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A N ADAPTATION FOR EARLY GROWTH IN TITS, PARUS SPP.

Those tits (Parus spp.) laying early each season in oakwood in southeast England are forced by shortage of food for egg-formation to produce eggs of the smallest viable size (Perrins 1970). Reduced egg-size is correlated with reduced growth on the part of the nestlings to which they give rise (Schifferli 1973). On the other hand, postponement of laying results in the nestling period no longer coinciding with the seasonal peak in the defoliating caterpillars on which the nestlings are fed, and the parents then have difficulty in feeding their young adequately (Lack et al. 1957, Perrins 1965). Both these effects (of egg-size and of timing) are most pronounced in large broods. The present report suggests an effect compensating for the reduced egg-size in early broods: a large brood provides a greater stimulus for parental foraging than does a smaller brood. This in turn benefits the late hatched nestlings in the large brood, enabling the young to attain quickly the weight appropriate to a larger egg-size in the first instance.

Data were obtained from nestbox broods of Blue Tits Parus caeruleus and of Great Tits P. major in Wytham Wood, Berkshire, in 1970 and 1971, and on Mahee Island, Co. Down, in 1974. I n 1970 and 1971 a total of 20 nests of the Blue Tit were visited daily from shortly before hatching began in the nest until all eggs in the clutch had either hatched or were clearly not going to do so; clutches took up to four days to complete hatching. Any chicks hatched since the previous visit 24 h earlier were weighed to 0-02 g on a spring balance; chicks were therefore on average 12 h old when first weighed. These weights were examined in relation to the number of young in the nest (a) for birds hatched on the first day on which eggs hatched in their nest, (b) for birds on the second day of hatching for the nest and (c) for birds hatched subsequently. Data from 1970 and 1971 were analysed separately (Table 1) since caterpillar numbers in the two years were very different (see below). In five of the six analyses the weight of the recently hatched young increased with the number of chicks in the nest at the time; in the sixth analysis- that for late hatched young in 1970-weights were lighter in the larger broods.

TABLE 1

Regressions of day 0 weight on nest contents for Blue Tits

Slope Intercept Significance Year glchick g level

First day of nest hatching 1970 0-046 0.648 P < 0.01 1971 0.01 3 0.929 n.s.

Second day of nest hatching 1970 0.036 0.724 n.s. 1971 0.039 0.748 P < 0.05

Later days of nest hatching 1970 -0.071 1.678 n.s. 1971 0.160 -0’518 P < 0.01

Note: The slopes and intercepts given are those calculated for regressions of the weights of individual chicks on their day of hatching (so that the chicks were on average 12 h old when weighed) regressed on the number of chicks of all ages in the nest at that time.

A similar effect can be shown for the Great Tit by a re-analysis of data collected in Wytham Wood by Schifferli (1973), who removed eggs of known fresh weight from nests shortly before hatching, and artificially incubated them; four to eight young were returned to nests to be reared by foster parents, selected without regard to the matching

524 SHORT COMMUNICATIONS IBIS 117

of brood-size with the original clutch-size of the fosterers or to the maintenance of sibling relationships. Examination of the growth-rates of these nestlings as a function of brood-size (Table 2) shows that early weight increases were higher for nestlings in the larger broods (correlation between growth rate and brood size = 0.477, P < 0-05), thus paralleling the results with Blue Tits.

TABLE 2

Growth rates in Great Tit broods in relation to brood-size

Brood-size Number of broods Growth rate g h-*

4 5 6 7 8

3 1 4 1 5

0.030 0.033 0.034 0.032 0-037

Note: Growth rates tabulated are average values over the period between the return of incubator hatched young to a foster nest and the time of first weighing 24-48 h later. The correlation between growth rate and brood-size was 0-477 (P < 0.05).

DISCUSSION

Five explanations could be postulated to account for these results: (1) chicks in larger broods hatch from heavier eggs; (2) birds laying large clutches do so because they are also more competent as parents; (3) the temporal pattern of hatching within a clutch is skew (for example, because incubation commences at a fixed time prior to clutch completion), so that larger clutches result in more young of greater than average age; (4) larger broods have smaller surface volume ratios and thus have lower respiratory weight losses; ( 5 ) young in the larger broods are fed more intensively by their parents, thus effectively anticipating their food shortage later in the nestling period.

The first, systematic variation of egg-weight with increase in brood-size, can be eliminated as an explanation. Such variation was specifically avoided in the Great Tit experiments by constructing each foster brood from equal numbers of chicks hatched from ‘heavy’ or from ‘light’ eggs. In the Blue Tit results the independent variable is the number of young in the nest at the time of weighing, rather than the final brood-size, and these two quantities were not necessarily correlated on the first two days of hatching. Furthermore, the differences between the third day results for 1970 and 1971 cannot be readily explained in terms of egg-weight patterns.

Parental competence can also be eliminated as an explanation. For the Blue Tits the differences between years are not readily explicable in such terms, whilst for Great Tits the experimental procedure made it as probable that a competent parent fostered a small clutch as a large clutch, thus precluding the explanation.

The third explanation-a skew distribution of hatching intervals-is also precluded by the experimental design in the case of the Great Tit, since the chicks were introduced simultaneously into the foster nest. For the Blue Tits the possibility can be eliminated by further analysis. Any effects due to skewness would be minimal in an analysis confined to the weights of the first n chicks to hatch in each nest, if n were small; here the weights of the first four chicks from each nest were analysed (Table 3). Heavy weights still pre- dominated in the chicks from the larger broods in both 1970 and 1971, significantly so in 1970. The results of Table 1 cannot therefore be attributed to variation in the mean age of the nestlings concerned.

