The use of artificial floating nest platforms as conservation measure for the common tern Sterna hirundo: a case study in the RAMSAR site Druzno Lake in Northern Poland

Figures & data

Figure 1. Nesting floating platforms, A (A), and B (B), and general view of all platforms location (C) in the Druzno Lake reserve (photos by authors).

Figure 2. Nesting floating platform A with adults on nests (A), eggs (B) and chicks (1–2-week-old) (C) of the common tern in 2019 (photo by authors).

Figure 3. The population dynamics of common terns breeding in the Druzno Lake reserve in 1999–2021. No data available for 2003 [indicated with X in the graph]. The red arrow indicates the construction of a one floating platform Type A, the purple arrow – two platforms Type B, for common terns bars indicate number of pairs, dots number of colonies.

Table I. The number and density of nests, mean clutch size and breeding success in 2017–2021 on the three nesting floating platforms in Druzno Lake reserve

Platform type	Year
	2017	2018	2019	2020	2021
Number of nests
A	1	3	37	11	15
B1	-	-	-	1	18
B2	-	-	-	22	14
Nest density per m^2
A	0.05	0.16	2.0	0.60	0.81
B1	-	-	-	0.10	1.71
B2	-	-	-	2.10	1.33
Mean clutch size ± SD
A	3.0 ± 0.0	2.3 ± 0.47	2.2 ± 0.90	2.3 ± 1.05	2.53 ± 0.83
B1	-	-	-	3.0 ± 0.0	2.94 ± 0.54;
B2	-	-	-	2.4 ± 0.83	2.14 ± 0.95
Hatching success (%)
A	100.0	100.0	67.1	48.0	63.2
B1	-	-	-	0.0	61.9
B2	-	-	-	75.5	66.7
Mean breeding output per nest
A	3.0	2.33	1.49	1.09	1.60
B1	-	-	-	0.00	1.77
B2	-	-	-	1.82	1.82

Table II. Recommendations for constructing and placing nest platforms for common terns

Recommendation	Note	Reference
Construction
Heavy construction of the platform with high buoyancy	prevention of the raft being buffeted by the strong winds at the exposed sites	Babcock & Booth 2020a
High sides/boards	minimize substrate erosion, protect nests and chicks against waves	Dunlop et al. 1991; Molina et al. 2009
Fine anchoring (e.g., concrete blocks)	strong winds and storms can damage platforms and/or pull them off the ground/move/rotate them	Dunlop et al. 1991; Molina et al. 2009
Chain connecting platform and anchor should be long enough to secure anchoring	protection against water level fluctuations	Dunlop et al. 1991
Drainage holes in the platform	protection against flooding of nests due to rainfall	Dunlop et al. 1991; Shealer et al. 2006
At least a 5 cm sand/gravel layer on the top of the platform	PCV sheet on the construction will prevent from sand spread	this study
Sides covered with metal sheets or a clear polycarbonate screen with a smooth surface	protection against mammalian predators; a smooth surface offers no footholds for the mammalian predators to climb	this study; Babcock & Booth 2020a
Boards of the platform should be secured to prevent chicks fall from the platform (but see the next point)	chicks that jumped to water are unable to climb back onto the platform	Dunlop et al. 1991; Lampman et al. 1996; Gach et al. 2018
A hinged flap fixed to the edge of the nest platform providing a shallow gradient which can be climbed by chicks	allowing unfledged chicks to climb back on the platform if they fall out, and being able to access water in hot weather	Molina et al. 2009; Babcock & Booth 2020a
Shelter structures for chicks (e.g., dry tree branches, small rocks) and/or deep, long and low chick shelters made of wood of PVC	hide from avian predators, excessive sunlight or strong wind; shelters shaped as a tunnel should be positioned at an angle away from the prevailing wind to maximise protection	this study; Richards & Morris 1984, Dunlop et al. 1991, Burness & Morris 1992, Babcock & Booth 2020a, 2020b
Location
Close to other platforms/natural colonies	social attracting	this study; Dunlop et al. 1991
Far from places exposed to human disturbance	minimize risks associated with human disturbance	this study; Babcock & Booth 2020a
Within the cost-effective distance from foraging areas	securing effective foraging and feeding chicks	this study; Babcock & Booth 2020a
Maintenance
Covering the platform, with e.g. a tarpaulin, until the common terns arrive	minimize risks associated with overcompeting terns by larger gulls	Babcock & Booth 2020a
Remove vegetation from the platforms before the breeding period onset	to maintain a comfortable open area with soft substrate	this study; Ramos & Del Nevo 1995; Denac & Božič 2019
Ongoing maintenance, e.g., repair of construction damage, sand replenishment	to maintain bird safety and attractiveness of the substrate	this study; Babcock & Booth 2020a
Monitoring
Remote control should be used, e.g., using drone, motion-activated cameras or camera	to minimize bird disturbance	this study; Chabot et al. 2015; Guilfoyle et al. 2017

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