COLONY DEVELOPMENT PHASE AND BREEDING PLUMAGE IN GREAT CORMORANTS Phalacrocorax carbo

Fabrizio Grieco & Isabella Veronesi

Netherlands lnstitnte of Ecology, P O. Box 40, NG-6666 ZG Heteren, tke NetHerlands

In the Great Cormorant Phalacrocorax carbo the amount of white feathers on head and neck is usually variable in time and among individual birds. The time of appearance of the white feathers in the adult bird is probably related to the response to daylight length and varies among individuals of the European populations of P. c. sinensis (Van Eerden & Munsterman 1986), while between-year differences in timing within individuals are not reported (A. Kortlandt, pers. comm.). Variation in the amount of white feathers has been described by several authors for both European races P. c. carbo and P. c. sinensis (see review in Marion 1995). However, this plumage feature is not longer now thought to be a reliable criterion to distinguish the two races, as it was before (Marion 1995). The variation in white is also related to age, 2 year old birds having less white than older individuals (Marion 1995). In this paper, phase of colony development was related to the variation in frequency of white-headed birds in a tree colony as a shortage of nest sites has occurred.

MethodsWe studied the tree colony of Val Campotto (Emilia-Romagna, Italy). Colony size, in terms of maximum nest count, is known for the years 1986-1996. Approximately every three weeks between January and May 1994 and 1996 we counted the number of birds (always n > 100) attending nests in the centre of the colony that showed any of the five types depicted in Van Eerden & Munsterman (1986) to categorise the amount of white feathers on head and neck. The phenotype sinensis is defined as the tpe-4 or -5 bird in Van Eerden & Munsterman ( 1986).

Results

The colony grew linearly from 12 nests in 1986 to 280 nests in 1994, then started declining (Fig. 1) and reached 200 nests in I996. Such a trend is very likely to be due to the collapse of the dead trees into the water (Grieco et at. in Press).

Figure 1. Size of Val Campotto colony in terms of maximum nest counts.

In 1994 the highest frequency of the phenotype sinensis was 37% (11 March), while in 1996 it was 5% (9 March). The frequency of the five categories between the two years with respect to peak dates (c ² = 19.6, df = 4, p < 0.001). Moreover, the frequency of bird types in late January and in early May was similar between the two years (January: c ² = 4.0, df = 4, n.s. and May: c ² = 0.9, df= 4, n.s., respectively), suggesting no annual difference in timing of appearance of nuptial plumage.

Between 1994 and 1996 the changes in the availability of nest sites were likely to force many birds not to breed or to move to other breeding sites. As younger birds may be the ones that first abstain from recruiting into the natal colony and move away when nest site become limiting, the increase in occurrence of phenotype sinensis in 1996 may be due to the older age of breeding birds. Further data from other colonies in different growing stages may confirm this theory.

Since variations in extension of white plumage on the head may be easily detected in colonies where nests are well visible, we suggest that this method may be used together with normal censuses in assessing population trends and estimating median age of the breeding Great Cormorants.

References

Grieco, F. E. Veronesi & S. Frugis in press. Suppl. Ric. Biol. Selv. 23.

Marion, L. 1995. Where two subspecies meet: origin, habitat choice and niche segregation of Cormorant Phalacrocorax c. carbo and P. c. sinensis in the common wintering area (France), in relation to breeding isolation in Europe. Ardea 83: 103-114.

van Eerden M. R. & Munsterman M. J., 1986. Importance of the Mediterranean for wintering Cormorants Phalacrocorax carbo sinensis. In Medmaravis & X. Monbailliu (eds.): Mediterranean marine avifauna. NATO ASI Ser., vol. G 12: 123-141, Springer Verlag, Berlin.