Abstract
The Sacred Ibis, an exotic and invasive species, has been nesting in the Venice lagoon since 2020, when there were 23 pairs in three colonies. In 2020-22, some data on the reproductive success of the species were collected through drone flights, in a colony located on a small, abandoned island. Reproductive success was high, with 1.6 – 1.7 young per pair in each of the three years. In 2022, a further five colonies were also surveyed, for a total of at least 188 pairs. The drastic increase in just three years observed in the Venice lagoon is destined to continue in the future, given the wide availability of food and nesting sites.
Riassunto
Insediarsi e competere: nidificazione in massa ed elevata produttività dell’ibis sacro Threskiornis aethiopicus in un’isola abbandonata della laguna di Venezia.
L’ibis sacro, specie esotica ed invasiva, nidifica nella laguna di Venezia dal 2020, quando erano presenti 23 coppie in tre colonie. Nel 2020-22 sono stati raccolti tramite voli effettuati con drone alcuni dati sul successo riproduttivo della specie, in una colonia ubicata in una piccola isola ab- bandonata. Il successo riproduttivo è risultato elevato, con 1,6 – 1,7 giovani involati per coppia in ciascuno dei tre anni. Nel 2022 sono state inoltre censite ulteriori cinque colonie, per un totale di almeno 188 coppie. Il drastico aumento in soli tre anni osservato in laguna di Venezia è destinato a continuare anche nel futuro, data l’ampia disponibilità alimentare e di siti di nidificazione.
Introduction
The Sacred Ibis Threskiornis aethiopicus, a non-native and invasive species for Europe, was observed for the first time in the lagoon of Venice in the late Nineties; the sightings became regular but scarce since 2010, with a marked increase in the following years (SCARTON & VALLE, 2020). In Italy, the Sacred Ibis nested for the first time in 1989; in 2019, 1,249 pairs nested at 31 colonies in Piedmont and Lombardy, where most of the Italian nesting population clumped (CUCCO et al., 2021). Nevertheless, this figure is already obsolete; in Lombardy alone about 1,100 nests were counted in 2021 (FASOLA et al., 2021) and in the same year in the Veneto region there were about 250 pairs (SIGHELE et al., 2022). Thus, the Italian population amounts nowadays to at least 1,500 pairs.
In the Venice lagoon nesting of the Sacred Ibis was presumed in 2017-2019 atseveral sites (SARTORI et al., 2020; SCARTON & VALLE, 2020) but without convinc- ing proof of reproduction. Only in 2020 nesting of the species was confirmed at three colonies, for a total of 23 pairs (SCARTON et al., 2021), risen to 216 pairs and four colonies in 2021 (SIGHELE et al., 2022).
Studying the reproductive success of a species in its very early stage of settlement in a new breeding area may give interesting insights about this biological aspect, allowing possible foresights about the growth of the population. Given the well known high adaptability of the Sacred Ibis to new contexts (MAILLARD et al., 2020), this could lead to a strong and quick increase of the population breeding in the Venetian Lagoon in the near future.
For these reasons, a detailed study on the reproductive success of the Sacred Ibis was made in one of the largest colonies occurring in 2020-22, using drone surveys. This method has been recently adopted extensively in the northern Adriatic (VALLE & SCARTON, 2021 and 2022) similarly to what has been done worldwide since many years in wildlife studies (see for a review MARCHOWSKI, 2021). The drone method allows rapid surveys, with little disturbance to the birds and gives the opportunity of collecting precise data which otherwise will not be possible to have, in colonies located on the top of trees and thick bushes, as is the case of the colony object of this study. Moreover, an update of the distribution and size of the Sacred Ibis colonies in the lagoon of Venice in the year 2022 is presented.
Study area and methods
The lagoon of Venice is the largest coastal wetland around the Mediterranean and is a site of international importance for the nesting and wintering of several waterbirds (SCARTON, 2017). All the lagoon, apart from the fish farms, was sur- veyed by boat to locate colonies of the Sacred Ibis in 2020-22.
