AbstractBreeding birds use a variety of cues to choose a nest site. For conspecific brood parasites, the number of eggs in a host nest may be an important indicator of nest stage (laying or incubating) and the resulting prospective value of the nest. In precocial birds, such as wood ducks ( Aix sponsa), a parasitic female should lay her eggs during the host’s laying stage to ensure hatch synchrony with the host. Incubation and hatching success may also be compromised in large clutches. Accordingly, parasitic females should respond to the number of eggs already present in a potential hosts’ nest and should preferentially lay eggs in nests with smaller clutches.

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We conducted a field experiment using simulated nests containing different numbers of “host” eggs to test this hypothesis. When offered a choice of nests containing clutches of 5, 10, 15, or 20 eggs, females were significantly more likely to lay eggs in the 5- and 10-egg treatments, laid more eggs in total in the smaller clutch treatments, and were more likely to incubate the nests in the 5- and 10-egg treatments. These results indicate that wood ducks are responsive to quantitative cues, such as the number of eggs in a nest, although we do not yet know if they are able to do so directly by numerical discrimination (i.e., counting). A central assumption in many areas of behavioral ecology is that individuals act as if assessing the profitability of different choices and then use this information to make choices that maximize prospects for success (e.g., mate assessment; and habitat selection; ).

In many situations, the differences are qualitative in nature (e.g., brighter, larger, closer). More challenging are cases where the choices differ quantitatively and require some ability to discriminate numerically.

Such ability would be valuable when incremental differences in quantity (number) of an item significantly influence the profitability of a choice. The extent to which animals use quantitative cues, possibly involving number sense or counting, has been well studied in the laboratory but has only recently attracted attention in field studies (;;;; ).The phenomenon of avian brood parasitism offers a novel opportunity to examine the extent to which birds may use numerical cues in decision making (; ). Brood parasitic species are different from most bird species with parental care because parasites lay their eggs in nests of hosts that already contain eggs.

Parasitic individuals are often constrained by the availability of active nests and the limited incubation period of the host; hence, the number of eggs in a nest may provide an important cue on the stage of the nest (laying or incubating) or the host readiness to incubate (;; ). Furthermore, the prospective value of a nest may vary directly with the number of eggs in a clutch because hatching success is often reduced in large clutch sizes (;;; ). Accordingly, we might anticipate that parasites would use the number of eggs in a nest to evaluate the potential utility of a particular nest.Obligate brood parasites rely completely on finding a suitable host nest (because they do not make their own nest) and so may be particularly sensitive to nest cues.

For example, brown-headed cowbirds ( Molothrus ater) examine the host’s readiness to incubate by evaluating both the egg-laying rate of the host and the number of host eggs in the nest (; ). The state of the nest, such as laying or incubation stage, may also be important for conspecific (nonobligate) brood parasites, particularly so for precocial birds. Evaluating the nest state may allow a female to ensure hatch synchrony with the host (the simultaneous hatching of eggs) that is critical for precocial birds because young that do not hatch with the host eggs will not survive. Nests with small clutch sizes may indicate that a host female has not initiated incubation, and any egg laid in the nest would therefore be synchronized with the host’s eggs.

Furthermore, the number of eggs in a nest may influence the potential hatching success of the nest, given that large numbers of eggs may inhibit the efficiency of incubation (,;,; ). Therefore, by laying in a nest with a smaller clutch size, a parasitic female could potentially increase the probability that her egg(s) will hatch.We explore these ideas here.

We examine the extent to which parasitic individuals assess the number of eggs in a nest, and specifically, we predict that parasitic females will prefer nests with smaller clutch sizes due to the higher potential for hatch synchronization and hatch success. We tested this hypothesis using simulated nests containing different clutch sizes. We were also interested in whether females use numerical cues not only to make choices about whether to use a nest but also to modify the number of eggs laid per nest as a function of different clutch sizes.

We examined this by evaluating whether females laid different numbers of eggs in a nest-box as a function of the original clutch size and as a function of rate of egg accumulation (how many eggs were laid per day). Because females might also respond to visual cues, such as the visual space the eggs occupy in the nest-box, we also examined whether the number of eggs laid was correlated with the area of the nest-box floor. We predicted that females would lay more eggs in larger boxes assuming that more space was available for eggs, and clutches of equal size would appear smaller in boxes with large floors than boxes with small floors. Study speciesWood ducks ( Aix sponsa) are cavity-nesting waterfowl that engage frequently in conspecific brood parasitism. Their natural history has been well documented (; Semel and Sherman 2001), and several studies have investigated qualitative nest-site cues, such as nest-box placement and visibility (; ), and the potential influence of philopatry and kinship (; ). Other qualitative cues, such as nest-site history and nest-site limitation (; ), have been well studied in other conspecific brood parasites.

As part of a long-term study of this population, we previously explored the effect of nest-site history on nest selection by wood ducks. We found that females did not select sites based on their own previous success or history at a nest but only on whether a site had been used in the past. The presence and number of eggs in a nest may provide an important cue on the history and successful use of a site.