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TABLE 3

Day 0 weights of the four smallest young in Blue Ti t broods in relation to brood-size

1970 Weights 1971 Weights > Median <Median > Median <Median

Brood size > Median 9 3 13 7 <Median 2 6 5 11

Fisher exact probability x2 = 2.61 P < 0.05 P= 0.10

Note: In 1970 median brood size was 7 and median weight was 0.90 g; in 1971 median brood size was 9 and median weight was 1.01 g.

Variation in surfacejvolume ratio with brood-size has been found to result in lower oxygen consumption per nestling in broods of Blue Tits and Great Tits 6-12 days old exposed to ambient temperatures (O’Connor 1975, Mertens 1969). However, very young nestlings of both species are brooded by the female almost continuously (Royama 1966) and surface/volume ratios are therefore of no significance to the young, and cannot result in differential weight losses in broods of different size.

There thus remains the hypothesis that newly hatched chicks in large broods were fed more intensively than were young in small broods. This explanation also accounts for the difference between 1970 and 1971, since in 1970 the density of defoliating caterpillars was only one-fifth of that prevailing in 1971 (G. R. Gradwell, pers. comm.); parents would therefore have had difficulty in meeting the food requirements of all their young at an early stage of the nestling period.

A hypothesis which would explain the differential response of the adults to each brood- size is that the adults take a finite time to change from incubating behaviour to feeding behaviour. If this is so, then reinforcement of the correct response to a begging chick will occur more frequently with the larger broods, and the weights of the chicks in such broods will increase more rapidly than is the case amongst the young of small broods. This was tested experimentally in 1974 by comparing the number of visits received by a chick on the first day that hatching had occurred in the nest with the visiting rate to newly hatched chicks in that nest 24 h later (Table 4). The results show that the later-hatched young receive more visits than did their earlier hatched siblings, thus supporting the hypothesis.

The pattern of weight increase with brood-size as a result of parental foraging rates increasing with experience may be regarded as an adaptation towards the rearing of large broods in the earlier part of the breeding season. When the food requirement of such broods are at their maximum (about day 12) the feeding rates required are so high that the parents tire towards the end of each day (Gibb 1955), and the young receive less food than they need for maximum survival. This problem is alleviated for late broods by an increase in egg-weight (Perrins 1965), allowing a larger hatching weight, faster growth and better fledgling survival (Schifferli 1973). For early broods, however, the females are already limited in their date of laying by a shortage of food for egg-formation (Perrins 1970) and the option of a compensatory increase in egg-weight is not open to them. Furthermore, any delay in laying leads to poorer breeding success anyway, since the nestling period is then retarded beyond the peak caterpillar period, requiring a reduction in clutch-size if any young are to be raised at all (Perrins 1965).

The data presented here, however, suggest that the foraging rates of the parents increase

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disproportionately rapidly with brood-size when the young are small, so that newly hatched young in large broods are fed more intensively than they would otherwise have been, thus achieving weights appropriate to larger egg-sizes in the absence of differential

TABLE 4

Feeding frequency in relation to recent parental experience of feeding young

Visits per nestling per hour (a) on the day hatching (b) on the day

Nest began in the nest following

a b C d e

2.4 5.4 _ - 3.0 2-8 2-0

6.0 20.0 5.3 3.0 7.5

Note: The rates of visiting measured are those to day 0 nestlings only; the older young in the nest on the second day were removed from the nests for the duration of the observation period since their larger size could have spuriously increased feeding rates. The probability that all five nests showed the same trend by chance is 0.03.

feeding rates. Such young consequently achieve greater growth during the early part of the nestling period, when the food requirements of the brood are within the foraging capacity of the parents, than do the young of smaller broods less likely to exceed a food limit late in the nestling period. The pattern is thus in line with the other adaptations displayed by tits in optimizing their breeding to the seasonality of their food supply.

I am indebted to Luc Schifferli for allowing me the use of his Great Tit data for the analysis presented here. I am also grateful to Drs D. G. Dawson, E. K. Dunn, A. Ferguson, C. M. Perrins and L. Sch8erli for helpful discussions, and to J. Forsyth and P. Mackie for help at Mahee Idand. The work in Oxford was carried out whilst in receipt of a Biological Scholarship from the NufEeld Foundation, whose support is gratefully acknowledged.

REFERENCES GIBB, J. 1955. Feeding rates of Great Tits. Br. Birds 48: 49-58. LACK, D., GIBB, J. & O m , D. F. 1957. Survival in relation to broodsize in tits. Proc. 2001. SOC.,

Lond. 128: 313-326. M w m s , J. A. L. 1969. The influence of brood size on the energy metabolism and water loss of

nestling Great Tits Parus m j o r major. rbii 111: 11-16. O’CONNOR, R. J. 1975. The influence of brood size upon metabolic rate and body temperature in

nestling Blue Tits Parus caeruleus and House Sparrows Passer domesticus. J. Zool., Lond. 175: 391-403.

PERRINs, C. M. 1965. Population fluctuations and clutch-size in the Great Tit ( P a m major). J. anim. Ecol. 34: 601-647.

PERRINS, C. M. 1970. The timing of birds’ breeding seasons. Ibis 112: 242-255. ROYAMA, T. 1966. Factors governing feeding rate, food requirement and brood size of nestling

Great Tits Parus major. Ibis 108: 313-347. SCHIFFERLI, L. 1973. The effect of egg weight on the subsequent growth of nestling Great Tits

Parus major. Ibis 115: 549-588.

Edward Grey Institute, Oxford

Department of Zoology, and

The Queen’s University, Belfast

18 July 1974

RAYMOND J. O’CONNOR