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Overlooked Costs of Coloniality: Mislaid Eggs and the Double Incubation of Separate Nests.


Although colonial breeding is widespread in vertebrates, the evolutionary drivers of group living remain poorly understood (Evans et al., 2016; Ward and Webster, 2016). Many studies have taken a cost-benefit approach to understand the evolution of coloniality (Jungwirth et al., 2015; Chappell et al., 2016; Evans et al., 2016; Ward and Webster, 2016). However, this approach is only effective if the impacts of colonial breeding on fitness are accurately assessed (Rolland et al., 1998). Group living is thought to benefit organisms by increasing access to limited resources, such as nest sites and opportunities for extra-pair copulations (Wagner, 1993; Mayer and Pasinelli, 2013), improving foraging efficiency due to the exchange of information and communal defense of resources (Brown and Brown, 1996; Bijleveld et al., 2010; Weimerskirch et al, 2010; Evans et al., 2016), and decreasing predation through enhanced vigilance, mobbing behavior, and the dilution of predation risk (Hamilton, 1971; Alexander, 1974; Hoogland and Sherman, 1976; Beauchamp, 2010; Jungwirth et al., 2015).

Nonetheless, colonial breeding often entails significant costs (Majolo et al, 2008). For example, coloniality can facilitate the transmission of parasites and increase competition for limited resources (Alexander, 1974; Hoogland and Sherman, 1976; Moller, 1987a; Rolland et al, 1998; Rifkin et al, 2012). Misallocation of parental care can also negatively impact the fitness of colonial organisms (Brown and Brown, 2001). For instance, intraspecific brood parasitism and the accidental adoption of young often occur more frequently in large colonies, where breeding density and reproductive synchrony are generally high (Pierotti, 1991; Brown and Brown, 1989, 2001).

In avian species that reuse nests from year to year, mislaying eggs in unoccupied nests adjacent to active nests and either abandoning or alternately incubating multiple clutches at the same time may represent an additional, yet overlooked, cost of colonial nesting. Given avian incubation is energetically expensive (Deeming and Reynolds, 2015), and eggs are nutritionally costly to produce (Monaghan and Nager, 1997; Monaghan et al., 1998), the waste of reproductive resources through mislaid eggs could reduce the reproductive success of colonially nesting birds. We explore the potential cost of colonial nesting by examining the frequency with which North American Barn Swallows (Hirundo rustica erythrogaster) laid eggs in empty nests located adjacent to active nests over a 6 y study and analyze the relationship between number of mislaid eggs and total available nests, which scales with colony size. In addition, we document two cases in which females were observed laying their eggs in two adjacent nests and alternately incubating both clutches at the same time.



North American Bam Swallows breed solitarily in human structures or in colonies containing over 200 breeding pairs (Cramp, 1988). In order to initiate breeding as early as possible following migration, they frequently reuse intact nests from previous breeding seasons (Barclay, 1988). The number of nests varies widely among sites, but usually exceeds the number of breeding pairs. In general larger colonies contain more nests that are located closer together (Moller, 1994; Brown and Brown, 1999; Turner, 2010). Repeated captures and observations throughout the breeding season have demonstrated that colony members are consistent through time, with very few transient individuals (Safran and McGraw, 2004; Safran, 2006). In addition they are highly faithful to their breeding sites, with less than 1% of adults changing breeding sites within or between seasons (Moller and De Lope, 1999). Some subspecies of Barn Swallow, such as the nominate one (i.e., Hirundo rustica rustica), show relatively high levels of intraspecific brood parasitism (16-22% of active nests; Moller, 1987b; Moller, 1989; Petrzelkova et al., 2015). However, a study in H. r. erythrogaster that performed parentage analyses with six microsatellite markers only identified 10 suspected brood parasites out of 432 nestlings (<2.31%) in Colorado populations (J.K.H. analysis conducted for Hubbard et al., 2015), confirming previous studies that have documented low rates of intraspecific brood parasitism in North American Bam Swallows (e.g., Shields and Crook, 1987).

