Differences in reproductive success and parental ...

Differences in reproductive success and parental qualities between habitats in the Great Tit Parus major R. RIDDINGTON* & A. G. GOSLERt Edward Grey Institute of Field Ornithology, South Parks Road, olcford OX1 3PS, UK

Reproductive success and parental attributes of Great Tits Parus major nesting in good and poor habitats were compared to investigate whether they Mered and, if so, whether parental or environmental variation contributed most to the differences. Monitored over 3 years, clutches in mature woodland were started earlier, were larger and produced larger broods of heavier chicks than those in marginal habitats such as gardens and hedgerows. Fledging success was significantly higher in woodland in 2 of the 3 years although egg weights were lower. Parents nesting in different habitats were very similar in body-size although those in woodland were in better condition. The prey delivered to nestlings in woodland was of significantly higher quality than that delivered in marginal habitats. Furthermore, the heritability of body-size (tarsus-length) was higher in woodland, suggesting that the environment limits nestling growth in poorer habitats. Reproductive success was considerably lower in poor habitats and appears to be constrained primarily by environmental quality, although this may operate partly through its affect on parental body condition.

woods. Nestling survival rates were much poorer in pinewoods. Cowie and Hinsley (1987) reported that both Great Tits and Blue Tits Parus cueruleus laid smaller clutches in gardens and managed to rear to fledging only half as many young per pair as they did in deciduouswoodland.However, Lemel(l989) found little evidence for a difference in reproductive success of Great Tits breeding in deciduous and coniferous forest in Sweden. He also suggested that males settling in richer habitats (deciduous woodland) did not have a higher resource-holdingpotential (Parker 1974)than those in poorer habitats. Despite this, morphological differences may exist between Great Tits in differentwoodland habitats. For example, Ulfstrand et al. (1981)found that bill- and winglengths of male Great Tits were significantly longer in deciduous than in coniferous woodland. Clearly, the. importance of differencesin habitat quality and of distribution to the population dynamics of tits is, as yet, unresolved since studies have yielded contradictory results. This paper compares several aspects of reproductive success, parental attributes, chick diet and heritability of bodysize of Great Tits in presumed good or poor habitats. The work was carried out as part of the Edward Grey Institute’s Iong-term population studies of tits in Wytham Woods near Oxford, U.K. (for background see Perrins 1965,1979, Lack 1966, Gosler 1993a). In particular, we assess whether parental or environmental attributes contribute most to observed differencesbetween habitats. The study increases our understanding of the effects of habitat fragmentation and heterogeneity that characterize many modern landscapes.

Most of western Europe is now characterizedby fragmented habitats of variable, but often low. quality. For example, small pocketsof ancient semi-naturalwoodland are the remnants of once extensive forest cover. To understand the dynamics of bird populations in these areas of fragmented habitat, it is important to investigate the patterns of movement between fragments and to recognize the costs and benefits incurred by individuals in areas of differing environmental quality. The genus Parus probably has been studied more intensively than any other group of birds during the last 60 years. Several of these studies have compared the ecology of Great Tits Parus major in different habitats, e.g. Lack (1955),Ulfstrand (1962). Perrins (1965), van Balen (1973). Ulfstrand et al. (1981) Berressem et al. (1983), Dhondt et al. (1984). Schmidt and Einloft-Achenbach (1984). Cowie and Hinsley (1987) and Lemel (1989). Some of these studies have compared attributes of populations and individuals nesting in different habitats. Van Balen (1973) found marked differences in the density of Great Tit pairs between mature oak Quercus spp. woodland (1.0-2.3 pairs/ha) and pine Pinus spp. woods (0.1-0.4 pairslha). At such densities, clutch-size was similar in both habitats; however, when population densities were similar, clutch-size was much greater in oak-

* Current address: Fair Isle Bird Observatory,Fair Isle, Shetland ZE2 9JU. UK.

