Ardea
Official journal of the Netherlands Ornithologists' Union

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van Balen J.H. (1973) A comparative study of the breeding ecology of the Great Tit Parus major in different habitats. ARDEA 61 (1-2): 1-93
In this report the breeding ecology of the Great Tit, Parus major, is compared in two types of habitat, oakwood and pinewood. The study is based on the striking difference in breeding density in two habitats, and is aimed at describing the feeding conditions and their effect on reproduction (especially the rearing of nestlings), in other words, at checking whether the observed breeding distribution has adaptive significance. Breeding data were obtained by regular nestbox: inspections, carried out over periods of 6 to 16 years in four areas, two pinewoods on sandy soil (Hoge Veluwe, Imbosch), and two oakwoods on more fertile soil (Oosterhout, Liesbosch). These areas ranged in size from 11.4 to 320 ha. As indicated in the literature, the breeding density in poorly developed pinewoods amounts to 1-4 pairs per 10 ha and in mature oakwoods on fertile soil varies from 10 to 23 pairs per 10 ha. The density figures for our study areas (Table 1) are in good agreement with these results. The feeding conditions were studied by sampling the caterpillars feeding in the canopy, which form the main type of prey fed to the nestlings. Sampling was performed by collecting, sorting, counting, and weighing the faecal pellets (frass) which drop from the canopy. Only those species and stages that were accepted as food, were sampled. Differences in feeding conditions between oakwoods and pinewoods were divided into differences in the level of caterpillar abundance and differences in the period of maximum abundance. The level of abundance is much higher in oakwoods than in pinewoods. This holds especially for the peak densities (Table 2), and to a lesser extent for the periods before and after the annual peak. The annual peak in caterpillar abundance in oakwoods usually occurs, in the second half of May. In the pinewoods the time of the annual peak varies between 10 June and 15 July. The timing of the breeding season was studied by analyzing the relationship between the period of maximal food requirements and the periods of maximal food availability. The timing mechanism has both proximate and ultimate aspects. The proximate aspect concerns the factors that directly determine the onset of laying and the development of the caterpillar populations. In oakwoods the onset of laying is affected primarily by the air temperature during the period from 1 March to 20 April, and the date of the caterpillar peak partly by temperatures during a much later period. This means that the hatching of the young tits can only be adjusted approximately to the period of optimal feeding conditions. The young hatch on average 7 days before the caterpillar peak, but this interval is rather variable (Tables 5 and 7). Evidence was obtained showing that the food requirements of the young are maximal shortly before and after fledging. On the other hand, it was concluded from the amount of weight lost by the female parents that they go through a critical period at the transition from the incubation to the nestling period. The breeding season is timed such that the caterpillar peak usually falls between these two critical periods. The onset of laying in pinewoods is affected by the air temperature in the same way as in oakwoods. The average laying dates of the first clutches in both habitats do not differ significantly. The average laying date in pinewoods (and consequently the average hatching date) is not correlated with the date of the caterpillar peak, which is primarily influenced by the air temperature during May and June. The caterpillar peak falls on average 41.5 days after the hatching of the first broods, but the dates vary widely from year to year. In many years repeat broods profit the most from the annual peak in caterpillar abundance. The size of first clutches in different habitats was compared, allowing for the effect of other factors on clutch-size. To a small extent clutch-size is affected by the age of the female, yearling females having slightly smaller clutches than older females. The effect of breeding density, laying date, and food supply was studied by multiple regression analysis. Clutch-size is inversely related to breeding density in all four areas, but the regression coefficient was significant only for Oosterhout. The effect of breeding density is greatest in the most sparsely populated habitats. In agreement with reports in the literature, the clutch-size in pinewoods and mixed woods is largest at the start of the season. In the oakwoods the clutch-size remains constant throughout April (Oosterhout) or shows a peak near the end of April (Liesbosch). The seasonal trend in clutch-size is related to the breeding density. High density causes a reduction in the size of the earliest clutches. Only slight indications were obtained for the existence of any proximate or ultimate effect of the food supply on clutch-size. At the densities occurring normally, the average clutch-size is similar in all habitats, but at equal density