The most common clutch size of North American Bam Swallows is four to five eggs, and the incubation period lasts approximately 14 d (Brown and Brown, 1999). While males occasionally assist with incubation, females spend considerably more time on eggs (Smith and Montgomerie, 1992). Females weigh 15-20 g and lay one egg per day, with an average egg mass of 1.87 [+ or -] 0.17 g (mean [+ or -] 1 sd, n = 581 eggs; A.K.H. pers. obs.). Therefore, for this short-lived songbird, with an average lifespan of around 3-4 y (Moller and De Lope, 1999), we assume each egg represents a significant resource investment for females (e.g., Monaghan et al., 1998; Visser and Lessells, 2001, Williams, 2005; Pellerin et al., 2015).


This study was conducted at 43 breeding sites in Boulder County, Colorado, U.S.A. (40[degrees]06'14"N, 105[degrees]10'16"W) from 2011-2016. Some sites were monitored across multiple years (2011: 23 sites, 2012: 27 sites, 2013: 23 sites, 2014: 11 sites, 2015: 4 sites, 2016: 23 sites). The breeding sites consisted of 42 barns and one underpass and contained an average of 30 [+ or -] 31 nests (mean [+ or -] 1 sd) and 10 [+ or -] 10 breeding females (mean [+ or -] 1 sd), with 3.5 [+ or -] 1.72 available nests per breeding female (mean [+ or -] 1 sd). We captured and uniquely color-banded the majority of Barn Swallows breeding at each site, matched breeding pairs to nests using observations, and monitored active and inactive nests every 2-3 d for the duration of the breeding season (May-Aug.). We quantified the number of breeding females at each site by observing banded and unbanded individuals. In several years we recaptured all, or nearly all, of the individuals at breeding colonies multiple times throughout the breeding season.


To calculate the rate of egg mislaying at each breeding site from 2011-2016, we quantified the number of times one or two eggs were laid in an empty nest in the vicinity of an active nest and subsequently abandoned during the spring egg-laying period. According to our field observations, one- to two-egg clutches generally belong to females that lay eggs in unoccupied nests adjacent to their own, as clutch size ranges from three to six in many North American Barn Swallow populations (Brown and Brown, 1999). Given intraspecific brood parasitism, which generally involves active rather than inactive nests (Moller, 1989), is relatively low (<2.31% of nestlings) in our populations, and our colonies were composed primarily of breeding residents throughout the breeding season, it is unlikely these eggs were laid by transient individuals. This conservative method of quantifying egg mislaying potentially underestimated the prevalence of mislaid eggs by excluding cases where more than two eggs were laid in an adjacent nest.

We fitted a linear model between number of breeding females and total number of nests (log transformed for assumptions of normality) to verify the number of available nests increased with colony size at each breeding site. We then modeled the number of mislaid eggs in: (1) one-egg clutches, (2) two-egg clutches, and (3) both one- and two-egg clutches as a function of total available nests using generalized linear mixed models (GLMM) that assumed a Poisson distribution. We included an offset term in each model for the number of breeding females, which allowed us to assess the number of mislaid eggs relative to the number of females at each site. This term controlled for the fact large colonies had more females, and therefore statistically more opportunities for females to mislay eggs compared to sites with fewer females. Our models also included site and year as random effects and total number of nests at each breeding site as a fixed effect.


The first observation of double incubation by a female Barn Swallow took place in a barn containing 72 total nests and 17 breeding pairs. On 3 June 2011, we observed the banded female laying a single egg in an intact nest from the previous breeding season. Over the following 3 d (4--6June), she laid three consecutive eggs in an immediately adjacent nest on the same beam of the barn. On 10 July and 11 July, the female was observed for several hours (by J.K.H.) alternating between the two nests and incubating both clutches.


