Conservation of heronry Ardeidae sites in North Italian agricultural landscapes

Biological Conservation 1992, 62, 219-228

Conservation of heronry Ardeidae sites in North Italian agricultural landscapes Mauro Fasola & Raffaella Alieri Dipartimento Biologia Animale, Pz Botta 9, 27100 Pavia, Italy (Received 24 June 1991; revised version received 18 October 1991; accepted 26 November 1991)

Internationally important populations of night herons Nycticorax nycticorax and of little egrets Egretta garzetta are concentrated within a 5000 km2 region of NW Italy, where the landscape is intensively cultivated for rice, natural biotopes are reduced, and human impact is high. From 1976 to 1990 the heron populations increased, despite a decrease in the number of heronries mainly due to habitat destruction. The heronry sites differed from the unoccupied sites, in particular in their greater protection from terrestrial predators and from human disturbance. The heronries were evenly distributed, and there were indications that the location of a heronry is determined by food competition with neighbouring heronries. Our results suggest the following for conserving the heronries in N W Italy, and in other zones where natural wet biotopes are scarce: protection of the existing heronries, and the active management of their habitat in order to maintain suitable characteristics; creation of a network of new breeding sites, spaced at 4-10 km in relation to the available foraging habitats, in zones without suitable sites for breeding. The protection of unoccupied, suitable sites near the existing colonies has a lower priority, because these sites will not be colonized.

one-third of the populations known to nest in the Palearctic (Fasola, 1983). Less abundant populations of other colonial herons (grey Ardea cinerea, purple Ardea purpurea and squacco Ardeola ralloides) breed in continental Italy, and cattle egrets Bubulcus ibis breed in Sardinia. The large populations are mostly concentrated in a 5000 km2 region of NW Italy, where agriculture is intensive, natural biotopes are reduced and human impact is high, but where large areas of rice fields provide the herons with extended foraging areas that supplement natural habitats. The heronries are the outstanding natural element of this agricultural landscape, and they therefore deserve a high priority in the conservation objectives for N Italy (Bogliani & Fasola, 1985). Thirty of the 34 heronries of NW Italy exceed the minimum criteria for inclusion in the list of internationally important bird areas (Ramsar Convention, Langeveld & Grimmett, 1990). The heronries in the rice fields region of NW Italy are a relict, but important, faunal component of the agricultural landscape; the herons feed

INTRODUCTION Colony sites of colonial waterbirds must be safe from predators, contain suitable breeding sites, and be within commuting distance of sufficient food supplies. The selection of sites has been stud~ed in colonial seabirds, Laridae and Sternidae, in various environments (Burger & Lesser, 1978; Gochfeld, 1983; Kotliar & Burger, 1986; Greer et aL, 1988, among others), whereas in colonial Ardeidae it has only been studied in some natural environments of North America, where the availability of suitable sites is not usually limiting (Beaver et al., 1980; Erwin et al., 1987; Gibbs et a/., 1987). A different situation exists in Italy and Ln most other European countries, where natural environments have been reduced by a long history of land reclamation. The night herons Nycticorax nycticorax and little egrets Egretta garzetta breeding in Italy constitute Biological Conservation 0006-3207/92/$05.00 ,© 1992 Elsevier Science Publishers Ltd. 219

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M. Fasola, R: Alieri

largely in the rice fields and depend entirely on residual patches of natural marsh for nesting. The heronries occupy small patches of scattered natural wetlands in the cultivated landscape. Traditional sites remain occupied for a long time, but cases of abandonment or displacement have been observed as a result of habitat destruction, direct human disturbance, or evolution of the vegetation towards a less suitable structure. The number of heronries decreased during the 1970s and 1980s, and the low availability of sites is likely to be a threat. Since 1972 we have studied the populations of nesting herons, the distribution and characteristics of their colonies and unoccupied sites, in order to construct a conservation plan. This paper aims: (1) to describe the colony site requirements of the five species of herons; (2) to assess the availability of alternative, suitable colony sites, in addition to the existing heronries, in a landscape largely modified by man; (3) to investigate the factors affecting colony location; (4) to analyse the influence of colony site availability on the populations; (5) to outline a scheme for the conservation of the internationally important heron populations of N W Italy.

