4.21 (1.06,24)

4.39 (1.10,103) 4.37 (0.93,54) 4.64 (1.08,14) 4.40 (1.05,17)

32.2 (31) 46.7 (15) 50.0(4) 38.0 (50)

83.3(6) 100.0(4) No data 90.0 (10)

71.4 (14) 60.0 (10) 40.0(5) 62.1 (29)

49.0 (51) 58.6 (29) 44.4(9) 51.7 (89)

Statistical comparisons: Clutches: clutches at 0-149 m o.d. (Kruskall-Wallis test, H = 13.7, Xi, P < 0.001) and 150-299 m o.d. (H = 6.13, P < 0.05) varied significantly with month of laying. Data pooled by altitude gave larger clutches in May than either in June or April (Mann-Whitney U-test, P < 0.001). There was no significant effect of altitude on clutch size in either separated or combined months. Broods: no significant trends by month or altitude. Success: success was significantly associated with month of laying (Xi = 10.88, P < 0.005) but not altitude (Xi = 0.89, n.s.).

although seasonal changes in brood size generally reflected those in clutch size, the differences were not formally significant (Table 1). Clutches of 6 eggs produced significantly larger broods (mean 5.11, s.d. 1.21, n = 27) than clutches of either 5 (4.40, 0.83, 59) or 4 eggs (3.25, 0.96, 12).

Breeding success To avoid bias against nests that failed, we calculated breeding success only for those found at the egg stage. Overall, 52% of 89 breeding attempts produced at least one fledged young with nests becoming increasingly successful as the season advanced (Table 1). Clutches of 6 eggs (86.6% success) were more likely to succeed than clutches of 5 (47.6%) or 4 (30.7%), probably reflecting seasonal trends in clutch size and success rates.

Diets The diets of adults and nestlings were broadly similar, with adult Diptera the commonest items (Table 2). Empididae, Syrphidae, Ephydridae, Trichoceridae, Tipulidae and Chironomidae were amongst the dipteran families taken. Adults and nestlings ate a similar proportion of aquatic prey (about 25%) although trichopteran larvae were more important to nestlings and ephemeropteran nymphs to adults (Table 2). DISCUSSION

Breeding abundance The breeding abundances we recorded (1-15 pairs per 10 km) were generally similar to those from other European studies: on fast rivers throughout Britain Grey Wagtails occurred at

48

S.J. Ormerod and S.J. Tyler Table 2. The diets of adult and nestling Grey Wagtails as shown by faecal analysis. The values are percentage contributions by each order, separated where appropriate into aquatic and aerial stage

Adults

Diptera Plecoptera Ephemeroptera Trichoptera Coleoptera Arachnoidea

Aquatic

Aerial

Aquatic

Aerial

1.8 8.5 7.1 7.6

55.2 7.3 3.9 1.6

0.7 5.9 1.5 17.0

52.5 9.5 8.0 2.3

Total items Number of faecal pellets


Nestlings

6.2 0.0

1.8 0.3

384 160

388 80

pairs per 10 km (Marchant & Hyde 1980a), other reported values being 3-25 in the New Forest (Tyler 1970), 0.9-8.5 in Wiltshire (Tyler & Tyler 1972), 12-19 in Switzerland (Schifferli 1972), 3.8-6.5 in Germany (Kolbe 1963, cited by Jorgensen 1977a), and 11.1 in Denmark (Jorgensen 1977a). However, marked differences were apparent between our various study plots, with Grey Wagtails particularly numerous on streams in the southern Wye catchment, despite the survey on these rivers being undertaken after the cold winter of 1982 (with January daily minima of -9 to -23°C), which might have been expected to reduce the Grey Wagtail population (Dobinson & Richards 1964; Tyler 1972; Marchant & Hyde 1980a). It was not entirely clear which factors most strongly influenced the distribution of Grey Wagtails in our study area. Along plots in the BTO Waterways Bird Survey, the mean abundance of Grey Wagtails exceeded 6 pairs per 10 km provided that river slopes were not less than 2.5 m km -1 (Marchant & Hyde 1980a). The lack of any significant relationship between slope and the abundance of wagtails in our study probably reflects the physiography of the streams: most exceed 2.5 m km -1 and they provide adequate feeding area. Similarly, Marchant & Hyde (1980a) recorded an increase in Grey Wagtail abundance as altitude increased from 0 to 300 m o.d. but we did not find a similar relationship, probably because steep and suitable streams occur in our area even near sea-level. In the New Forest (Tyler 1970) and Wiltshire

