Breeding biology of the Great Cormorant Phalacrocorax carbo in the tree-nesting colony of Val Campotto (N-E Italy)
Fabrizio Grieco
A method for studying the structure of a colony in trees
In 1986, the Great Cormorant established a new breeding colony at Val Campotto (Emilia Romagna, North East Italy). Apparently, this species had not bred in the Italian mainland for several centuries, since the last certain record dates back to the 16th century. However, it is quite likely that the Cormorant had been present in a more recent past. It is plausible that breeding colonies were present at least in Tuscany and Sicily, until the 19th century.
The establishment of the colony at Val Campotto offered the chance of studying the factors influencing the colonization of new areas and the growth of the breeding colonies of this species.
In particular, we wondered to what extent the availability of suitable nesting sites would limit the growth of this new colony. The Cormorants at Val Campotto started nesting in a group of decaying trees within a flooded area. The progressive collapse of the trees would cause a rapid decrease in the number of structures available for nesting. What would the response of the Cormorants be in terms of choice of the nest site and breeding performance?
Since the year of foundation of the colony, ornithologists have observed that some individuals with immature plumage (i.e., brown back and whitish belly instead of glossy black as in the adults) were breeding, often successfully. This is quite a rare event in natural populations. Why did it occur at Val Campotto, and so frequently? Was it related to the early stage of development of the colony?
The Cormorants usually breed once in a season. In a few cases in the wild, and in zoos, they have been reported to breed twice, apparently as response to the high density of food available in the environment. We therefore looked at whether some of the Cormorant pairs at Val Campotto bred twice a year.
A method for studying the structure of a colony in trees
The use of photographs to 'mark' the cormorant nest sites was feasible at Val Campotto because all trees were dead at the start of the study, so that the nest were well visible at distance. Since 1992 I have taken pictures to all the trees in the fishing pond. The progressive colonization of the trees during the same year can be studied by taking pictures at regular intervals (e.g. every three weeks).
At each survey, a scheme has been made where the location of old (circles)and new nests (dots) is recorded.
The location of the nests differed among years (e.g.., nests were in higher position in early years, when trees were still high) and within years (e.g., nests built late in the season were closer to the water than those built earlier). To describe the structure of the nest sites, I defined five categories of structures, according to the form of the branches that sustained the nests:
The last three categories refer to nests being built on broken trees, sloping or horizontal, and stumps. The fifth category - nest on stumps emerging from the water - represents the final stage of utilization of trees by the Cormorants.
The data collected at Val Campotto lead us to the following conclusions:
The colony appeared to be limited by the availability of nest sites. Two facts support this view:
1) nest sites of different structure were occupied in a density-dependent fashion. When new nest sites become available following colonization of a new area, the Cormorants did not re-occupy low-quality nest sites on stumps (Grieco 1999);
2) the pattern of decline of colony size (in terms of maximum number of nest in any year) followed that of the estimated available nesting space in the flooded area:
The establishment of a new subcolony in 1997 led to an increase in total colony size (open dot in Figure above).
young Cormorants in the new colony on living trees
3) The nesting and successful breeding attempts by immature-plumaged cormorants appear to be related to the good feeding conditions at Val Campotto. Surprisingly, the decline in mean nest site quality was not followed by a decrease in the number of breeding attempts by the immatures. This suggests that age at first breeding in this species is not simply related to the availability of nest sites, and recruitment is influenced by the interaction between colony growth, food levels, and nest site quality.
4) The high breeding success reported for many years of study (from 1989 to 1997) did not change as the quality of nest sites declined. Also, the number of breeders and breeding performance was unrelated to the estimates of fish harvest in the period 1992- 1997. Prey abundance seemed not to influence the decrease in colony size.
5) There is some evidence that Cormorant pairs attempted to breed a second time after a first, successful attempt early in the season. In the following picture, a pair is establishing a nest site while a young is not yet fledged. The pair tolerates the young, suggesting that the two adults are actually the parents of the young:
This pair failed raising a new brood, but we know similar cases where the pair raised successfully young for the second time in one year.
In short, the colony of Val Campotto provides an example of a breeding colony where size is not an appropriate estimate of its actual state. Although breeding numbers were declining, the colony works probably as a source for other, more recent settlements of the Cormorant in the Po Delta area.
Grieco F., S. Frugis. & R. Groppali. 1993. La colonia di Cormorani Phalacrocorax carbo nidificanti nell'Oasi di Val Campotto (Ferrara). Pianura 5: 21-31.
Grieco F., S. Frugis & R. Groppali. 1995. Studio della colonia di Cormorani Phalacrocorax carbo nidificanti in Val Campotto (Ferrara): una proposta metodologica. Suppl. Ric. Biol. Selv. 22: 563-568.
Grieco F. 1994. L'occupazione di nidi incustoditi da parte di conspecifici nella colonia di Cormorani Phalacrocorax carbo di Val Campotto (Ferrara). Riv. It. Orn. 64(1): 71-76.
Grieco F. 1994. Fledging rate in the Cormorant Phalacrocorax carbo at the colony of Val Campotto (Po Delta, NE Italy). Avocetta 18:57-61.
Grieco F. 1995. Nest site selection by Cormorants Phalacrocorax carbo at the colony of Val Campotto (NE Italy). Avocetta 19: 194-200.
Grieco F. & E. Veronesi. 1995. Accertata nidificazione di Cormorano Phalacrocorax carbo in canneto a Val Campotto (Emilia Romagna). Riv. It. Orn 64 (2): 168-170.
Frugis S., F. Grieco & E. Veronesi. 1995. Evoluzione della colonia di Cormorani Phalacrocorax carbo di Val Campotto (Ferrara). Avocetta 19: 35.
Grieco F., S. Frugis & E. Veronesi. 1995. La nidificazione di cormorani in abito di immaturo nella colonia di Val Campotto. Avocetta 19: 43.
Grieco F. 1995. Nidificazione, modalita', tempi e riuscita nella colonia di Campotto. In Quaderni del Museo di Campotto n. 7 - Il Cormorano. Alfa Editoriale, Ferrara.
Carpegna F., F. Grieco, M. Grussu, E. Veronesi & S. Volponi. 1997. The italian breeding population of Cormorants. Proc. 4th Eur. Conf. on Cormorants. Suppl. Ric. Biol. Selv. 27: 76-80.
Grieco F., E. Veronesi & S. Frugis. 1997. The Cormorant colony of Val Campotto: recent changes in structure and breeding biology. Proc. 4th Eur. Conf. on Cormorants. Suppl. Ric. Biol. Selv. 27: 461-466.
Grieco F. & I. Veronesi. 1998. Colony development phase and breeding plumage in Great Cormorants Phalacrocorax carbo. Corm. Res. Gr. Bull. 3: 22-24.
Grieco F. 1999. Nest site limitations and colony development in tree-nesting Great Cormorants. Waterbirds 22(3): 417-423.
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