METHODS Census of heronries From 1972 to 1990 we surveyed the occupation of all the heronries within the 5000 km 2 region of N W Italy covered by rice cultivation (Fig. 1). The heronries (Table 1) are usually mixed, with two to four species; little egrets and squacco herons are always associated with night herons, while grey and purple herons nest either in mixed or in monospecific aggregations. The heronries are situated in wet patches of alder Alnus glutinosa and of bushy willow Salix eaprea, and in mixed broadleaf woods, with the exception of one colony of purple herons in reedbeds. During the b r e e d i n g season the herons mostly forage for food in the rice fields (which cover on average 25% of the region of intensive rice cultivation), and less abundantly in rivers, marshes and irrigation canals (Fasola, 1986). Colony turnover rates were computed between consecutive years by T = I/2(S1/N1 + $2/N2), where S1 is the number of heronries existing only in the first year, N1 the total number of heronries in the first year, $2 the number of heronries exist-

ing only in the second year, and N2 the total number of heronries in the second year (Erwin et al., 1981). Censuses were conducted by ground counts of the nests. A complete census was accomplished in 1981, whereas some colonies were missed in the other years. A population index was calculated as total nests in the colonies censused in a given year, divided by the total nests in the same colonies in 1981 (by definition, the index for 1981 is 1). The index was calculated only for those years when more than 75% of the heronries had been cenSused, because fluctuations in the single colonies may not match the trend of the entire population, and the index calculated from counts of a small number of heronries could be misleading. Study of site selection The factors affecting the distribution of the heronries and site selection were studied in that portion of the study area (2200 km2, province of Pavia, Fig. 1) for which aerial photographs and detailed maps (1:10 000 scale) were available. On the basis of the aerial photographs we located all the potential heronry sites (i.e. all the patches of reedbeds, marsh with bushy or arboreal vegetation, and natural woods, greater than 1 ha), to study site selection. We measured the on-site characteristics listed in Table 2 in all the heronries and in all the patches in which nesting herons were not present, during the nesting seasons (May to July) from 1986 to 1988. Surface area, length and width, and the percentage of the perimeter protected by canals or by other water bodies (an estimation of the protection of the site from terrestrial predators), were measured on 1:10000 maps. • Heronries present from 1972 ~

Ricefieldsre$ior [ !~, ~ ~

• to 1990 Intensive study O Disappeared from 1972 to r'="l area *Newheronrles 1990 f._._ Ma,orrivers /

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1. Region of intensive rice cultivation in N Italy in which the heronries were censused, and where the distribution of heronries and site selection were studied (rectangle). Fig.

Conservation of N Italian heronries

'Disturbance' is a composite index of the human disturbance recorded at the sites during the nesting season. The index, based on the presence and utilization of roads and paths and on the presence of tracks within the sites, may range from 0 (absence of disturbance) to 1 (presence and high utilization). 'Diversity' is the Shannon index, calculated from the surface areas of the habitat types ~vithin each site. 'Buildings' is an index, ranging from 0 to 154, based on the number of surface anits of 1 ha occupied by roads, buildings and other man-made structures, within 700 m of the centre of each patch. When appropriate, the percentage variables were normalized by arcsine square root transformations, and the other variables by logarithmic transformations. The differences between heronry sites and unoccupied sites were tested by single factor Analysis of Variance, and analysed by stepwise Discriminant Analysis. The contribution by a variable to the discriminant function is expressed by (a) the function coefficient which allows the calculation of the values of each case on the discriminant function from the original variables, but neither its absolute value nor its mathematical values indicate the importance of the variable; and (b) the correlation coefficient showing the degree of association between the variable with the function, and its negative or positive contribution.