Our second observation of double incubation occurred in the same barn, which contained 86 nests and 19 breeding pairs in 2016. From 6-8 June 2016, we observed a banded female Barn Swallow laying three consecutive eggs in a nest. She then proceeded to lay two additional eggs in a separate nest located half a meter away on the same rafter of the barn (Fig. 1). To track incubation behavior, we installed thermocouple temperature probes (OM-EL-USB-TC, Omega Engineering, Stamford, CT, U.S.A.) with data loggers in the two nests. The probes were placed within fake eggs that were painted to resemble Barn Swallow eggs in coloration, patterning, size, and shape and filled with lubricant that closely matched the thermal properties of albumen (Ardia et al., 2009). We used Rhythm 1.0 and Raven Pro 64 1.5 to analyze temperature fluctuations and measure the duration and frequency of onand off-bouts from the nest, specifying a minimum off-bout duration of 2 min and a minimum cooling slope of 0.25 C/min in Rhythm to quantify bout length (Cooper and Mills, 2005; Ardia et al., 2009; Cooper and Voss, 2013). The loggers recorded temperature data ever)1 min for 5 d (22-26 June, n= 3953 observations) before removal. We used times of sunrise and sunset to divide the incubation data into daytime and nighttime periods. From the diurnal data, we averaged temperature, on-bout length, and off-bout length for the 5 d and compared nests using paired t-tests. As female Barn Swallows typically remain on the nest at night, we also quantified the number of nights the female spent on each nest.

Additionally, we carried out behavioral observations to confirm the same female incubated both nests. S.P.T. observed the female for 1 h on 22 June, approximately 12 d into the incubation period, while K.M. and M.H. observed the female for another hour on 25 June, 15 d into incubation. The female switched back and forth between the nests numerous times, always exiting the barn before incubating the other clutch. While her social mate accompanied her as she entered and exited the barn, he never assisted with incubation during either observation. All statistical analyses were performed in R version 3.4.3 (R Development Core Team, 2017) using the lme4 (Bates et al, 2015) and nlme (Pinheiro et al., 2017) packages. The results are presented as mean [+ or -] 1 SD.



We document 85 cases of Barn Swallows laying, and subsequently abandoning, eggs in unoccupied nests located adjacent to active nests during the 6 y study. Fifty of these cases occurred in one-egg clutches and 35 occurred in two-egg clutches. These 85 incidences of egg mislaying comprised 8.09% of the 1051 breeding attempts that we monitored. Over the 6 y study, the average clutch size of our populations was 4.5 [+ or -] 0.78 eggs (mean [+ or -] 1 SD, n = 545 clutches). Of the nests that contained at least one egg that successfully hatched, only 0.18% were one-egg clutches, 1.46% were two-egg clutches, and 6.79% were three-egg clutches. The relatively low frequency of hatching in one- and two-egg clutches supports our interpretation the majority of these clutches represent cases of mislaid eggs.

Although the total number of nests exceeded the number of breeding pairs at each site, we found a strong positive linear relationship between number of breeding females and total available nests, indicating that number of nests increased with colony size (Fig. 2; n= 111, b= 0.09, SE = 0.008, F= 120.25, df = 67, P < 0.001). In addition we documented a significant positive relationship between total available nests and number of mislaid eggs while controlling for number of breeding females in one-egg clutches (Fig. 3A; GLMM with Poisson distribution: total nests, n= 111, b= 0.015, SE = 0.005, F=9.65, P = 0.002), two-egg clutches (Fig. 3B; GLMM with Poisson distribution: total nests, n= 111, 6=0.013, SE = 0.006, F= 5.5, P = 0.02), and both one- and two-egg clutches combined (Fig. 3C; GLMM with Poisson distribution: total nests, n= 111, b= 0.016, SE = 0.005, F= 15.46, P < 0.001).


In 2011 two of the three eggs in the larger clutch successfully hatched on d 18 of incubation. Although the female attended both nests during the incubation period, the one-egg clutch failed to hatch.