the clutch-size in oakwoods is considerably larger than in mixed or pinewoods (Fig. 7). The food brought to the nestlings was studied by observing the feeding process at close range, i.e. in a glass-backed nestbox. During each observation period the weight of each prey was estimated; the feeding frequency was recorded automatically during longer periods in a large number of broods. Caterpillars form the main type of prey in both habitats. In oakwoods, where Tortrix viridana and Operophtera brumata are extremely abundant during the first half of the breeding season, the relation found between caterpillar abundance and food composition indicates that the level of abundance is far in excess of the food requirements during the annual peak as well as during part of the decline in caterpillar abundance. In pinewoods, where Panolis flammea is predominant at the time of the annual peak, the frequency of its occurrence in the food can reach very high values. Its percentage in the food increase with increasing Panolis density. Other types of prey are mainly fed when the above-mentioned preferred species are not available. The daily food consumption was computed from the observed prey weight and the recorded feeding frequency, these quantities being inversely related. The food consumption increases sharply during the first week of life, and thereafter remains constant. Early oakwood broods receive the largest amount of food; in late oakwood and all pinewood broods the level of food provision is much lower, which probably is the cause of the mortality observed in these broods. At equal brood-size the food consumption is only slightly higher in oakwood than in pinewood. Tits in pinewoods have longer feeding days than those in oakwoods, mainly due to extended feeding in the evening, but in spite of this they are unable to satisfy their young completely. Days with low food consumption are usually followed by mortality, which brings the number of nestlings into balance with the feeding capacities of the parents. Nestling survival is very high in early oakwood broods, and decreases in later broods (Tables 19 and 20). In pinewoods survival is on average much lower, with slight increases at the start and in the middle of the season. The annual variation in nestling survival is also very low in early oakwood broods, and increases in late oakwood broods and in all pinewood broods, indicating that feeding conditions are less reliable for the latter. In late oakwood broods and early and late pinewood broods nestling survival is positively correlated with the feeding conditions. Starvation plays a major role in most of the cases in which nestling mortality occurs. Nestling survival is not related to brood-size. In all areas the largest broods are the most productive (Tables 22 and 23). This was an unexpected finding, since feeding conditions should be worse for large than for small broods. The reasons for this discrepancy are discussed. The growth curve for well-fed nestlings is sigmoid in form (Fig. 24), with the highest growth rate between day 2 and day 9. The growth rate in this period is higher in oakwood than in pinewood. The effect of several factors on growth rate was investigated by multiple regression analysis. The caterpillar abundance has a significant effect in both habitats, whereas brood-size is only operative in the oakwood. The effect of caterpillar abundance is increasingly apparent the more the feeding conditions deteriorate (Table 24). Many broods were weighed between the 15th and the 18th day of life, these weights being considered to represent the body weight at fledging. Fledging weights are highest in early oakwood broods, but the differences between the categories are small and often not significant. The annual variations in weight also suggest that feeding conditions are most reliable for the early oak broods, and much more variable in the other categories of broods. The seasonal trends in fledging weight generally correspond with the trends in feeding conditions, but several unexplained deviations occurred. The effect of several factors on fledging weight was examined by multiple regression analysis. The effect of caterpillar abundance is only significant in pinewoods, and increases the more the feeding conditions deteriorate (Table 27). In three out of four categories of broods, body weight was negatively correlated with brood-size. Furthermore, high nestling mortality is associated with low fledging weight, irrespective of the size of the brood and the level of food abundance. Several explanations for the latter relationship are suggested. No effect of air temperature or breeding density on fledging weight was found. In the discussion the difference in feeding conditions between oakwoods and pinewoods, and its consequences for the timing of the breeding season, are treated. The Great Tit is evidently adapted for reproduction in deciduous habitats; the pinewood populations have not adapted their breeding season to local feeding conditions, but start breeding at the time appropriate for breeding in oakwoods. The consequences of this inadequate timing are evident throughout this report.


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