However, the extent to which wood ducks, or any other conspecific brood parasite, might use quantitative cues such as the number of eggs has largely remained unexplored. Warm ginger caramel spice cake pecans. METHODS Experimental nestsWe experimentally manipulated the number of eggs present in a nest-box at the beginning of the breeding season. Our intent was to provide females with a choice of boxes that varied in the number of eggs in the nest.

Wooden eggs (Howee’s Inc., Joplin, MO), which were similar in size and weight to wood duck eggs, were used to simulate a nest. Wooden eggs were approximately 50.8 mm in length and 34.0 mm in breadth and weighed approximately 40 grams. Wood duck eggs are approximately 50.1 mm in length and 38.2 in breadth and weigh approximately 40.5 g. Wooden eggs were painted beige (KILZ paint, color “Popcorn”) to match the most common color of wood duck eggs in our population (Odell NS, personal observation). During 2004, 2 treatments were contrasted; artificial manipulations simulating clutches of small size (5 eggs), and heavily parasitized clutches (20 eggs). Previous studies indicate that a female wood duck typically lays 10–12 eggs in a single clutch. Nests with 13 eggs or more are considered parasitized (Semel and Sherman 1992).New nest-boxes (40 boxes, 20 for each condition) were erected to control for previous nest-box history.

Each nest-box was built to be the same height from the bottom to the top of the box (37.2 cm) and the same bottom area (547.4 cm 2) of the box to control for spatial cues. Twenty boxes were placed at a site with a high density (38.7 boxes/km) of nest-boxes (Conaway Ranch, Woodland, CA), and 20 boxes were placed at a site with a medium density (18.2 boxes/km) of nest-boxes (Hedgerow Farms, Winters, CA); both sites contained nest-boxes prior to this experiment.

Each nest-box was attached to a tree using 2 lag bolts at an approximate height of 1.5 m within sight of water and within 10 m of another nest-box to control for environmental effects on nest-site preferences. The simulated nests contained only shavings and the artificial eggs. The eggs were positioned in the middle of the nest-box on top of the shavings. Nest-site use, the number of eggs, and the nest state (laying or incubating) were recorded weekly. Females were deemed to be incubating when they were on the nest during the day, the eggs were warm, and the eggs were covered in down feathers.In 2005, 4 treatments were evaluated; (1) clutches of small size (5 eggs, to simulate a situation in which the host is still laying), (2) average (typical) size (10 eggs, simulating a clutch near the completion of egg laying with incubation soon to begin), (3) lightly parasitized nests (15 eggs, in which incubation should have started), and (4) heavily parasitized (20 eggs, in which incubation should have started or the nest is abandoned).

A heavily parasitized nest would represent a nest where either several parasitic females had laid eggs or 1 parasitic female laid her whole clutch in the box.Nest-boxes from 2004 ( n = 40) were reused for the experiment in 2005, and 20 new boxes were erected at Conaway Ranch to increase sample size (total nest-boxes = 60). New nest-boxes were similar but larger than the previously erected boxes (bottom of box area was 726.3 cm 2).

Treatments were randomly assigned among boxes to control for previous nest-box history, nest-box size, and philopatry.Female wood ducks cover their eggs during absences from the nest, both before and during incubation. As artificial eggs were placed uncovered in nest-boxes, changes in egg position in the nest indicated that a female had investigated the nest. During each nest-box visit, we recorded any visual changes in egg position, such as whether eggs were buried in shavings or uncovered. This provided an additional means by which to assess whether females visited and assessed experimental treatments, even when egg laying did not occur. We caution that we cannot determine with absolute certainty that all movements of eggs were by wood duck females.

Predators or other bird species may have inspected the boxes. However, predation on nests is uncommon in this population , and predation attempts were typically indicated by egg loss, destruction, or marks (e.g., tooth or bill marks) on eggs. Extended observational watches at nest-boxes indicated that only wood ducks entered nest-boxes during this period of the year. Accordingly, changes in egg position could not be readily attributed to other species.

Non–experimental nestsWe also followed a large number of non–experimental nests. These were nests where birds selected a nest-box under natural conditions (no manipulation or other interference). We collected data on these nests during 7 field seasons (2000–2006) to compare the behavior of birds in our clutch size experiment with natural patterns of egg laying.Nest checks were performed at least biweekly throughout the nesting season (February–August) at 4 field sites: (1) Conaway Ranch (lat 38°386N, long 123°420W, USA), (2) Putah Creek (lat 38°310N, long 121°465W, USA), (3) Hedgerow Farms (lat 38°360N, long 121°590W, USA), and (4) Russell Ranch (lat 38°323N, long 121°525W, USA).

Each site encompassed 2–5 km of riparian habitat along a stream (Putah Creek, Russell Ranch) or slough (Conaway ranch, Hedgerow farms). During nest checks, females were identified via United States Geological Services’ metal leg bands, and eggs were counted, numbered, weighed, and measured. Hatching success (number of ducklings hatched) was determined by subtracting the number of ducklings hatched from the final clutch size.As the nest-boxes of the natural nests were not uniform, we examined the potential relationship of nest-box size and clutch sizes. The bottoms of nest-boxes were measured for width and length (cm).

