4.1 Artificial nest survival
Interestingly, corresponding trade-off between food abundance and nest
predation risk was not evident at the habitat-level, because ponds rich
in food also have the lowest nest predation rates. The wildlife camera
data show that the nests on the shores of seasonal, beaver or man-made
ponds had higher survival than nests on the shoreline of permanent
lakes. Because we tried to keep the nest cover constant between the
experimental nests, this difference in survival rates probably arises
from more heterogeneous shoreline habitats of ponds and/or the
availability of other abundant food resources ponds offer for predators.
It has been suggested that nest survival is a combination of large-scale
environmental factors and local nest-site characteristics. Landscape
productivity can affect general predator and prey abundance, but at the
nest-site level vegetation and nest location might affect nest
detectability and predator behaviour (Ringelman et al., 2018). For
example, predator foraging in the landscape may be concentrated at
habitat edges (Andrén, 1995), such as the interface between terrestrial
and aquatic habitats. The occurrence of the edge effect may depend on
the predator community and predator behaviour (Pasitschniak-Arts et al.,
1998), and for instance whether productive wetlands attract and support
high number of predators (Stephens et al., 2005).
In the Evo area, mammalian predators have been found to occur more often
around beaver ponds than permanent lakes (Nummi et al., 2019b). Still,
higher predator occurrence around the beaver ponds was not reflected in
the nest predation results, indicating that indeed some habitat-related
factors there are working in favour of higher nest survival. It is
possible that variability in the shoreline creates circumstances under
which predators probably are not able to form long-term search images,
i.e. circumstances resembling those considered by Nams (1997) for prey
aggregated in space or time. We suggest that because predators may use
spatial memory to improve searching efficiency (Phillips et al., 2004),
their search around permanent lakes is more regular and effective than
around temporally unpredictable ponds. This underlines the potential
importance of seasonally flooded ponds for breeding ducks, especially in
agricultural areas.
Overall, nest predation risk was lower in forest compared to shoreline
nests, indicating occurrence of the edge effect between terrestrial and
aquatic ecotones. The pattern was not depending on the fields in the
landscape but emerged in both forested and agricultural landscapes.
Several currently threatened duck species in Finland are typically
nesting near the shoreline (e.g. common pochard Aythya ferina ,
tufted duck), and may suffer from stronger nest predation rates than
more flexible nesters (e.g. mallard and teal; e.g. Pöysä et al., 2019).
Nesting in forest may be safer, but on the other hand, newly hatched
ducklings will have to move a long way into water (Pöysä & Paasivaara,
2006).
Increased predator abundance and diversity are typical especially for
fragmented landscapes (Andrén 1995; Pasitschniak-Arts & Messier, 1995).
The pattern is particularly pronounced in agricultural landscapes, where
there are already high numbers of predators, such as corvids (Andrén
1992; Roos 2002; Holopainen et al., 2020a). The results support these
observations: nest predation risk was higher in the agricultural
landscape, where high rates of corvid predation and richer predator
communities were observed with similarly executed wildlife
camera-artificial nest experiments (Holopainen et al. 2020a, 2020b).
Wildlife camera studies conducted both with artificial nests (Holopainen
et al. 2020b) and natural nests (Bell & Conover, 2023) have proven that
after the initial depredation event, disturbed nests are often visited
by multiple secondary predators. Multiple mammalian visits lead not only
to an increased egg depredation rate, but also higher mortality risk for
the incubating female. Indeed, hens often abandon (partially) depredated
nests and even if incubation is continued, hatching success rate is low
(Ackerman et al., 2003; Bell & Conover, 2023).
In Europe, the overall predator populations have increased during last
decades threatening bird populations (Roos et al., 2018). In addition to
native species, invasive alien predators such as raccoon dog have
dispersed widely (Bonesi & Palazon, 2007; Kauhala & Kowalczyk, 2011).
Raccoon dog nest predation can be destructive on islands (Dahl & Åhlén,
2018), but its role as mainland duck nest predator has remained unclear
(Kauhala, 2004; Kauhala & Auniola, 2001; Nummi et al., 2019a;
Sidorovich et al., 2008). Our nest predation results were based on
detailed predation information derived from camera trapping. Corvids and
raccoon dog were responsible for most of the nest depredation occurred
at experimental nests mimicking the situation in the early stage of egg
laying (Holopainen at al., 2020a). Without cameras, predator
identification is uncertain as it relies on the remains of eggshells or
other cues on the nest site (Larivière, 1999).
We recognise that artificial nests give an uncertain reflection of
actual nest predation, and thus the intention in this study was not to
evaluate actual predation rates but only to study the habitat-specific
relative predation risk. Many important differences exist between real
and artificial nests that decrease the correspondence and are thus
recommended to consider whenever conducting artificial nest experiments
(Butler & Rotella, 1998; Whelan et al., 1994; Wilson & Brittingham,
1998; Pärt & Wretenberg, 2002). Effort was put in to tackle the
uncertainties: real mallard eggs were used and the species observed in
the camera pictures are known predators of real duck nests (Opermanis et
al., 2001; Pöysä et al., 1997), and therefore it is assumed that the
observed species do not differ from the actual nest predator assemblage.
As Anthony et al. (2006) showed with dusky Canada geese (Branta
canadensis occidentalis ) artificial nests can be used to identify the
potential nest predator species and that the predator species ratios can
correspond those of the real nests. Our artificial nest density was low
ensuring that observations were independent. The lacking hen problem was
avoided by focusing only on the early egg-laying stage when females are
not on their nests, so the set-up resembles the actual situation.