Analysis of heronry distribution We measured, on the 1:10 000 maps, three factors, external to the colony sites but which may affect the distribution of the heronries among the suitable sites. (1) the 'Distance' of each heronry, and of each unoccupied patch, from the nearest heronry; (2) the 'Foraging habitats', i.e. the surface area covered by rice fields, rivers, canals, and ponds, which are the habitats in which herons feed in our study area (Fasola, 1986), and which were measured in each 1 km2 unit of the study area of 2200 km2. The habitats available to a heron nesting in a heronry or in a patch were assumed to be the sum of the aquatic habitats within a 5 km radius from the centre of the site, a range below which most night, grey and other herons forage during nesting (Custer & Osborn, 1978; Van Vessem et al., 1984);

221

(3) the 'Nesting pairs', i.e. the total number of herons nesting within 10 km (double the foraging distance of most herons) of the centre of a heronry or of a patch; analogous measures of the abundance of nesting pairs and of the competition for food have been adopted by Furness and Birkhead (1984) and by Gibbs et al. (1987). The values of these three variables for heronries and unoccupied patches were submitted to Discriminant Analysis. In order to test the regular spacing of the heronries, we compared the actual inter-heronry distances with the nearestneighbour distances of a random sample of the available heronry sites. The sample was obtained by selecting at random 17 (as many as the existing heronries) from the 43 available sites (the 17 heronries, plus the other 26 unoccupied sites that had been found to be suitable as heronry sites). The distances of the available sites were obtained from 100 replications of the random selection.

RESULTS

Populations trends and heronry turnover The populations of night herons and of little egrets were stable until 1986, and have subsequently increased; grey herons have increased since 1978; purple herons decreased until 1985, and have subsequently been stable (Fig. 2). The turnover of the grey heron colonies was mainly due to the settlement of new heronries by the species, during the years of population increase from 1985 (Fig. 2), with a net gain of eight grey heron colonies from 1976 to 1990 (Table 1). However, most of the new settlement occurred in colonies already occupied by night herons and little egrets, and only two new, small heronries were found. On the other hand, the turnover of purple heron colonies was mainly due to its disappearance from mixed heronries, while its population was decreasing between 1976 and 1985 (Fig. 2), with a net loss of two heronries. Night herons and little egrets abandoned 16 heronries owing to the destruction of the sites for land reclamation, direct human disturbance, and the succession of the vegetation towards dry woodland (Table 1); the five cases of heronry disappearance

M. Fasola, R. Alieri

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Table 1. Populations of nesting herons, number of heronries, and causes of heronry turnover, within the region of intensive rice cultivation in NW Italy

No. heronries 1990 Total no. nests 1990

Night heron

Little egret,

Squacco heron

Grey heron

Purple heron

All herons

26 16 602

25 7 612

11 208

16 2 842

10 114

35

9

2

9

2

9 2 1 5

New heronries 1972-1990

7

Heronries disappeared (1972-1990) because of: Habitat destruction Direct disturbance Changes of vegetation Unknown

8 2 1 5

2

All little egret heronries were mixed with night herons. Table 2. Characteristics of the 17 heronries and of the 133 unoccupied sites

Heronries (n = 17) Alder woods (ha) Reedbeds (ha) Willow bushes (ha) Willow woods (ha) Mixed dry woods (ha) Acacia woods (ha) Water (ha) Grass (ha) Total area (ha) Area suitable for nests (ha) Length (m) Width (m) Protected perimeter (%) Disturbance (index) Diversity (index) Buildings (index)

3-22 1-45 2.31 0-30 1.22 0 1.69 0.24 10.4 7-06 664 196 91.8 0.09 1.15 42.4

Unoccupied sites (n = 133)

(12.8-0) (8.6-0) (13.2-0) (3.9-0) (7-7-0) (0-0) (21.5-0) (4-8-0) (43.0-2.6) (14-9-2.2) (2 300-200) (400-90) (100-19) (0.33-0) (2-52-0) (95-0)

1.08 0.64 0.78 0-69 1-95 1.35 0.13 0.03 6.68 5-84 437 184 53.6 0.37 0.62 52.4

(18-0) (12.3-0) (18 -0) (30-0) (32.3-0) (22.9-0) (6-0) (2.57-0) (60-0.35) (48-0) (2 000-60) (650-50) (100-0) (1-0) (2.33-0) (121-0)

*** * *** NS NS *** *** * *** ** *** NS *** *** *** *

Averages (and ranges) are given. Significant difference tested by ANOVA: ***, p < 0.001; **, p < 0.01; *, p < 0.05; NS, not significant.