In 2016 the 5 d of incubation data obtained from the temperature data loggers revealed the female consistently attended both nests throughout the incubation period (Fig. 4). However, on average, she spent more time incubating the nest with the larger clutch size. The three-egg nest, excluding the thermocouple egg, was maintained at a warmer average temperature than the additional nest with two eggs (Nest 1: 30.6 [+ or -] 3.1 C vs. Nest 2: 27.0 [+ or -] 3.7 C; paired t-test: P = 0.04, w=5 d; Fig. 5A). In addition the female incubated the three-egg nest for longer on-bouts (Nest 1: 999.1 [+ or -] 260.4 s vs. Nest 2: 523.1 [+ or -] 674.6 s; paired t-test: P = 0.08, n = 5 d; Fig. 5B) and left the nest unattended for significantly shorter periods of time (Nest 1: 508.4 [+ or -] 119.3 s vs. Nest 2: 1346.9 [+ or -] 295.5 s; paired t-test: P = 0.003, n = 5 d; Fig. 5C) than the nest with two eggs.

At night the female alternated between the two nests, spending three of the four nights incubating the three-egg clutch and one night incubating the two-egg clutch. Patterns of hatching success reflected the female's level of attendance at the two nests. Although the three eggs in the original nest hatched on 29 June (19 d into incubation) and the nestlings successfully fledged, the two eggs in the adjacent nest failed to hatch.


We present evidence the potential to mislay eggs in large colonies may constitute a novel cost of group living in colonially nesting birds. Over the course of the 6 y study, we detected 85 cases of Barn Swallows laying, and subsequently abandoning, one to two eggs in empty, unattended nests located adjacent to active nests. The majority of these observations occurred in large colonies (>50 nests), and the rate of egg mislaying relative to number of breeding females was positively correlated with the number of nests at each breeding site. While it is possible females laid these eggs in separate nests intentionally (e.g., if the eggs were defective), the fact they continued to incubate them for the entire incubation period in the two cases of double incubation suggests that the eggs were mislaid. Given larger colonies often contain more nests that are located closer together (Moller, 1994; Brown and Brown, 1999; Turner, 2010), we hypothesize females in larger colonies have a higher probability of confusing nests during the egg-laying period and unintentionally laying their eggs in adjacent unoccupied nests.

In birds the production of eggs is both energetically and nutritionally costly (Monaghan and Nager, 1997; Monaghan et al, 1998). Increased reproductive effort through egg production can negatively influence female body condition, chick-rearing ability, and future reproductive success and survival (Williams, 2005). For short-lived organisms like Bam Swallows, the waste of reproductive resources and loss of offspring through mislaid eggs may have important impacts on reproductive success. Importantly, our conservative method of quantifying the number of mislaid eggs likely underestimated their prevalence by excluding cases where more than two eggs were laid in an adjacent nest.

Although it is unclear what proportion of these mislaid eggs was immediately abandoned or incubated and failed to hatch, we observed two females in different years alternately incubating separate clutches at the same time. To our knowledge these are the first documented observations of a single bird with largely uniparental incubation alternately attending two nests during the incubation period. Yasue and Dearden (2006) reported the simultaneous biparental incubation of two nests by a pair of Malaysian Plovers Charadrius peronii. Similarly, Piersma (2008) described a female House Martin Delichon urbica provisioning nestlings at two nests attended by different males, attributing the observation to polyandry or helping at the nest. Here, polyandry and helping at the nest can be ruled out as potential explanations of double incubation due to the observed egg-laying schedules and lack of additional mates.