Throughout this period six colony sites were surveyed: 1) Ca’ Pasqua: a very small islet between two rivers, close to the south-western border of the Lagoon (see SCARTON et al., 2021 for more details); 2) San Giuliano and Buel del Lovo: two small, abandoned islets in the northern Lagoon, both covered with thick arboreal vegetation; 3) Aleghero: a dredge island of a few hectares, sparsely vegetated, located very close the town of Chioggia; 4) Sacca San Biagio: an islet of about four hectares in the outskirt of Venice, where only a temporary depot of urbane waste occurs. Most of the islet is covered with grass and some bushes; 5) San Giorgio in Alga (45°25’29” N, 12°17’32” E), where most of the present study took place. This island is located about 1.5 km SW of Venice and is less than two hectares in size, surrounded by a wall of bricks. It was abandoned almost a century ago, but it still hosts some remains of artefacts. Elder Sambucus nigra, with Black Locust Robinia pseudoacacia, Tamarisk Tamarix gallica and Field Elm Ulmus minor largely cover the island.
The study of the reproductive success was conducted during the 2020-2022 breeding seasons, within the framework of a broader project aimed at monitor- ing the breeding species of herons (SCARTON et al., 2020), only at San Giorgio in Alga. Data were collected exclusively using drones (DJI Mini in 2020, and DJI Mini2 in 2021-2022) according to protocols previously validated in the study area (VALLE et al., 2021; VALLE & SCARTON, 2022). No ground surveys were conducted by the authors in order to avoid disturbance to the breeding birds. Nests were searched and monitored flying at elevations above ground level (AGL) higher than the agitation distance (i.e., adults opening wings to cover newly hatched chicks in response to low AGL overflights) for the species (pers. obs.). According to the current recommendations (HODGSON & KOH, 2016; VALLE & SCARTON, 2018), the drone was launched at least at 150 m from the colony, from a boat, in order to minimize disturbance to birds. The drone reached the vertical point of the colony (where nest attendance was checked) through a lawn-mower flight pattern, flying 70 m AGL. Then, the drone was slowly driven once more through a lawn-mower flight pattern to an altitude of 15 m AGL (which allows clutches, eggs and chicks to be clearly detected upon post-processing, when not incubated by adults; Fig. 1 and Fig. 2) and it slowly flew over the colony at a speed of 2-5 km/h.
We did not plan to assess clutch size nor hatching success to avoid low altitude drone flights during the vulnerable phases of egg laying and hatching, whereas we recorded fledging success, since juveniles are easily detectable by drone surveys and apparently indifferent to the latter. Fledging success was defined as the number of chicks that survived to 3 weeks of age per nesting pair, which was estimated for each colony from the third survey (at 21 days from hatching). These surveys, which were carried out from laying up to three weeks of age, permit only a conservative estimate of productivity, but we chose not to fly over older chicks that would be capable of flying away in response to drone intrusion. In the post-processing phase, counts were performed by two observers using imagery obtained stitching videos with the free software ICE (Microsoft’s Im- age Composite Editor, release 2.0; https://www.microsoft.com; VALLE, 2021). Categorical data are presented as percentages, and continuous data as mean + 1 standard deviation. Statistics were done with SPSS and the map with QGIS 3.14 Madeira.
Results
The Sacred Ibis were found breeding for the first time on San Giorgio in Alga in 2020; it cannot be excluded the species had nested in previous years, since the island had not been visited before. On July 14th, the drone flight shown the occurrence of 10 nests with 16 chicks of different ages.
In 2021, a massive breeding was found on May 9th 2021. On May 30th, 187 nests of Sacred Ibis were distributed in eight sub-colonies (mean + SD = 21 + 13, range = 6 – 45 nests per sub-colony), scattered on the vegetation at 1 – 5 m from the ground. The colony was formed by three nuclei: an early (estimated laying period: early April), large group of 118 nests; a medium (estimated laying period: late April) nucleus of 39 nests and a late (estimated laying period: late May), small one of 30 nests. Most nests were clumped together in tight aggregations, frequently in contact with each other. Overall, clutch size was 2.5 + 0.6 eggs (range 1 – 3; n = 39), with a significant difference between mid vs. late nesters (2.8 + 0.4 vs. 2.4 + 0.6; unpaired t-test conducted on square root transformed variable, P =0.032); no data on clutch size was available for early nesters. Hatching success of early + mid nesters was 2.3 + 0.6 chicks per clutch (range = 0 – 3), whereas no data are available for late nesters. Fledging success was only available for early and mid-nesters: 1.8 chicks per nesting attempt.
In 2022 (Figg. 1-4), breeding was confirmed in the same colony site with 102 nests on June 5th, scattered in six sub-colonies (mean + SD = 17 + 14, range = 3 – 42 nests/sub-colony) with a similar pattern of the previous breeding season. Associated colonial species were Little Egrets Egretta garzetta and Grey Herons Ardea cinerea. Fledging success of a sample of 27 nests was similar to the previous year, with 1.7 chicks fledged per breeding attempt for early nesters (estimated laying date: early May).