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rey Heron

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Population trends and turnover rates of the heronries. Turnover rates are plotted jointly for the night heron and the little egret because all the colonies which shifted sites were mixed.

for which the cause was unknown may have been due to human disturbance, which was difficult to document. Night herons and little egrets settled seven new heronries, but suffered a net loss of nine

heronries, despite the increase in their populations. We conclude that the heronries had a high site fidelity, if undisturbed and when populations are stable. The observed turnover of the heronries

223

Conservation of N Italian heronries

was mainly determined by: (1) increase of the population of a species and its settlement in other, already settled, mixed colonies (grey heron); (2) decrease of the population of a species and its abandonment of colonies (purple heron); (3) disappearance of heronries when their sites were destroyed or became otherwise unsuitable (night herons and little egrets); (4) settlement of new colonies that replaced a part of the disappeared colonies. Considering all the species of herons from 1976 to 1990, the loss of 17 heronries was only partially compensated by the nine new ones. The net loss of eight heronries despite the general population increase suggests a shortage of suitable colony sites that could replace those lost for land reclamation and human disturbance. H e r o n r y sites

The area in which we analysed the distribution of the colonies and the availability of alternative sites (the south east portion of the rice cultivation region, Fig. 1) included 17 heronries and 133 unoccupied patches of natural vegetation. The heronries differed significantly from the unoccupied sites in 13 of the 16 variables that describe the environmental characteristics within the sites (Table 2). A Discriminant Analysis between heronries and unoccupied sites was performed using six of the 13 variables with significant differences; the other seven variables were excluded owing to their high multi-collinearity. The Discriminant Analysis distinguished the heronries from the other available but unoccupied sites by a function that included only four variables, the other two variables submitted to the analysis were not retained by the stepwise procedure in the discriminant function (Table 3). The heronry sites had a positive centroid on the discriminant function, and the correlations of the variables with the function show that the presence of a heronry was correlated with high values of the four discriminating variables (Table 3), which are all related to the absence of h u m a n disturbance and of terrestrial predators within the site. The heronry sites had a high portion of their perimeter protected by water, a low level of disturbance, and were covered by vegetation typical of wet soils (alders and willows). On the other hand, the presence of 'buildings', a measure of the potential disturbance around the site, showed only a low difference between heronries and unoccupied sites. The classification procedure based on

Table 3. Discriminant Analysis between the 17 heronries and all the 133 unoccupied sites, based on site characteristics

Correlation with function

Protected perimeter 0.70 Disturbance -0.68 Alder woods 0-66 Willow bushes 0.63 Acacia woods --0-51 Buildings --0-02 Constant -0.28 Centroid of heronries Centroid of unoccupied sites Canonical correlation Percentage of cases correctly classified

Unstandardized function coefficients (units and transformations)

* * * *

0-40 (%, arcsin) -1.73 (index) 1-41 (ha, log) 1.14 (ha, log)

1-26 --0-23 0.48 74.4

The significant(p < 0-0001) function derived by the analyses was based on the four variables marked * the discriminant function correctly classified 77% of the 150 heronries and unoccupied sites. Of the 133 unoccupied sites, the 107 that were correctly classified may be defined 'unsuitable'. The other 26 unoccupied sites were not discriminated from the heronries, and may therefore be defined as 'suitable' alternatives, on the basis of their environmental characteristics (Fig. 3). H e r o n r y distribution

