MOULT IN FIVE SPECIES OF CORVIDAE IN BRITAIN
Authors: Seel D. C. 
Published: August 2010

This paper presents objective methods for measuring moult and uses them to describe the pattern and rate of the complete moult as a component of the annual cycle in Corvus corone, C. frugilegus, C. monedula, Pica pica and Garrulus glandarius in Britain.

The basis of the methods used to measure the rate of moult was the dry weight of the whole plumage and its component tracts and feathers. Within whole tracts of flight feathers showing diversity of size, the longer feathers grew in length a little faster than the others by accumulating feather material very much faster. Thus, for the flight feathers at least length by itself was a poor indicator of size, but, if first related to weight, could be used to predict weight. Hence, to enable realistic comparisons to be made between different flight feathers and tracts of flight feathers on a given individual bird or on individuals of different species, a ‘units’ system was created which would take account of such variations in size. This information provided a standard against which the growth of new flight feathers in a specimen collected in the field could be measured. The progress of moult in each tract was indicated by the sum of the units. In addition, for both the flight feathers and all other tracts, the duration of moult was measured from the start and finish in each tract as indicated by the proportion of birds having either growing or entirely fully‐grown feathers in each tract.

In the annual cycle body weight and abdominal fat weight were at a maximum in mid‐winter and at a minimum in mid‐summer in Corvus; variations in these items were apparently absent in G. glandarius. In Corvus probably only post‐second‐year birds breed, but in P. pica some second‐year individuals do so as well; also, increase in testes size in second‐year males was greatest in P. pica.

The patterns of the relative seasonal timing of the start and finish of moult in each tract were most similar in the three Corvus species, in which moult in the whole plumage began in the primary tract and finished in tracts on the body; P. pica differed principally in the extended growth of the secondary tract; G. glandarius differed in that moult began in the dorsal tract. By summing the data from individual tracts, a measure of the rate of growth in the whole plumage was obtained: all five species moulted at essentially the same time of year.

In the primary, secondary, tertiary and rectricial tracts the seasonal accumulation of dry weight of feather material in each tract followed a sigmoid pattern; in the alula tract dry weight increased most rapidly at the beginning. These patterns were correlated with the numbers of feathers growing at different stages in each tract. The primary tract accumulated dry matter at the highest relative rate, while the rectricial, secondary, tertiary and alula tracts did so at successively lower rates. The maximum relative rate of increase in dry weight in any given tract of flight feathers was fairly similar in all five species, but was possibly related inversely to body weight. Maximum absolute rates of increase in dry tract weight were also calculated.

During the springtime period when non‐moulting and moulting birds were present simultaneously in the population, no difference could be detected between these groups in any species in body and fat weights; in individuals obtained together with evidence of reproductive activity, few had begun primary moult, but in post‐second‐year C. frugilegus with branchers, males began before females; in C. corone and C. frugilegus testes weight was lower in moulting than in non‐moulting birds.

In conclusion, some aspects of moult for further research are suggested.