Area was estimated by multiplying width by length. Statistical analysisWe first used contingency tests (log likelihood chi square) to test for differences among study sites (Conaway Ranch, Hedgerow Farms) and years (2004 and 2005) in the response of wood ducks to the experimental treatments. Our response variable in each test was type of nest use (incubated, egg laid but not incubated, and not used). As there were no differences among sites (likelihood ratio χ 2 = 4.88, degrees of freedom df = 2, P = 0.09) or years (likelihood ratio χ 2 = 2.50, df = 2, P = 0.29) in the type of use of boxes with experimental treatments, we pooled data for all study sites and years.We also used contingency tests (log likelihood chi square) to evaluate female use of boxes with different treatments (again indicated by type of use—incubated, egg laid but not incubated, and not used). We further compared the types of use (laid in only versus incubated) using contingency tests.

Differences in the number of eggs laid per treatment were examined using one-way analysis of variance (ANOVA). We refer to 2 measures of clutch size: (1) only the eggs laid by females (hereafter “added” eggs) and (2) total clutch size, which includes both the added eggs and the original experimental artificial eggs (this would reflect the total number of eggs in the nest visible to a visiting female).For all boxes that were used, we plotted the laying behavior of females in each box. We calculated the total clutch size of each nest, the number of eggs laid per box, the length of time the box was being used (egg-laying activity), and the rate of egg laying. We used one-way ANOVA to compare each response variable among treatments.

Egg deposition rates were calculated by dividing the number of eggs laid in a box by the number of days the nest was active. Types of use by experimental treatment. This figure shows the number of boxes that were not used (no eggs were laid), the number of boxes where an egg was laid, and the number of boxes where female(s) laid an egg in the box and incubated the nest.As a second assessment of the response to the clutch size treatments, we compared visual changes in the arrangement of eggs in the box (covered in shavings or uncovered).

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All treatments exhibited evidence of having being explored by females; over half of all nests (53 of 100) had eggs covered in shavings at some point. However, we found significant differences among treatments in the degree to which females covered eggs (likelihood ratio χ 2 = 24.65, df = 3, P 0.75). Do females modify the number of eggs they lay as a function of the number of eggs in a nest?The average number of eggs laid in experimental boxes differed significantly among treatments ( F = 3.37, df = 3, 33, P = 0.03). More eggs were laid in the smaller clutch treatments as the number of eggs per treatment declined in direct relation to the number of experimental eggs in each treatment (mean ± standard error SE: 5-egg treatment: 11.4 ± 1.3, n = 18; 10-egg treatment: 8.3 ± 1.7, n = 10; 15-egg treatment: 5.3 ± 2.2, n = 6; and 20-egg treatment: 3.0 ± 3.1, n = 3). Linear regression of the number of eggs laid against the number of experimental eggs in each treatment suggests that for each incremental experimental egg, approximately 0.58 fewer eggs were laid by wood duck females (regression equation: 14.23 − 0.58 × No.

Eggs in Treatment; F = 10.68, df = 1, 35, P 0.15), suggesting that there may be an upper limit to the number of eggs that females will lay in a nest site. Total clutch sizes by experimental treatment. The x axis shows the experimental treatments. The y axis shows the total number of eggs laid by females in each box.

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The regression line illustrates the decline in the number of eggs laid by females in each successive experimental treatment.Although females clearly differed in the number of eggs added to each treatment, the rate at which they did so did not differ among treatments. Egg addition rates (the number of eggs added per day) of the manipulated treatments did not differ significantly ( F = 0.80, df = 3, 29, P 0.50; mean eggs/day ± SE: 5-egg treatment: 0.39 ± 0.08, 10-egg treatment: 0.48 ± 0.10, 15-egg treatment: 0.29 ± 0.12, and 20-egg treatment: 0.20 ± 0.18). Likewise, the lag time (number of weeks between the start of the experiment and the week the first egg was laid) did not differ significantly among treatments ( F = 0.80, df = 3, 33, P 0.50; mean lag ± SE: 5-egg treatment: 5.3 weeks ± 1.8, 10-egg treatment: 9.3 ± 2.6, 15-egg treatment: 8.7 ± 2.9, and 20-egg treatment: 4.0 ± 4.5). Are experimental nests similar to non–experimental nests?Unmanipulated clutch sizes of wood ducks in our population varied considerably from 1 to 58 eggs but most nests ranged from 6 to 27 eggs. Experimental nests contained total clutch sizes between 8 and 29 eggs when artificial eggs were included. Considering only eggs added to nests (excluding experimental eggs), the average number of eggs in experimental nests was significantly less than that in non–experimental nests ( t = 2.82, df = 151, P. Frequency distribution of clutch sizes.

(A) Clutch sizes in experimental nests (artificial eggs plus “added” eggs) and (B) clutch sizes of non–experimental nests in our populations.Hatching success of eggs varied significantly with clutch size in non–experimental nests (polynomial regression, r 2 = 0.073, F = 21.67, df = 552, P 40 eggs) were never incubated.