0.6 - 15.8

(Tyler & Tyler 1972), Grey Wagtails were more common along base-rich streams than along soft-water streams, possibly because of greater food availability in the former (Sutcliffe & Carrick 1973; Townsend, Hildrew & Francis 1983). There was some evidence from our study to support this suggestion: Grey Wagtails were commonest along hard-water streams in the Wye system, in which some invertebrates were also relatively abundant (see Fig. 2 and Brooker & Morris 1980; Ormerod 1985b; Ormerod et al. 1985). However, in view of the aerial component in the diet of Grey Wagtails, measures of aquatic invertebrate abundance do not necessarily reflect the food available. For example, trees surrounding rivers often contribute a large proportion of the insects in the riparian zone (Mason & Macdonald 1982) and Grey Wagtails probably exploit such prey. Indeed, both in this study and on soft waters throughout mid and north Wales, Grey Wagtails appeared to select streams which had treelined banks and areas of riffle, irrespective of hardness, acidity or the abundance of aquatic invertebrates (Ormerod & Tyler 1985). Similar habitat selection occurs in Grey Wagtails breeding in Denmark (Jorgensen 197Th) and wintering in East Africa (Tyler & Ormerod 1986). Further research on the reasons for such choice is in progress.

Breeding biology The timing of laying shown by Grey Wagtails in our study, with peaks in late April and in May, was closely similar to that given by


Breeding Grey Wagtails 49 Tyler's (1972) sample of 636 nest record cards. Our data also support Tyler's conclusion that the second peak represents replacement first clutches, rather than second clutches, because the interval between the 2 peaks (4 weeks) is less than that required for the successful production of a first brood (minimum 28 days plus transition to independence). Clearly, many pairs attempt to breed in April despite the high probability of failure (about 2 in 3), presumably because, when successful, this strategy somehow enhances fitness. There was evidence that air temperature prior to the breeding season had some influence on the onset of laying. Similar relationships with spring temperature have been described in other passerines, including differences in relation to altitude (Coulson 1956; Perrins 1965). We do not know whether seasonal or altitudinal patterns in air temperature affected our birds directly, such as through energy demands and body condition, or indirectly, through food availability. In other birds, the breeding season often coincides with a period of peak food availability for rearing chicks (Lack 1954, 1966; Perrins 1970) and the same appears true for Welsh Grey Wagtails: 'in the Wye catchment, the number of aerial insects around the main river increases between April and July, remaining high until September (Morris 1981). However, small upland tributaries warm up most slowly in spring, with consequent effect on invertebrate development and emergence (Ormerod 1985b). The only identifiable influence on clutch size in our study was laying date, with the largest clutches occuring in May. Similar seasonal

changes occurred in Grey Wagtails breeding in Denmark (Jorgensen 1977a) and Switzerland (Schifferli 1972). Whilst we cannot rule out the influence of seasonal changes in food availability, it was noteworthy that the largest clutches were laid when breeding success increased markedly. The greatest cause of failure for breeding Grey Wagtails is predation (Tyler 1972), although its incidence declines markedly with seasonal progression as nests become overgrown and hidden (Tyler, unpublished). We, therefore, suggest the possibility that Grey Wagtails make their largest reproductive investment when the probability of failure is relatively low. In June, despite the low incidence of failures, clutch size declined, possibly because birds were breeding for the second or third time. Comparing our study with others, the mean clutch size laid by Welsh Grey Wagtails was smaller than the means in Denmark and Switzerland, similar to those in Belgium and Scotland, but larger than the mean for Britain as a whole (Table 3). Tyler (1972) suggested that latitudinal influences were apparent in Grey Wagtail clutches within Britain and the high proportion of southern clutches in her sample might explain the difference from this study. However, there is no simple latitudinal trend in Europe as a whole (Table 3) and the influence of local factors requires further study.

Diet In other insectivorous passerines, including Pied Wagtails Motacilla alba, faecal analysis faithfully reproduces diet when either aquatic

Table 3. Clutch size in studies of Grey Wagtails. The values are means, with standard deviations and sample sizes in parentheses. Probabilities refer to significance tests of differences from our study, based either on Mann-Whitney U-test (n < 50) or approximate t-test (n > 50); n. s. = not significant

Source

Clutch

Denmark (Jorgensen 1977a) Switzerland (Schifferli 1972) Scotland (Nicoll 1979) Belgium (Herroelan 1955) This study Britain (Tyler 1972)

5.45 (0.88, 126) 5.35 (0.77, 151) 5.16 (0.57, 43) 5.12 (1.03, 41) 5.07 (0.56, 147) 4.93 (0.70, 203)

< 0.001