The simultaneous attendance of multiple nests has also been reported in several avian species that initiate second breeding attempts while feeding large nestlings from their first brood, thereby minimizing the interval between breeding attempts (Westmoreland, et al., 1986; Batchelder et al, 2012). However, unlike rapid renesting, the double incubation of separate nests in Barn Swallows appears to be maladaptive. Avian incubation requires significant energy expenditure (Deeming and Reynolds, 2015) and limits the amount of time available for foraging, territorial defense, preening, and other activities necessary for self-maintenance and survival (Deeming and Reynolds, 2015). The double incubation of multiple nests likely delayed the hatch date of the two successful nests and caused the hatching failure of the additional nests by not maintaining the eggs within the optimal temperature range suitable for development (>36 C in most avian species; Conway and Martin, 2000). In particular the hatching failure of the additional nests can be partially attributed to the fact the females only attended a single nest throughout the night, and ambient temperature often dropped at night, exposing the unattended nests to harsh conditions for extended periods of time.

While the mislaying of eggs and alternate incubation of multiple nests undoubtedly represents a waste of reproductive resources, the incidence of this behavior appears rather low in our populations (8.09% of monitored breeding attempts). In addition outside of the two observations of double incubation, we lack fitness data to indicate whether females that mislaid eggs fledged fewer offspring than females that did not, or whether those offspring were of lower quality. Male incubation, while uncommon, has been documented in North American Bam Swallows (Smith and Montgomerie, 1992) and could reduce the cost of mislaying eggs if males assist with incubation in cases of double incubation. In particular there were a few instances where the temperature of both nests in our 2016 observation of double incubation appeared to increase simultaneously. While these fluctuations may have occurred in response to changes in ambient temperature, it is possible that male incubation was involved. When compared to other demonstrated costs of colonial breeding in swallows, such as elevated rates of ectoparasitism (Brown and Brown, 1986; Moller, 1987a; Shields and Crook, 1987), increased competition for limited resources (Hoogland and Sherman, 1976; Moller, 1987a), and higher rates of intraspecific brood parasitism (Moller, 1987a), it is therefore possible the energetic costs of mislaying eggs are relatively minor. However, further studies on the prevalence of mislaid eggs, and the specific costs incurred by females that mislay their eggs (e.g., average time to renesting, lifetime reproductive success, recruitment rates of fledglings from nests that were alternately incubated) are necessary to fully evaluate the importance of this phenomenon relative to other costs of colonial breeding in swallows.

Both cases of double incubation occurred in dense breeding colonies (2011: 72 total nests, 2016: 86 total nests), where the probability of mistaking a neighboring nest for one's own may be high. Combined with our finding that the rate of egg mislaying, relative to the number of breeding females, was positively correlated with total available nests, this study indicates mislaying eggs in unattended nests and alternately incubating multiple clutches may be deleterious consequences of colonial breeding in species that tend to reuse nests from year to year. Investigations into the characteristics of females that mislay eggs (e.g., age, experience), the timing of egg mislaying relative to egg production (e.g., does the first or last egg have a higher probability of being mislaid?), and the specific mechanisms underlying higher rates of egg mislaying in large colonies could be fruitful areas of further research. In particular the relationship between the spatial density of nests and the probability of mislaying eggs remains unclear.

Ultimately, direct observational evidence of females laying eggs in unattended nests is needed to clarify many aspects of this behavior and verify that resident individuals were responsible for the abandoned eggs documented in this study. While egg laying is often difficult to observe in wild populations, the use of radio frequency identification technology to remotely track female visits to multiple nests by attaching transmitters to breeding females and setting up receivers around various areas of the barn could potentially shed light on these remaining questions. Nonetheless, this study provides new insight into the implications of colonial breeding for reproductive success, further challenging our understanding of the conditions under which group living may evolve. Overall, our findings suggest the increased probability of misallocating parental care and wasting valuable reproductive resources in large colonies may negatively impact the fitness of colonially breeding birds in ways that have been largely overlooked to date.

