British Birds. Laying dates of four species of tits in Wytham Wood, Oxfordshire E. K. Dunn

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1 British Birds VOLUME 69 NUMBER FEBRUARY I976 Laying dates of four species of tits in Wytham Wood, Oxfordshire E. K. Dunn It has been argued by Perrins (970) that laying a large clutch imposes a considerable extra load on the food-gathering abilities of a small bird. From this, he suggested that the date at which individual birds in a population begin laying is related to the food supply at the start of the breeding season: egg-laying cannot begin until food has become sufficiently abundant for each female to find enough to start producing eggs without risk to her own body maintenance. Jones (97) provided indirect support for this hypothesis by demonstrating that small female Great Tits Parus major tend to lay earlier than large ones, probably because smaller birds require less food for body maintenance (Gibb 957). Perrins (in prep.) has extended this interpretation to explain why the smaller tit species lay earlier than the larger ones. Although the tits of the genus Parus have been the subject of detailed research (Lack 97, Barnes 975) our knowledge of their laying dates is surprisingly incomplete. It is known that in deciduous woodland Blue Tits P. caeruleus start laying before Great Tits (Lack 966, Frederiksen et al. 97). Coal Tits and Marsh Tits P. palustris, however, breed rather sparsely in deciduous woodland and little has been published on their time of laying. The data collected by Frederiksen et al. in Denmark suggest that Coal and Marsh Tits begin laying at about the same time as Blue Tits, but their sample sizes were small. Ryves (9) has shown that in north Cornwall Coal Tits fledge before Blue Tits, though this does not necessarily imply an earlier laying date for Coal Tits since they usually lay the smaller clutch; in other words, a Coal Tit could start laying at the {Brit. Birds, 69: 5-50, February 976] 5

2 6 Laying dates of tits same time as a Blue Tit and still fledge its young earlier. Thus there is no conclusive evidence that either Coal Tits or Marsh Tits differ significantly in their laying dates from Blue Tits. The study of tits in nest boxes in Wytham Wood near Oxford, started by Lack in 97, had, by 975, yielded records of laying dates for Coal Tits and 80 for Marsh Tits. Here, these are compared with the now more extensive data on Blue Tits and Great Tits, and with published information on Long-tailed Tits Aegithalos caudatus in Wytham (Lack and Lack 958, Gaston 97). METHODS For each species in each year, the mean date on which the first egg was laid, excluding repeat and second clutches, is taken as a measure of the timing of breeding. Following Kluyver (95) and Lack (966), laying dates have been numbered from st April, which is, so that 0th April is 0 and st May is and so on, and the means are taken to one place of decimals. The nest boxes in Wytham are distributed over eight areas of woodland which differ to varying extents in elevation, slope, aspect and vegetation. These differences are reflected in the laying dates of the tits which inhabit each area, though in the cases of Blue and Great Tits, which breed in relatively large numbers throughout the wood, laying dates for the whole wood are little affected by area-specific differences.* RESULTS The data are shown in table. Coal Tits and Marsh Tits were only occasionally recorded in nest boxes before 959. The year before, Dr G. M. Perrins had begun to expand the study by putting up boxes over a much wider area of the Wytham estate and, since 96, about 900 boxes embracing all the 0 ha of woodland have been available to the tits. In addition, laying dates tended to be more advanced in these early years of the study (97-6: Blue Tits.0, Great Tits 6.9; 96-75: Blue Tits 7.0, Great Tits 0.8). Thus, laying dates for as a whole tend to underestimate the real gap separating Coal and Marsh Tits from the other two species. A more meaningful comparison uses only those years when all four species were recorded breeding in nest boxes. The table shows that there was considerable variation in the order of laying over the 8 years concerned, but Coal Tits appeared on average to lay first, a day before Marsh Tits, two days before Blue Tits, and almost six days before Great Tits. Matched pairs t-tests showed that each of the three smaller tits laid significantly earlier than Great Tits. Each year's laying dates of the small breeding populations of Coal and Marsh Tits have been corrected for area differences by weighting means, but those of Blue and Great Tits have not.