The reasons why the present sites are occupied by the heronries whereas the other similarly suitable sites remain unoccupied may lie in environmental factors external to the sites themselves. The distribution of the heronries and of the suitable sites (Fig. 3) suggests that the heronries are spaced with some evenness, whereas most suitable, unoccupied sites are clumped and are close to existing heronries. As already shown by Fasola and Barbieri (1978), heronries are close together in zones with abundant foraging habitats, and more widely spaced in those with few foraging habitats. As the heronries in the western part of the study area, which also has more aquatic habitats, are distributed more closely together (cf. Figs 3 and 4), this highlights the even spacing of the heronries. The nearest-neighbour distances of a random sample of suitable sites approached a Poisson distribution, as expected for random distances; some observed differences from a Poisson distribution are due to the fact that inter-site distances are not

M. Fasola, R. Alieri

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Conservation of N Italian heronries 0.25

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random within the study area, because the sites are concentrated to the west, and widely dispersed to the east. On the other hand, the inter-heronry distances approached a normal distribution, but were truncated towards low distances (Fig. 5(a)). The average distance between heronries was greater (5.7 km) than that of the suitable sites (4.1 km), and the two distributions differed significantly. This pattern indicates a non-random, even-distance settlement of the heronries among the suitable sites. The heronries had more aquatic habitats within the foraging radius compared with the unoccupied sites (Fig. 5(b)). The importance of even spacing and of aquatic habitats on the distribution of the heronries among the available sites is confirmed by a second Discriminant Analysis performed between the 17 heronries and the 26 suitable sites. Three variables were included in the Discriminant Analysis, the first describing the availability of aquatic habitats within the foraging range ('Foraging habitats'), and the other two possible competition on the foraging grounds with the herons nesting in neighbouring sites ('Distance' from the nearest heronry, and the number of 'Nesting pairs' within a 10-km radius, Table 4). The heronries were significantly discriminated from the suitable, unoccupied sites by their greater distance from the neighbouring heronry, and by a greater abundance of foraging habitats (Table 4). Distance from the nearest heronry was highly correlated with the discriminant function; thus those sites which are suitable for the environmental characteristics within the site itself are not occupied because they are too close to existing heronries. The discriminant function of Table 4 was used to evaluate those zones where the creation of new

suitable sites could provide effective possibilities for the settlement of other heronries. The value on the discriminant function of Table 4 was calculated for each of the 2200 km 2 of the study area. From these values, we estimated the likelihood (conditional probability, Norusis, 1986) that each 1-km2 unit could belong to the heronries group, or to the suitable but unoccupied g r o u p - - t h a t is the probability that a new, suitable site set up in that unit could be occupied by a new heronry. We introduced the necessary and obvious limitation that no heronry can settle where foraging habitats are few. None of the existing colonies had less than 20% of aquatic habitat in its foraging radius, and therefore we prudentially assigned those units with less than 10% of aquatic habitat within 5 km to a separate category, that of zones which are not

Table 4. Discriminant Analysis between the 17 heronries and the 26 suitable, unoccupied sites, based on the environmental factors external to the sites Correlation with function

Distance Foraging habitats Nesting pairs

0.77 * 0.20 * --0.13

0-36 3.37

Constant Centroid of heronries Centroid of unoccupied sites Canonical correlation Percentage of cases correctly classified

Unstandardized function coefficients (units and transformations)

(km) (%, log)

--6.70 0.54 --0-36 0.41 72.1

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suitable because of scarce foraging habitats. Most breeding herons forage within a distance of 5 km from their colonies (see Methods). Large portions (33%) of the study area appeared suitable for the creation of new sites (Fig. 6). In the zones with less foraging habitat, the areas for new sites were situated a long way (about 10 km) from the existing heronries (e.g. on the east part of our study area, Fig. 6), whereas in zones with abundant foraging habitats new nesting sites could be profitably created at shorter distances (about 4 km) from existing heronries.

