Artificial nest experiment
Experiments using artificial clutches enable the separation of the contributions of crypsis (Haskell 1996) and parental incubation behaviour (Cresswell 1997) to nest success. For each real nest (hereafter called ‘incubated nest’), we assigned a pair of artificial nests (not incubated, and therefore not affected by parental activity; Cresswell 1997) 7·5 ± 2·7 m (mean ± SE, range 5·5–15 m) from the incubated nest. For each pair of artificial clutches, one was covered by nest material taken from the spatially associated incubated nests, and the second was left uncovered (although we added some nest material around this clutch to mimic the scent and appearance of a real mallard nest (Guyn & Clark 1997). This experimental design enabled us to test the anti-predation function of clutch covering (survival of covered vs. uncovered artificial nests), as well as the contribution of parental incubation behaviour to the nest success (survival of incubated vs. covered artificial clutches; we assumed that an equivalent or lower quality of clutch crypsis is provided by the female feather colouration compared to nest material [see below]).
Artificial nests were constructed from dead vegetation shaped into a cup closely resembling natural mallard nests. Real inactive nests baited with an artificial clutch have previously been used to assess the effect of parental activity on nest survival in open cup shrub-nesting birds (Cresswell 1997; Komdeur & Kats 1999; Weidinger 2002). In the case of ground-nesting birds, clutch colouration rather than nest design is expected to be a crucial clue for predator nest disclosure (i.e. Weidinger 2001). Thus, we assume that the use of human-made artificial nests does not seriously affect the results of this experiment.
Clutches of artificial nests consisted of four brown chicken eggs. In the additional experiment performed in year 2006 on the same study plots (data not included in the results), artificial clutches baited with chicken eggs had a survival rate comparable to those baited with mallard eggs (28 artificial nest couples χ2 = 0·65, d.f. = 1, P > 0·40; in fact mallard eggs were slightly more predated than chicken eggs, 19 vs. 16 nests predated, Kreisinger & Albrecht unpublished data). Hence, the difference in eggshell colouration between chicken and greenish mallard eggs is unlikely to bias our results.
Nest sites for artificial clutches were chosen to approximate the nest site of the associated incubated clutch in features that could affect the probability of predation (e.g. vegetation structure, height and density, proportion of dead vegetation, distance to the closest tree, shrub and water). We took special care not to manipulate or damage the vegetation around all nests. To control for possible bias in this aspect, vegetation concealment was measured for each clutch at the beginning of each experiment. A 20 × 20-cm grid composed of eight white 5 × 5-cm squares was placed directly on the nest bowl, and the percentage of white squares obscured by vegetation when viewed from 1 m directly overhead was scored. The mean vegetation concealment of all nests was 50·7 ± 24·0% (mean ± SE) with no differences in vegetation concealment between incubated and artificial clutches in each triplet (6·3 ± 8·7%[mean ± SE], Repeated Measures anova, F2,173 = 0·658, P > 0·5).
The relative proportion of time that a female spends on the nest differs noticeably between the pre-incubation and incubation stages (Afton & Paulus 1992). Therefore, in this experiment, we only included real nests that survived to day 3–5 of incubation (based on the floating test of Westerkov 1950). Clutches found during pre-incubation or very early incubation (< 3 days) are often abandoned (e.g. Johnson, Nichols & Schwartz 1992), and therefore, were not included in our experiment since the influence of parental behaviour cannot be assessed for abandoned nests. Incubated nests included in this experiment were a random sub-sample of all mallard clutches found in our localities which fulfilled the criteria mentioned above.
Nests triplets were checked twice, the first time after 5·7 ± 0·8 (mean ± SE) and a second time after 11·4 ± 1·1 (mean ± SE) days from the start of the experiment. When we approached the nest, the female duck usually left the nest suddenly without covering eggs with nest material. When this happened, we covered clutches with nest material after the nest check to mimic the behaviour of an undisturbed female leaving the nest during incubation recesses (Caldwell & Cornwell 1975). During the second check, successful artificial clutches were removed.
Nests were defined as predated if the clutch was damaged or at least one egg was missing. This criterion makes our results rather conservative, because incubated nests with partial clutch loss, where the female continued the incubation were classified as predated (two incubated nests, one and three eggs missing, respectively). Nevertheless, in these cases we could not distinguish between partial clutch predation (i.e. Ackerman et al. 2003) and other causes of eggs loss (such as the ejection of an egg with a broken shell by the female). We excluded two nest triplets for which the incubated nest was abandoned by the female during the experiment and the clutch was found intact during a subsequent visit. The final analysis included 60 nest triplets.
Experiments using artificial nests increase natural nest densities, which may draw predators and lead to an artificial increase of predation rates. However, mallards already breed in high densities in our locality. As only a small proportion of incubated nests were included in the experiment (15–18% mallard nests found), nest densities were only increased by the experimental setup by 30–35%. Similarly, the mean distance between neighbouring natural nests (10–20 m) was only increased slightly by our experiment. We assumed that this modest increase in nest densities did not alter natural predator densities or predation rates, since experiments based on much larger differences in duck nest densities show no consistent effect of nest densities on predation rates (see Ackerman, Blackmer & Eadie 2004 for review).