BIJLSMA RG (1979) AThe ecology of the Hawfinch Coccothraustes coccothraustes on the Zuidwest-Veluwe, with special reference to the. LIMOSA 52 (1): 53-71.

Hawfinch Coccothraustes coccothraustes In an area of ? 200 km2 in the central part of the Netherlands, the ecology of a small population Hawfinches Coccothraustes coccothraustes was studied during the period 1973-1978, with special reference to the breeding biology. A detailed description of the study area is given (fig. I). The number of pairs was determined by means of localization of territories. The reliability of the census depends on (a) the familiarity of the observer with the area to be censused (if not, an error of >50% can occur), (b) the familiarity of the observer with sound and behaviour of the Hawfinch (if not, an underestimation of 100% is usual), and (c) the condition of the deciduous woodland with regard to the foliage (if not leafless, an experienced observer who is abreast of the exact location of all breeding pairsstill makes an error of 30-80%). With this in mind, the census results of 1973-1974 are unreliable (except for a few colonies), that of 1975-1977 being very much better and resulting in 69,92 and 126 breeding pairs respectively: The nests of pairs in colonies can be traced by following nestbuilding activities or by systel11atic searching when the trees are still leafless. Localization of nests of solitary breeding pairs is extremely difficult because ofthe stalking way of life of these pairs during the breeding season. After breeding has begun, however, the male makes long and directed flights from and to the nest. By following such a line in the right direction, the nest can be found quite easily. In winter, most Hawfinches live in flocks, the size of which and the frequency of occurrence is given in table I and probably depends on the abundance of food. Flocks of 2-5 birds are most common but 52 specimens. in one flock have been seen. Feeding in winter is restricted to ground level. Immediately after a rise in temperature in January of February the males start singing and displaying. At first the female is highly agressive but eventually the elaborate and very ritualized display of the male appeases the female and before taking up the breeding-territory pair formation has taken place. Sometimes the males carry out exploring flights to the breeding places long before final settlement in March. Once the pairs are established the males in colonies become very territorial, especially in the immediate vicinity of the nest site which is chosen by the male. Solitary breeding pairs are utmost obscure and, in contrast to colonybreeders, lack territorial behaviour completely. As a result, tracing of these pairs in the early stage of the breeding cycle is very difficult. The breeding pairs are unevenly distributed over the study area (fig. 2). Most pairs and all colonies (in which the nests or territories are within 50 m of each other) are sit.uated in a rather small corridor of predominantly deciduous woodland from Ede southwards to Wageningenand from Wageningen eastwards to Renkum and Heelsum(see fig. I and 2). Taole 5 gives the distribution of 470 breeding pairs over a number of breeding habitats. Gardens, parks, deciduous woodland mixed with conifers and, to a smaller extent, pure deciduous woodland are preferred. A small number of territories is sited in pure coniferous woodland, most of which in 1977 when the cone crop of Pseudotsuga menziesii was abundant. The breeding places in coniferous woodland are not occupied every year, as can be seen from table 2. The number of pairs per colony varies greatly from year to year (table 3). These fluctuations are probably due to the exchange of pairs between colonies as the total number of pairs fluctuates much less (table 4). At .the end of March and the beginning of April the males start nestbuilding which is taken over by the female shortly afterwards. For the construction of the nest, oak twigs are preferred (86% of 62 nests). The nest itself is a rather loose structure, somewhat resembling that of a Mistle Thrush Turdus viscivorus. Most nests are constructed in Oak Quercus robur and Beech Fagus sylvatica but a wide variaty of other tree species occur (tabel 6). Mean nest height is 14 in and varies from 7-19 m. The majority of the nests is made 10m or. more above ground level (table 6). Hawfinches are very consistent as regards to the pcisition of the nest in the tree. Nearly all nests in broad-leafed trees are sited on a flat fork at the top; in conifers most nests are c.onstructed against the main trunk on a side branch and seldom on a side branch without support of the main trunk. Mean intern est distances in four colonies varied between 28 and 43 m with 10 mas a minimum and 50 m as a arbitrary maximum. Solitary pairs usually have their nests I km or more apart. During the period 1975-1977 62layings were controlled, 82% of which in 1977. Most clutches are completed in the latter half of April and during May (71%) but the total length of the laying season lasts 4Y2 months from the beginning of April up to and including the first half of August (fig. 3) Mean clutch size is 4,6 per clutch, is largest in April and May (table 7) and larger in colonies (x=4,7; n=26) than in solitary pairs (x=4,3; n= 15). Of 62 layings started, 48 (= 77%) produced fledged young. Mean number of fledglings for all 62 pairs is 3,4 (table 7). The length and timing of the breeding season is strictly related to breeding in colonies or not (fig. 5). The pairs breeding in colonies have a highly synchronized breeding cycle (nearly all clutches completed in May), so that most eggs hatch in May and June which is the period of maximum abundance of the two main prey species, i.e. the caterpillars of Tortrix viridana and Operophtera brumata. As a result, breeding success is very high. Solitary breeding pairs have a much more extended laying season with a breeding success onethird smaller than that in colonies. The synchronization of the breeding cycle can be brought about by the .courtship-feeding of the cock which has to stimulate the sexual development of the female at the .correct time in relation to maximum food supply at hatc1ing time. Probably more experienced birds can perform this act better than recruits. It is theorized that most colony-breeders are experienced birds and most solitary pairs recruits as older females lay larger clutches than younger ones and clutch size in colonies is larger than outside. However, differences in food supply can not be excluded. It is uncertain whether breeding success is linked with the size of the colony. In four colonies with three, five, eight and 13 pairs respectively the breeding success was 66;7%, 73,9%, 78,9% and 77, 1% respectively, the differences being not significant (Spearman-test: P>O,IO). Colony size never exceeds a limited number of pairs (2-8, exceptionally more). It is .to be expected that natural selection has brought about the correct colony size in order to obtain the highest breeding success. If more breeding pairs are permitted to breed inside existing colonies; these probably render too conspicious which will result in a higher predation pressure. It is unknown in which way the number of breeding pairs per colony is limited but territorial behaviour probably plays an important role. The behaviour of Hawfinches during the breeding season is very obscure but as soon as the young have fledged, they render conspicious again. As a result predation of (especially young) Hawfinches during the postfledging stage is exceptional high (fig. 4, table 8). The family remains intact and in the immediate vicinity of the nest for a period of 10-15 days after which the young become independent and dispersion begins. Most dispersion takes place from August to November in a wide variaty of directions in order to locate good food sources outside the breeding places. A smaller number of (probably young) birds migrate southwestwards (table 9). Now and then movements of an invasion type occur; the last irruption in the Netherlands being that of 1969 - 1970. Most of the time these movements are extremely local in occurrence and in all probability linked with food supply

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