CONCLUSIONES Y RECOMENDACIÓNES

that solitary carrion crow nests may be counted as rook nests but this should not be a problem (and was not in this study) as

verification of the species is just a matter of waiting and observing.

A second class of problems which might lead to error in estimating rook population size concerns the nests. Just what

constitutes a rook nest may be far from clear. Nest structures in %

rookeries at the time of counting can be of four general types: ' 1) a well-formed structure, solidly built, which would clearly

accommodate one rook plus brood; /

2) a vertical mass of twigs up to several feet tall;

3) a laterally sprawling mass of nest material which could have room for several pairs of rooks to nest in; and

4) a less solid structure having the appearance of a partly built nest which might eventually be used for nesting by one pair of rooks.

Structures of type 2 are a consequence of birds building a nest using the nest(s) of a previous year as a foundation. Those of type 3 probably result from this as well but also from birds building nests so close together that a large platform is gradually created, lype 4 structures are in all likelihood partly built nests. It is equally likely that they will be eventually completed or remain

unfinished. They may be the first attempts at nest building by young rooks (Nau 1960) or they could be renewed attempts at constructing a nest by adults.

In the surveys all structures of type 1 were counted as one ;|

nest (equivalent to two breeding birds). Nests of type 2 were also counted as one. Type 3 structures presented more problems. If

discrete nests were at all obvious within these then each was counted as one. This was occasionally an easy judgment to nake as the shapes of individual nests could be observed by changing one's angle of view from the ground. If strips of daylight were discernible through these conglomerations then this also served as a criterion for counting more than one nest. In some instances it was not possible using these standards to separate the large m s s of nesting mterial into individual nests. In these cases a somewhat arbitrary

definition of the number of nests they contained was used. This was

the mxi m u m number of nests that could be contained if each nest was

1 m apart from any other nest in the structure. Such a definition is unlikely to lead to gross errors in estimting the number of breeding pairs. Depending on its appearance a type 4 structure could either be counted as a nest or not. If no daylight was visible through the structure from any angle, thus suggesting that a nesting platform had been built, then it was counted as a nest. If daylight was

discernible through it then it was not counted as a nest. Again

there was room for error here but in most cases it would probably be of a mgnitude of much less than 5% of the total count for a rookery.

These comments lead on to third category of possible inaccuracy in the estimation of the breeding population size. That is the question of whether all nests counted as such are in fact occupied by a breeding pair of rooks. With structures which have the appearance of being partly built nests it is difficult to ascertain whether these will be eventually used for breeding purposes. If young birds are responsible for building them then there is perhaps an even chance of this being so. It is possible that these

season, and so will not be used to rear young. The chances of this

leading to a large margin of error are slight, however, as rrrast nests |

(in deciduous trees at least) are destroyed by the elements after the breeding season, and also any nests which do survive until the

following spring will either be used as foundations for new nests or will be dismantled for the material for new nests to be built

elsewhere in the rookery. Roebuck (1933) suggested that adult pairs of rooks may build two nests in a season and only use one of them. Again this would not appear to be a serious problem in estimating

breeding numbers. Unused nests would probably be quickly plundered p

for the material they contained, given that there is usually competition for this (Roskaft 1982).

The counting of nests in pine trees can be especially difficult. In particular, the problems just discussed concerning nest structures are exacerbated. In the present study the same criteria used to count nests in deciduous trees were used for coniferous ones but instead of looking for daylight through and

between nests it was necessary to attempt to discern "spaces". For i

this reason there is perhaps a greater margin of error in nest counts 4

made in coniferous trees but again it is unlikely to be greater than 5%, and probably much less - especially in small trees and small rookeries. It is also difficult (but not impossible) to discover

previously unrecorded rookeries if they are built in coniferous trees 4

as the nests may not be visible from even very short distances.

A problem in large, sprawling rookeries is keeping track of which nests have been counted and which have not. This is a matter of organization and memory, but if there was any doubt then the count

was abandoned and started again. i

I

The timing of a rookeries census is important. The time of g

the year when rooks begin to build their nests depends on several

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exogenous factors, including daylength and temperature (see e.g. Busse 1965). The nest building period may be prolonged. The best

time for counting nests is therefore as late as possible. This J

ensures that all nests built by adults will be counts, while very 4

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late nests built by inexperienced birds who will probably not breed t

will not be included in the count. In the present study all counts /

were made as late in the spring as was feasible, i.e. before leaf

cover obscured nests in deciduous trees. This varied from late March ÏÎ

to as late as mid May, but most counts were made in mid to late April each year.

The number of nests in every tree in each rookery was recorded on cassette tape using a hand held microrecorder.

Binoculars (lOx) were used when necessary. As well as number of nests, the species of tree in which these nests were contained was also recorded. This data was transcribed on to paper the same day and the date of counting noted. The geological substrate on which each rookery stood was obtained from the Ordnance Survey 1:63 360 (1 in to 1 mile) Geological Survey map for Fife. The altitude (to the nearest 25 feet above sea level) of each rookery was also ascertained using O.S. 1: 25 000 maps. A rookery was defined as one or more rook nests greater than 100 m distant from any other rook nest, the

standard B.T.O. definition.

RESULTS

A list of N.E. Fife rookeries is given in %>pendices I and 1 II. These indicate the reference number of each rookery, its O.S.

national grid map reference, the number of nests it contained in the

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