DISCUSSION In agreement with a study by Gibbs et al (1987) on the distribution of heronries in North A m e r i can coastal zones their characteristics in our study area differed from the unoccupied sites; the heronries were evenly distributed and there were indications that their distribution is determined by the availability of foraging habitats and by the presence of other heronries i e by competition for

food. The sites selected differed mainly in their better protection from terrestrial predators and from human disturbance. The safety of the site is important for successful nesting by colonial waterbirds (Burger, 1981; Parsons & Burger, 1982; Gochfeld, 1983; Greer et al., 1988; Parnell et a l , 1988). •The belief (Birkhead & Furness, 1985) that nesting populations of colonial waterbirds are affected mainly by food arises because in largely natural landscapes the availability of nesting habitats does not seem to limit the distribution of seabird colonies and heronries (Gibbs et al., 1987). However, in the intensively cultivated landscape of Northern Italy, the limited availability of suitable colony sites does influence the distribution of heronries. We believe that the availability of sites in our study area is representative of many European zones profoundly modified by human activities. Even partially natural zones in Europe (e.g. the great river deltas) offer wide foraging habitats, but they provide few and scattered breeding sites suitable for the preferentially arboreal herons, such as night herons, little egrets, squacco and grey

Conservation o f N Italian heronries

herons. The availability of sites may therefore limit their populations in these regions. In our study area the number of heron populations increased despite a decrease in the number of heronries. Availability of suitable sites has therefore not been the major determinant of the population trends during the 1970s and the 1980s. The area of cultivated rice in NW Italy has fluctuated yearly by less than 10% over the last two decades (Ente Nazionale Risi, pers. comm,), and the changes in the heron populations cannot be related to this low variation in foraging habitats. It is possible that the heron populations may have been limited by factors external to the breeding areas, for example, survival during the preceding winter in Africa, which may determine the European populations in the next breeding season (Den Held, 1981). Although the numbers of herons in our study area remained abundant despite the loss of colony sites, the reduction of heronries resulted in an increase in the breeding populations of night herons, little egrets and grey herons in the fewer heronries. At present 33% of our study area seems to be suitable for the settlement of new heronries, due to the presence of foraging habitats and to the distance from neighbouring heronries. The creation of suitable colony sites would probably produce a redistribution of, and possibly an increase in, breeding herons.

227

active management of the sites is necessary, because tree-nesting herons prefer the intermediate stages in the evolution of the natural wet biotopes (in our study area, alder and willows thickets on wet soil, Fasola & Alieri, in press), whereas they avoid both the early and the mature stages (in our study area, reedbeds and mixed woods, respectively). (2) Creation of new sites in areas where foraging habitats remain unexploited owing to the distance from suitable nesting sites. A network of suitable colony sites should be established, at inter-site distances varying in relation to the density of foraging habitats. In our study area the optimal inter-site distances range from 4 km in the rich zones to 10 km in zones with few foraging habitats. Suitable sites closer than these limits have a high probability of remaining unoccupied. On the other hand, if suitable sites are scattered at distances greater than 4-10 km, the breeding population could be limited by the difficulty of exploiting available but too fardistant foraging areas. (3) The protection of unoccupied, suitable sites within 4 10 km from existing colonies has a lower priority, as it is unlikely that these sites will be colonized. These alternative sites should be used only when an existing heronry is destroyed.

ACKNOWLEDGEMENTS A SCHEME FOR PROTECTING HERONRY SITES

Our results suggest that heronries, in zones where natural wet biotopes are s c a r c e - - a n d particularly in the cultivated landscape of N W I t a l y - - s h o u l d be protected by the following courses of action, in order of decreasing priority. (1) Protection of those existing heronries occupying the best sites distributed at optimal distances. The colony sites should be managed in order to maintain suitable characteristics, which in the case of the tree-nesting herons in our study area are: a total surface area of suitable habitat (alder woods or willow bushes) of at least 2-6 ha (Table 2); complete protection by water along the perimeter; and absence of disturbance within the site. Disturbance in the vicinity of the site is less important. These requirements must be strictly adhered to, but protection would not be expensive, due to the limited surface area and to the low agricultural value of the wet residual areas. An

This study was partially funded by the Regione Lombardia and the Provincia di Pavia, as applied research for the protection of the heronries by the creation of special nature reserves. Our work was greatly helped by many, especially by Francesco Barbieri, Giuseppe Bogliani, Luca Canova, Armando Gariboldi, and Daniela Zandonella.