In the same year five additional colonies were active (Fig. 5): 1) Ca’ Pasqua: eight nests were found on the top of small trees and bushes, along with those of several species of herons; 2) Aleghero: 38 nests, placed on the ground. No other colonial species was nesting; 3) Sacca San Biagio. Due to the late timing of visit, we can only estimate the occurrence of 10 pairs; no other colonial species was nesting; 4) San Giuliano: 10 pairs were estimated, with nests placed on trees among those of other species; 5) Buel del Lovo, with at least 20 pairs in a multispecific colony. In the last two sites, drone flights could not be done due to local restrictions.
Thus, in 2022 there were six colonies with 188 nests, but this should be seen as a very conservative estimate due to incomplete coverage of the last two colonies above quoted. The mean number of nests per colony in 2022 was 31.3 + 36.4 (N=6), and the median was 15, since more than 50% of pairs clumped in just one colony. Four colonies were on secluded, small island; one on a dredge island and the last one on an islet with vegetated ground.
The distance from each colony to the closest ranged from 1.4 to 7.8 km (2.9 + 2.75, n=4); three colonies were clumped around Venice and two around the town of Chioggia (Fig. 5) Assuming a feeding range of 10 km, a distance reported in Italy for ecologically similar species such the colonial herons (FASOLA, 1983), in Fig. 5 the estimated feeding areas for the nesting adults is reported; in both cases the area encompasses urban settlements, tidal flats and saltmarshes, but negligible extensions of fish farms.
Conclusions
Despite referring to just one site, the reproductive success observed in the Ve- netian lagoon in the very first years of settlement seems similar or higher than to those observed elsewhere. As summarized by COCCHI et al. (2020), in the natal breeding range the reproductive success is often less than 1 juv/nest; in French colonies it was 1.3 juv/nest and 1.7 juv/nest in Italian colonies.
While in the first two years since its establishment the Sacred Ibis nested in the Venetian Lagoon only in already existing colonies of other herons and cormorants, in 2022 the species began to nest also alone. According to CUCCO et al. (2021) all the Sacred Ibis nesting in NW Italy settled only in colonies with other colonial herons; the same finding was reported for the Lombardy alone for the years 1989- 2021 (FASOLA etal., 2021). Nevertheless, the occurrence of monospecific colonies of Sacred Ibis in Italy has been already reported by CASTIGLIONI etal. (2015). Clumping of most of the colonies around the towns of Venice and Chioggia suggests a strong dependance of the species from urban sites and feeding sources available there, but a detailed study on the trophic ecology of the Sacred Ibis in the Venetian Lagoon is needed. As it was easily foreseen in a previous paper (SCARTON etal., 2021), the population of the Sacred Ibis nesting in the Venetian lagoon has been growing up quickly, taking advantage of the availability of many suitable nesting sites (abandoned islands, dredge islands, peripheral urban sites) and the occurrence of several feeding sources, both natural (agricultural fields and ditches, tidal flats) and anthropogenic (urban waste).
1: Buel del Lovo; 2: San Giuliano;
3: San Biagio;
4: San Giorgio in Alga; 5: Aleghero;
6: Ca’ Pasqua.
Not all the colony sites were used each year.
Dashed line: estimated feeding areas (10 km radius) for birds nesting at colonies 1-4 and 5-6.
Following its inclusion in 2016 in the list of invasive alien species (IAS) of Eu- ropean Union relevance, in Italy an Action Plan on the Sacred ibis was recently approved (COCCHI et al., 2020); this document suggests several actions for the control of the species, including capture of birds, egg removal and adult killing. For the Veneto region, the document specifies activities such as a control finalized to the species eradication, the local eradication and monitoring. None of these measurements has been adopted so far, and their enforcement in the near future seems also unlikely. This will give the Sacred Ibis the possibility to expand further in the Venetian lagoon, with effects on the autochthonous fauna of this interna- tionally important wetland which, at the moment, remain completely unknown.
Acknowledgments
M. Bon (Natural History Museum of Venice) kindly reported us the occurrence of Sacred Ibis at San Biagio; M. Cucco (University of Piedmont) gave us some recent information on the Sacred Ibis population in NW Italy. Two referees made useful comments.
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