t Author for correspondence. 371

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STUDY AREA A N D METHODS The Wytham Woods study area comprises approximately 320 ha, largely deciduous woodland (see Gosler 1990 for habitat details). Approximately loo0 nestboxes are located within this study area, and standardized monitoring during the tits’ breeding season provides data on all nesting pairs using the boxes (Perrins 1979). Largely for logistic reasons, the woods are divided into nine nestbox areas or ‘rounds’. Data presented in this paper were collected from three of these rounds, known as Extra, Great Wood and Common Piece (Minot 81Perrins 1986).Together these contain almost 300 nestboxes and include examples of all the major woodland types found on the Wytham estate (see Table 1)and thus form a representative sample of the whole study area. These data are here referred to as ‘Wytham’data. To compare the Wytham Great Tit data with those from poorer habitats, an additional round of 155 nestboxes (‘external round’) was established outside the Wytham Woods study area during the winter of 1988-1989. Nestboxes were located in a variety of habitats within 2 km of the Wytham Woods boundary. ‘Garden’habitats ranged from large village gardens (often with trees and bushes or scrub cover) to small town gardens with little mature vegetation. ‘Woodlands’ comprisedsmall (1-10-ha) copses. These were mostly deciduous and typically dominated by oak or Ash Fraxinus excelsior with a well-developed understorey. ‘Sub-optimal’ habitats were considered a priori (although based on studies elsewhere) to be intermediate between gardens and woodland in their ability to provide food of sufficientquantity and quality for tits during the breeding season. This category contained areas such as hedgerows, farmyards,areas of scrub with scattered trees, orchards and churchyards. The ranking in quality of these habitats between gardens and woodlands largely reflects the differencesin tree density between them. Data presented here are primarily concerned with the garden and sub-optimal habitats, so for present purposes they are grouped together as ‘marginal’habitats. These marginal sites contained 90 nestboxes (58% of the total), the remaining 65 being in woodland fragments. The majority (75%)of nestboxes were erected specifically for this project. All nestboxes in gardens were of wooden construction with an entrance hole of about 32 mm diameter, a volume of at least lo00 cm3and an opening lid. Most nestboxes in the other areas were of the concrete, predator-proof design used in the main Wytham study area and described in Perrins (1979). All nestboxes were visited weekly from the beginning of April, and the following information was recorded: (1)date of the first egg, calculated assuming that females lay one egg each day: (2) mean egg weight, from a sample of unincubated eggs: (3) clutch-size: (4) hatching date, dculated by using a growth curve (Gibb 1950)to age chicks: (5) brood size: (6) fledgingsuccess,calculatedas the number fledged from nests which fledged any young since differencesin predation rates among box types would make invalid a comparison of the rates of total nest failure: (7) the nestlings were measured and ringed on their 15th day, by which time their weight

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Table 1. Characteristics of Great Tit nestbox: rounds. Wytham Woods, OxJordshire (Gosler 1990 and unpubl. data) No.

Round

Habitata

(ha)

nestboxes

Extra Common Piece Great Wood

2, 3, 4, 5, C 2, 3, c 1. 5

60.7 19.0 25.9

138 SO 105

Area

Altitude 92-165 84-122 61-92

(m)

a Habitat types: 1, ancient semi-naturalwoodland: 2. Sycamore Acer pseudoplatanus woodland: 3. young, broad-leaved plantation:4. beech woodland 5, scrub: C, major stands of conifer;c. broad-leaved with occasional single or a few conifers.

has stabilized. Each chick was weighed on a Pesola spring balance to the nearest 0.1 g. Tarsus-length was measured on a stopped rule to 0.5 mm (Gosler 1991). These procedures resulted in the lay-dates of a few unusually early nests being missed (especially in 1990). Similarly, lay-dates of very late broods (discoveredafter hatching) were sometimes missed. This resulted in small differences in sample size between lay-dates and 15th-daydates. However, although we assume that any biases caused by this would be similar in all habitats, so that comparisons made between habitats should, nonetheless, be valid. the means show some anomalies (see further discussion in Results). Parents were trapped at the nest after the chicks were 6 days old, and the following data were collected: (1)age (first year or older) and sex (Svensson 1984): ( 2 ) wing-length, maximum chord measured on a stopped rule to 0.5 mm: ( 3 ) tarsus-length, as for chicks: (4) moult (in first-yearbreeders, the number of retained juvenile greater coverts was recorded as this may indicateenvironmentalstress during moult [Gosler 19911): (5) fat score measured as the amount of fat in the tracheal pit recorded on a six-pointscale, 0-5: (6)muscle score, a measure of the shape of the pectoral muscle and prominence of the sternal keel, taken on a five-point scale, 0 - 2 (including half points), which may be a measure of protein reserves (Gosler 1991): (7) weight, as for chicks, and time of weighing. For a more detailed discussion of the condition scores used, see Gosler (1991). Seven of these parameters were considered to reflect the breeding successof a pair (lay-date,mean egg weight, clutchsize, brood-size,day-15 date, mean day-15 chick weight and number fledged), and these were compared between the Wytham and marginal sites. These comparisons were made as a series of unbalanced two-way ANOVAS by using the Generalized Linear Model (GLM) procedure of the MINITAB statistical package (Ryan et al. 1985). These allowed us to examine simultaneously the effects and interaction of habitat (Wytham or marginal) and year on the variable under investigation. Three measurements of adults which reflect body-size (wing-length,tarsus-length and weight) were compared be-