Acknowledgments.--We would like to thank 1. Levin for access to her 2015 breeding data, R. Safran for her support, and the site owners of the barns included in this study for generously allowing us to study the Barn Swallows nesting on their property. Special thanks to D. Ardia for teaching us how to construct and use the thermocouple eggs and R. Terrien for helping us process the temperature data. Funding for this project was provided by the National Science Foundation (NSF-GRFP grant to SPT, NSF-GRPF and NSF-DD1G lOS-1602400 grants to AKH, and NSF-REU award 1553835 to KM), as well as grants to MH from the Undergraduate Research Opportunities Program and the Verderber Undergraduate Research Fund at the University of Colorado al Boulder. Research was approved by the University of Colorado Institutional Animal Care and Use Committee (IACUC) protocol no. 2498, and banding was conducted in accordance with state regulations and US federal bird banding permits.


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SHEELA P. TURBEK (1,2), AMANDA K. HUND (2), KELLEY MCCAHILL, and MARA HERNANDEZ Department of Ecology and Evolutionary Biology, University of Colorado, Boulder 80309



Department of Biology, Truman State University, Kirksville, Missouri 63501

(1) Corresponding author:

(2) These authors contributed equally to the manuscript

Caption: FIG. 1.--The two adjacent nests that were alternately incubated by a single female Ham Swallow in Boulder County, Colorado, in 2016. Nest 1 contained three eggs and Nest 2 contained two eggs, not including ilie thermocouple eggs that were installed to track incubation behavior. The two alternately incubated Barn Swallow nests observed in 2011 were located in similar positions on the other side of the barn

Caption: FIG. 2.--The relationship between number of breeding females and total number of Barn Swallow nests al 43 sites monitored from 2011-2016 in Boulder County, Colorado. Several sites were monitored in multiple years. The number of breeding females was positively correlated with total number of nests (n = 111, b = 0.09, SE = 0.008, F= 120.25, df=67, P < 0.001). The points depict the raw data, the black line represents the model results, and the gray band represents the 95% confidence interval

Caption: FIG. 3.--The relationship between total Barn Swallow nests and number of mislaid eggs at 43 sites monitored from 2011-2016 in Boulder County, Colorado. The number of mislaid eggs was positively correlated with total nests when accounting for number of breeding females at each site in (A) one-egg clutches (GLMM with Poisson distribution: total nests, n = 111, A= 0.015, SE = 0.005, F= 9.65, P = 0.002), (B) two-egg clutches (GLMM with Poisson distribution: total nests, n = 111, b = 0.013, SE = 0.006, F= 5.5, P = 0.02), and (C) both one- and two-egg clutches combined (GLMM with Poisson distribution: total nests, n = 111, b= 0.016, SE = 0.005, F= 15.46, P < 0.001). We included number of breeding females as an offset term and site and year as random effects, as several sites were monitored in multiple years. The points depict the raw data and the lines represent the fitted values from the models

Caption: FIG. 4.--A representative trace plot from the 2016 observation of double incubation in Boulder County, Colorado, demonstrating that the female Barn Swallow alternated between the two nests throughout the incubation period. The incubation data presented here were collected on 23 June (d 13 of incubation)

Caption: FIG. 5.--Box and whisker plots of diurnal (A) average temperature, (B) on-bout length, and (C) offbout length during the final days of double incubation for the two Barn Swallow nests observed in Boulder County, Colorado, in 2016. The midline represents the median, the borders of the boxes depict the first and third quartiles, and the whiskers represent the range of the data. Points that lie outside 1.5 * interquartile range are plotted separately. Nest 1 contained three eggs, whereas Nest 2 contained two eggs. The three-egg nest was maintained at a warmer temperature (paired t-test: P = 0.04, n = 5 d), incubated for longer on-bouts (paired t-test: P = 0.08, n = 5 d), and left unattended for significantly shorter periods of time (paired t-test: P = 0.003, n = 5 d) than the nest with two eggs
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Author:Turbek, Sheela P.; Hund, Amanda K.; McCahill, Kelley; Hernandez, Mara; Hubbard, Joanna K.
Publication:The American Midland Naturalist
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2019
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