Compared to natural nests, the use of cameras may affect nest survival,
typically by decreasing the predation rate (Richardson, et al, 2009);
this is a potential shortcoming that could not be avoided. It is also
acknowledged that this study design potentially emphasises the role of
visual predators, such as corvids, as nests were not necessarily hidden
as efficiently as a dabbling duck hen’s nest would be. High corvid
predation rates may also be expected to occur at the early real nests,
as the duck nest predation rate in North America during the early part
of the breeding season was observed to positively relate to American
crow (Corvus brachyrhynchos ) activity (Johnson et al., 1989).
The correspondence of the artificial nests with actual nest success
cannot be assessed, but brood production rates in the study areas have
been measured, which can be used as a rough estimate of predation
pressure. While there are still uncertainties in this method, we
emphasise that the problems underlined by the earlier studies have been
considered and the differences between real and artificial nests were
accordingly minimized; thus, we suggest that our data are suitable for
detecting trends in predation rates in relation to habitat (Wilson &
Brittingham, 1998).
Invertebrate food abundance
As expected, ponds (seasonal, beaver and man-made) were more
invertebrate-rich habitats than permanent lakes, while contrary to the
hypothesis, the percentage of field land around the wetlands did not
influence invertebrate index. Selecting a pond instead of a lake as a
breeding habitat would thus simultaneously minimize nest predation risk
and maximize food availability in any landscape.
Habitat use of duck pairs was not associated with invertebrate food,
whereas duck broods preferred habitats richer in food. The number of
broods at the water bodies was only weakly dependent on the number of
pairs, which can be a reflection of differing habitat requirements of
pairs and broods (Holopainen et al., 2015) or high nest predation
and brood mortality. Sjöberg et al. (2000) showed for mallards that all
lakes used by pairs are not suitable for broods, the difference in lake
use between pairs and broods being due to food limitation at the brood
stage (Gunnarsson et al. , 2004). In boreal lakes food limitation
can be intensified due to food competition between ducks and fish (Nummi
et al., 2016). Income breeders, like teal, seem to avoid brood-stage
food limitation by congregating in beaver ponds and seasonal ponds where
invertebrate production is high and the habitat structure favourable for
brood foraging (Nummi & Hahtola, 2008; Nummi & Holopainen 2014). In
Evo it is known that teal brood production is following the flood
dynamics created by the beaver and spring floods (Holopainen et al.,
2014).
Interestingly, the results did not show that duck pairs or broods used
ponds more than permanent lakes. This contradicts the earlier long-term
results from the Evo area (Nummi & Holopainen, 2014). It is possible
that ducks visit food-rich ponds for foraging in very short periods,
reducing the ability to detect them there (Nummi et al., 2019c).
Waterbird species may also differ in their ability to respond to
environmental factors, such as habitat variability (Wiens, 1976; Nummi
& Pöysä, 1997). For example, lapwings (Vanellus vanellus ) are
known to nest in higher densities around flooded footdrains, and chicks
forage on the wet mud around these wet features supporting invertebrate
rich habitats (Eglington et al., 2008, 2010).
Conservation implications
Successful management of ducks would demand understanding of the
relationship between habitat availability and predators (Drever et al.,
2004). This study emphasizes the benefits of the availability of
different water body types for breeding ducks. It showed that flooded
and/or seasonal ponds might be especially good habitats. It seems that
two important limiting factors of the breeding season – nest survival
and amount of invertebrate food – are higher there than on permanent
lakes.
Kubelka et al. (2018) showed that shorebirds have experienced a
worldwide increase in nest predation over the past decades and that the
pattern is especially pronounced in the high northern latitudes. 12 of
the 19 duck species living in Finland are already classified as
threatened to some degree by the Finnish red list (Lehikoinen et al.,
2019), underlining the urgent need for conservation actions. These
results indicate that while duck pair and brood densities are higher in
an agricultural landscape, brood production seems to be higher in
forested landscapes with lower nest predation rates. Thus, duck species
nesting at eutrophic lakes in agricultural areas and preferring
especially shorelines as nesting places, may suffer from high nest
predation rates, which may contribute to the declining population trends
(Lehikoinen et al., 2016; Pöysä & Linkola, 2021). It is
suggested that the nest predation pressure around these lakes has
increased due to the appearance of alien predators (Holopainen et al.,
2021; Pöysä & Linkola, 2021) and the disappearance of protective
umbrella species; for instance, loss of black-headed gullChroicocephalus ridibundus colonies removes a local protective
“umbrella” of mobbing gulls, in a way that is thought to expose the
nests of associated waterbird to greater predation threat (Pöysä et al.,
2019). Controlling predators, especially alien species, would thus be an
important conservation action to improve duck breeding success (Dahl &
Åhlén, 2018; Garrettson & Rohwer, 2001; Jaatinen et al., 2022).
Seasonal pond ecosystems in the boreal biome remain poorly studied, even
so that in Finland the habitat type does not have a conservation status
evaluation done due to lacking information (Lammi et al. 2018).
Nevertheless, seasonal pond habitats are commonly not protected. The
loss of seasonal ponds has been dramatic in boreal biome due to
drainage, destruction and water regulation (Colburn, 2004), also in
Finland (Kuusisto et al., 1998). Furthermore, it is predicted
that climate change will reduce the extent of snowmelt dependant spring
flooding in the future (Veijalainen et al., 2010). In addition to
wetland restoration and blocking up drains, the lack of flooded waters
may be mitigated by favouring beavers (Hood & Bayley, 2008; Nummi &
Holopainen, 2020) or man-made wetlands (Danell & Sjöberg, 1982;
Eglington et al., 2008; Čehovská et al., 2022).