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tween Wytham and marginal sites. Although the relationship between body-size and individual quality or resourceholding potential is not understood. body-size is probably an indicator of status (Garnett 1981). Wing- and tarsuslengths and two of the measures of body condition (muscle score and greater covert moult) were compared between sites in an unbalanced two-way ANOVA (as above). Weights and fat scores were analysed in a similar way except that these were first adjusted for the effect of time of day by analysis of covariance (ANCOVA) as these measures are known to be strongly time dependent (Gosler 1991). To test the hypothesis that poorer feeding conditionsmight affect nestling growth and subsequently the similarity between them and their parents (van Noordwijk et al. 1988). the heritability of tarsus-length was estimated in each habitat by the regression of brood mean tarsus-length on midparent mean (i.e. the mean of the two parents) tarsus-length. The heritability (hZ)was then taken as the slope of the regression line and these were compared by ANCOVA. Tarsuslength was preferred for this analysis because, unlike winglength and weight, it does not vary after day 15 (Garnett 1981. van Noordwijk et nl. 1988). In 1991, faecal sacs were collected from chicks on their 15th day to compare their diets in the different habitats. These were preserved in 70% ethanol to prevent decay and refrigerated until analysed to prevent evaporation of the preservative. Since caterpillars (Lepidoptera larvae) are the main and preferred nestling food of Great Tits (see reviews in Gosler 1993a,b), our analysis concentrated on the relative abundance of these in the diet and followed the methods of Gosler (1987). Each sample was examined under a microscope and scored for the relative abundance of six prey categories: (1) caterpillar spiracles; (2) caterpillar mandibles; (3) non-caterpillar prey (NCP); (4) flies (Diptera);(5) beetles (Coleoptera): (6) spiders (Araneae). NCP is an estimate of the percentage of identifiable items that are not from caterpillars (hence a score of 0 = absent, 1 = about 20% of identifiable material is non-caterpillar, 2 = about 40%, 3 = about 6O%, etc.). A11 other categories are simple relative abundance scores on a scale of 0-5. where 5 was the maximum of the specified type found in any sample. Thus categories (1)and (2) are independent assessmentsof the quantity of caterpillar prey in the sample, NCP is a generalized index of the non-caterpillar prey, whilst the remaining three categories indicate the abundance of specific non-caterpillar prey. Each sample was given a unique and arbitrary code number so that it could be analysed ‘blind’ to prevent any observer bias of the data. Comparison of these data between Wytham and marginal sites was made by Kruskal-Wallis tests.

RESULTS Reproductive success Table 2 presents the results of comparisons between sites with regard to reproductive parameters. Eggs were heavier

373

in marginal sites than in Wytham in all years. Clutches in Wytham were larger and started earlier (significantlyso in 2 years). the biological significanceof this is discussed below. Brood-size was larger and fledged young were heavier in all years in Wytham than in marginal sites. In 2 years (1990, 1991).fledging success was considerably better in Wytham than in marginal sites. In 2 years, Wytham nestlings reached their 15th day earlier than did nestlings elsewhere (significantly so in 1990). whilst in 1989 they were slightly later. This contradicts the differences observed in lay-date. The discrepancy of about 2 days in 1989 is due to the loss by desertion or predation of clutches or broods from the Wytham sample. Had we excluded failed nests from the calculation of lay-dates, the difference in 15th day between habitats would have been more or less equivalent to the difference in lay-dates, less any difference in clutch-size between habitats. In summary, of the seven measures of reproductive success analysed. Wytham pairs appeared to be significantly more successful in all regards except their egg weights and, perhaps, breeding dates than in marginal sites. However, because we have no information on the availability of nest sites other than nestboxes in marginal habitats, and therefore on the probability of a nest being detected there compared with Wytham, our conclusionsregarding first-egg and 15th-day dates are somewhat equivocal. O u r results, however, are comparable with earlier comparative studies of the timing of breeding, which also have used data only from nestboxes. Previous work on the breeding biology of the Great Tit has shown that timing has a strong influence on other reproductive parameters and is critical for their success (Kluyver 1971). For example, earlier-fledged young tend to be heavier and have a higher probability of surviving to breed, and while later-laid eggs tend to be heavier, later clutches and broods tend to be smaller (reviews in Perrins 1970, 1979. Gosler 1993a). Hence the difference between sites in clutch initiation could explain some of the other differences observed between sites. To investigate this, we carried out a further series of analyses on egg weight, clutch-size,broodsize, day-15 weight and number fledged, in which the firstegg date (lay-date) was entered as a covariate to remove its effect (Table 2). Lay-date had a highly significant effect on clutch-size, brood-size and number fledged but no effect on day-15 weight. Although differences between sites in the timing of breeding could explain some of the variation between them in the other parameters, significant differences still remained between them in all five parameters tested.