REFERENCES

Beaver, D. L., Osborn, R. G. & Custer, T. W. (1980). Nestsite and colony characteristics of wading birds in selected Atlantic coast colonies. Wilson Bull., 92, 200-20. Birkhead, T. R. & Furness, R. W. (1985). Regulation of seabird populations. In Behavioural Ecology, ed. R. M. Sibly & R. H. Smith. Blackwell,Oxford, pp. 145-67. Bogliani, G. & Fasola, M. (1985). Le specie animali da proteggere in Italia: considerazioni sulle prioritY. In Atti H Convegno Societg~ Italiana Ecologia, Parma, ed. A. Moroni, A. Anelli & O. Ravera. pp. 1059 61. Burger, J. (1981). A model for the evolution of mixed-species colonies of Ciconiiformes. Quart. Rev. BioL, 56, 143-67. Burger, J. &Lester, F. (1978). Selection of colony sites by common terns Sterna hirundo in Ocean County, New Jersey. Ibis, 120, 433~,9.

228

M. Fasola, R. Alieri

Custer, T. W. & Osborn, R. G. (1978). Feeding habitat use by colonially-breeding herons, egrets and ibises in North Carolina. Auk, 95, 733-43. Den Held, J. J. (1981). Population changes in the purple heron in relation to drought in the wintering area. Ardea, 69, 185-91. Erwin, M., Galli, J. & Burger, J. (1981). Colony site dynamics and habitat use in Atlantic coast seabirds. Auk, 98, 550-1. Erwin, M., Spendelow, J. A., Geissler, P. H. & Williams, B. K. (1987). Relationships between nesting populations of wading birds and habitat features along the Atlantic coast. In Waterfowl and Wetlands Symposium, ed. W. R. Whitman & W. K. Meredith. Delaware Department of Natural Resources, pp. 56-67. Fasola, M. (1983). Nesting population of herons in Italy depending on feeding habitats. Boll. Zool., 50, 21-4. Fasola, M. (1986). Resource use of foraging herons in agricultural and nonagricultural habitats in Italy. Colonial Waterbirds, 9, 139-48. Fasola, M. & Alieri, R. (in press). Nest site characteristics in relation to body size of herons in Italy. Colonial Waterbirds. Fasola, M. & Barbieri, F. (1978). Factors affecting the distribution of heronries in Northern Italy. Ibis, 120, 337-40. Fasola, M., Barbieri, F., Prigioni, C. & Bogliani, G. (1981). Le garzaie in Italia, 1981. Avocetta, 5, 107-31. Furness, R. W. & Birkhead, T. R. (1984). Seabird colony dis-

tribution suggests competition for food supplies during the breeding season. Nature, Lond., 311, 655-6. Gibbs, G. P., Woodward, S., Hunter, M. L. & Hutchinson, A. E. (1987). Determinants of great blue heron colony distribution in Coastal Maine. Auk, 104, 38-47. Gochfeld, M. (1983). Colony site selection by least terns: physical attributes of sites. Colonial Waterbirds, 6, 205-13. Greer, R., Cordes, C. & Anderson, S. (1988). Habitat relationships of island nesting seabirds along coastal Louisiana. Colonial Waterbirds, ll, 181-8. Kotliar, N. B. & Burger, J. (1986). Colony site selection and abandonment by least terns Sterna antillarum in New Jersey, USA. Biol, Conserv., 35, 1-21. Langeveld, M. J. & Grimmett, R. F. A. (1990). Important Bird Areas in Europe. Wetlands for the Shadow List of Ramsar Sites. International Waterfowl Research Bureau, Slimbridge, pp. 1-64. Norusis, M. J. (1986). SPSS Advanced Statistics. SPSS, Chicago, Illinois. Parnell, J. F. et al. (1988). Colonial, waterbirds management in North America. Colonial Waterbirds, ll, 129-69. Parsons, K. & Burger, J. (1982). Human disturbance and nesting behavior in black-crowned night-herons. Condor, 84, 184-7. Van Vessem, J., Draulans, D. & DeBont, A. F. (1984). Movements of radio-tagged grey herons Area einerea in the breeding season in a large pond area. Ibis, 126, 576-87.