3 *qying dates of tits 7 "able i. Mean annual laying dates of four tits Parus spp in Wytham Wood, Oxfordshire "he dates are calculated from ist April, which is I, so that 0th April is 0 and st May is, and the means are taken to one place of decimals T ear COAL TIT P.m tor MARSH TIT P. palu. stris BLUE TIT P. caendeus GREAT TIT P. major > i ' ' '9-... i-i ' i '5 i 8 7 ' 8 ' '5 ' i ' ' ' » IEANS aid -5 TOTALS JEANS FOR SEARS - KCOMMON Coal Tits also laid significantly earlier than Blue Tits (t =.5, j < 0.05, -tailed), but not earlier than Marsh Tits, which, in turn, dd not lay significantly earlier than Blue Tits.

4 8 Laying dates of tits DISCUSSION Laying date and clutch size It used to be thought (Dr J. Gibb, in Lack 955) that the Blue Tit's larger clutch might explain why it started laying a little before the Great Tit in deciduous woods. By starting earlier, its young would therefore hatch at about the same date as those of the Great Tit, and both would benefit from the peak emergence of caterpillar food. Perrins (965, 970) suggested diat other factors were likely to be involved, and a simple relationship between clutch size and laying date is unlikely. If the clutch sizes of the four Parus species in Wytham are listed in order of the laying dates found here, no consistent trend emerges: Coal Tit 9.7, Marsh Tit 7.7, Blue Tit 0.7, Great Tit 9.5. (The last two values are from data for 97-6 in Lack (966), the odier two from Wytham nest record cards for ) Laying date and body weight The laying dates follow in the same sequence as the body weights of the species concerned. Average winter weights of birds caught in Wytham are: Coal Tit 8.9 g (0 birds), Marsh Tit 0.5 g (8), Blue Tit 0.7 g (50), and Great Tit 9. g (90). (The figure for Marsh Tits was calculated from recent records, and those for the other species from data in Owen (95).) The Great Tit, which is by far the heaviest of the four species, lays markedly later than the other three. Furthermore, the lightest tit species in the wood, the Longtailed Tit, weighing on average 7.7 g ( birds, this study), is by far the earliest breeder, sometimes starting as early as the end of March, and on average probably not later than about st April (Lack and Lack 958, Gaston 97). Thus the general pattern emerges of progressively earlier laying with descending order of body size, which agrees with Perrins' (in prep.) predictions. Unfortunately Willow Tits P. montanus, which at 0.o g (0 birds, this study) are intermediate in weight between Coal Tits and Marsh Tits, do not breed in the Wytham nest boxes and therefore there are no records of their laying dates in the wood. Laying date and habitat Van Balen (97) has shown that the maximum food supply in oakwoods is much larger, and peaks earlier, than that in pinewoods. For several years, Lack (955, 958) compared the laying dates of tits in a number of broadleaved (mainly oak Quercus robur) and coniferous (mainly Scots pine Pinus sylvestris) habitats in the south of England. In Lack's analysis, the data for broadleaved woods were lumped for the purpose of comparison with conifers; he found that there were no consistent differences between laying dates in the two sorts of habitat. In table, the laying dates for 99-5 of Great