Parents The breeding Great Tits in Wytham tended to contain proportionately more first-year males that those using marginal sites. This tendency was statistically significant only when data from all years were combined. However, no such difference was found in the female population (Table 3). Apart from female wing-length, which was significantly larger in

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Table 2. Reproductive success of Great Tits in Oxfordshire, 1989-1 991, in Wytham Woods (mature woodland) and external round (marginal habitats). The table shows mean values t s.d. (sample size in parentheses may vary for logistic reasons [see text] and due to losses) of each parameter, for broods in each area

by-date (external round) (‘aprildate’) (Wytham) Habitat, F,.,,, Egg weight (9)

=

1989

1990

1991

24.7 t 11.30 (17) 22.7 f 6.29 (102)

27.3 t 6.22 (20) 16.3 f 7.59 (80)

27.4 f 9.10 (17) 21.7 f 6.75 (57)

31.5, P < O.OOO1; year. F,,,,,

=

2.17, n.s.; habitat x year, F2,,87= 5.85. P < 0.01

1.740 f 0.096 (11) 1.678 f 0.128 (110)

1.706 f 0.129 (22) 1.724 f 0.127 (15) 1.616 f 0.114 (57) 1.677 f 0.121 (70) Habitat. F1.279 = 6.77. P < 0.01: year, F,.,79 = 1.23. n.s.: habitat x year, F2,,79= 2.81. n.s.

Clutch-size

7.95 t 1.64 (20) 8.48 f 1.75 (112)

Habitat, F,.,,,

=

6.77 k 1.77 (22) 8.88 f 2.32 (104)

23.2, P < O.OOO1: year, Fz.I,p= 1.5. n.s.: habitat x year, F,,,,,

Brood-sizea

6.86 f 1.88 (21) 7.62 f 1.99 (108)

Habitat. F,,,,,

=

26.8. P < O.OOO1: year, F,,,,,

Day-15 date (‘aprildate’)

= 0.05,

58.7 f 9.29 (17) 59.5 f 6.31 (94)

Habitat. F,.269= 14.3, P < 0.01; year, F,.,, Day-15 weight (g)

=

=

n.s.: habitat x year, F,,,,,

6.71 f 2.09 (14) 7.79 f 2.40 (56) =

5.91. P < 0.01 64.0 f 8.11 (12) 60.2 f 6.36 (53)

16.8. P < O.OOO1; habitat x year, F2.,69= 8.4, P < O.OOO1

17.5 i 1.70 (17) 19.0 f 1.08 (80)

7.12 f 1.69 (17) 7.28 t 2.18 (93) =

2.8. n.s.

17.3 f 1.86 (10) 18.5 f 1.74 (56)

25.7, P < O.OOO1; year, F2.1M= 0.8. n.s.; habitat x year, F,,,M = 0.3, n.s.

Number fledgeda Habitat. F,.,,,

=

5.79 f 2.22 (19) 8.91 f 2.03 (91) 59.1 f 6.08 (17) 49.9 f 5.94 (82)

17.0 t 4.57 (17) 18.7 t 1.13 (90) Habitat, F,,,,

7.65 f 1.94 (17) 9.17 f 1.93 (60)

26.7. P < O.OOO1: year. F,.,,,

5.41 f 2.24 (17) 8.61 f 2.11 (82) = 2.16.

ns.; habitat x year, F,,,,5

5.10 f 2.18 (10) 7.40 k 2.23 (50) =

7.4. P < 0.001

Note. In addition. ANCOVAS were carried out. with lay-date as a covariate. habitat as a factor and egg weight, clutch-size. brood-size. fledging weight and number fledged as dependent variables, to test for differences between habitats, while holding lay-date constant. The results are summarized as follows. Egg weight: habitat, F,,,,, = 5.7, P < 0.02: lay-date. F,,,,, = 4.34. P < 0.05.Clutch-size: habitat, F,,,,, = 13.3. P < O.OOO1: lay-date. F,,,,, = 46.9. P < O.OOO1. Brood size: habitat. F,,,,, = 14.8. P < O.OOO1: lay-date, F,,,,, = 28.5. P iO.OOO1. Day-15 weight: habitat, F,.,,, = 15.4, P < O.OOO1; lay-date, F,,,,, = 0.9. n.s. Number fledged habitat, F,,,,, = 11.4, P < 0.001; lay-date. F1,225 = 35.9. P < O.OOO1. Statistics show results of unbalanced two-way ANOVA, with habitat. year and an interaction term, habitat x year, as factors in the model. a Brood-size and number fledged refer to successful broods only (see text).

Table 3. Age of breeding Great Tits 1989-1991 in Wytham Woods and external round (marginal habitats) study areas. Age is given as l y (Erst year birds) or 2 y + (older). A xz test (d.j. = 1 ) was carried out to compare age ratios in the two areas in each year Males Wytham

External

Year

ly

2y+

1Y

2Y+

1989 1990 1991

32 46 26 104

55 47 27 129

2 5 3 10

14 11 8

A11

Females

33

Wytham

External

x21

P

1Y

2Y+

1Y

2Y+

3.6 1.8 1.7 6.8

ns. n.s. n.s.