Bird community of the Douro Estuary Local Nature Reserve (Vila Nova de Gaia) and mouth of Granja stream (urban stream, Porto) (2016-2017)

Study Extent

The study was conducted in and around the RNLED, covering 138 ha of several substrates, and in the mouth of Granja stream, covering 8 ha.

Sampling

Nine counting points were established in the study area (7 in RNLED and 2 in Granja stream), using Google maps and ArcGIS 9.3 (Environmental Systems Research Institute, Redlands, CA, EUA). The points had to fulfill two criteria: (1) to maximize the area covered and, (2) to cover the different substrate types used by the birds. Thus, they were established in a way that each covered at least one of the pre-defined substrates (but usually more). Between them, they enabled the detection of all aquatic birds present during counts in their field of vision (complete counts method). In total, 31 counts were performed in 2016-2017 (20 in RNLED and 11 in Granja stream). From December 2016 to February 2017 (wintering period), 12 counts were conducted in RNLED (6 at low-tide and 6 at high-tide) and 7 in Granja stream (4 at low-tide and 3 at high-tide). In March and April 2017 (migration period), 8 counts were performed in RNLED (4 at low-tide and 4 at high-tide), and 4 in Granja stream (2 at low-tide and 2 at high-tide). Counts were performed around the tidal peak (± 2 hours) in days with good weather conditions (i.e., without heavy rain, strong wind and fog). To cover the daily heterogeneity of the birds' behaviour, the counts were performed between 06:57h and 17:21h, except in the Granja stream where all counts were made between 12:50h and 16:16h. The counts were carried out with binoculars (Opticron 10×50) and a telescope (Swarovsky 20×60). All the individuals present on each substrate were quantified by direct counting or estimated (in the case of large flocks) (Bibby et al. 1992). Additionally, the behaviour of the focal birds, (feeding or resting and/or preening), the substrate type(s) used (assessed in the field, using natural markings), as well as any disturbance events (e.g., people, domestic animals), were recorded. The birds were identified to the species level, the exception being the juveniles of the large gulls (Larus michahellis and L. fuscus), which were grouped under the name of “Larus sp. (juv)”. Tidal levels and general weather conditions were taken from online data (http://www.tabuademares.com/pt/porto/porto-rio-douro and https://www.ipma.pt/en/otempo/ prev.localidade.hora/, respectively). The substrate types were defined visually, based on two field visits carried out at each local, one at high tide and the other at low tide, to estimate the respective areas. The types considered were: (a) sand with grass shrubs (sand with portions of grass and/or shrubs; RNLED, only), (b) sand, (c) muddy sand (mixture of sand and silt, with the predominance of the former), (d) mud (RNLED, only), (e) sand with rocks (mostly sand with some natural rocks; Granja stream, only), (f) rocks with sand (mostly rocks with some sand; Granja stream, only), (g) rock (natural rocks or human-made rock walls), (h) salt Marsh (mud soil flooded and drained by tides; RNLED only), (i) dry dock (old construction to repair artisanal boats; Granja stream, only) and (j) water (open water, river or sea).

Quality Control

All identifications were made by qualified technicians.

Method steps

1. To be completed

The taxonomic coverage of this dataset spans 1 phylum, 1 class, 4 orders, 8 families, 21 genera and 26 species.

1. Chordata
   rank: phylum
2. Aves
   rank: class
3. Anseriformes
   rank: order
4. Charadriiformes
   rank: order
5. Pelecaniformes
   rank: order
6. Suliformes
   rank: order
7. Anatidae
   rank: family
8. Charadriidae
   rank: family
9. Recurvirostridae
   rank: family
10. Scolopacidae
    rank: family
11. Laridae
    rank: family
12. Ardeidae
    rank: family
13. Threskiornithidae
    rank: family
14. Phalacrocoracidae
    rank: family

The study area covers the terminal part of the Douro Estuary, encompassing the Douro Estuary Local Nature Reserve (RNLED, Reserva Natural Local do Estuário do Douro), located some 500 m from the mouth of the estuary (Vila Nova de Gaia municipality) and the mouth of an urban stream (Granja stream) located in the northern part of the estuary, 2 km upstream (Porto municipality).