5 Laying dates of tits 9 Tits and Coal Tits in Mousehall, a Scots pine plantation in Norfolk, are compared with the corresponding figures for Marley Wood in Wytham; there is again no difference (matched pairs t-test). Lack concluded that Great Tits, and also Blue Tits, are primarily adapted to broadleaved woods and unable to adjust their laying dates to coniferous woods. Van Balen (97) replicated Lack's findings for the Great Tit and provided support for Lack's conclusion by demonstrating that the tits' mean hatching date in Dutch pinewoods correlated well with the earlier caterpillar peak in nearby oakwoods, but not at all with the peak in the pinewoods. The Great Tits were apparently unable to time their breeding to profit from the later emergence of caterpillars in the coniferous habitats. Lack did not attempt to assess the situation for Coal Tits, which are known to prefer coniferous habitats (Partridge 97). The Marley data for this species were very limited for 99-5 but there Table. Mean annual laying dates of Great Tits Parus major and Coal Tits P. ater in Marley Wood, Oxfordshire, and Mousehall, Norfolk Data for Coal Tits in Marley from this study, but not weighted for area, the rest from Lack (955). The dates are calculated from st April (see table ) GREAT TTT COAL TIT Year Marley Mousehall Marley Mousehall 99 '95 '95' 95a l8 MEANS DIFFERENCES C- days.8»9-. days is some indication (table ) that laying tended to be earlier in Mousehall than in Marley. Thus the Coal Tit may be able to start breeding earlier in its preferred coniferous habitats in spite of the fact that food during the spring is probably much more abundant in broadleaved woods. ACKNOWLEDGEMENTS I wish to thank Derek Onley, who first suggested this topic to me a long time ago, and Dr C. M. Perrins, who suggested it independently. The data have been collected by numerous fieldworkers climbing an estimated 60 km of ladder in Wytham Wood since 97. I wish to thank Dr Perrins, Dr A. W. Diamond, Dr L. Partridge and M. C. Garnett, who kindly read and criticised an earlier draft.

6 5 Levying dates of tits SUMMARY Few data have previously been published on the breeding seasons of Goal Tits Parus ater and Marsh Tits P. palustris in deciduous woodland. From a long-term study of tits in Wytham Wood near Oxford, typical laying dates of these two species have been calculated, along with those for Blue Tits P. caeruleus and Great Tits P. major. Comparisons between these and dates for Long-tailed Tits Aegithalos caudatus, which also breed in the wood, suggest that the order of laying follows a sequence of increasing body size, beginning with the Long-tailed Tit. This trend is consistent with current theory on die timing of breeding seasons. The Coal Tit fits into the sequence of laying despite the fact that its preferred habitat is coniferous woodland. However, it is suggested that its breeding season may start slightly earlier in coniferous than in deciduous woodland. REFERENCES BARNES, J. A. G. 975 The Titmice of the British Isles. Newton Abbott. FREDERIKSEN, K. S., JENSEN, M., LARSEN, E. H., and LARSEN, V. H. 97. 'Nogle data til belysning af yngletidspunkt og kuldstorrelse hos mejser (Paridae)'. Dansk Orn. Form. Tidsskr., 66: GASTON, A. J. 97. 'The ecology and behaviour of the Long-tailed Tit'. Ibis, ii5:0-5>- GIBB, J 'Food requirements and other observations on captive tits'. Bird Study, : JONES, P. J. 97. 'Some aspects of the feeding ecology of the Great Tit Parus major L.\ Unpublished Ph.D. thesis, University of Oxford. KXUYVER, H. N. 95. 'The population ecology of die Great Tit Parus major L.' Ardea, 9: -5. LACK, D 'British tits (Parus spp) in nesting boxes'. Ardea, : 'A qualitative breeding study of British tits'. Ardea, 6: Population Studies of Birds. Oxford. 97. Ecological Isolation in Birds. Oxford. and LACK, E 'The nesting of the Long-tailed Tit'. Bird Study, 5: -9. OWEN, D. F. 95. "The winter weights of titmice'. Ibis, 96: PARTRIDGE, L. 97. 'Habitat selection in titmice'. Nature, 7: PERRINS, C. M 'Population fluctuations and clutch-size in the Great Tit Parus major L.\ J. Anim. Ecol., : 'The timing of birds' breeding seasons'. Ibis, : -55. (in preparation) Tits. London. RYVES, B. H. 9. 'Fledging of tits in north Cornwall'. Brit. Birds, 7: 77. VAN BALEN, J. H. 97. 'A comparative study of the breeding ecology of the Great Tit Parus major in different habitats'. Ardea, 6: -9. Dr E. K. Dunn, Edward Grey Institute, Zoology Department, South Parks Road, Oxford oxi PS