Skip to main content

Overlooked costs of coloniality: Mislaid eggs and the double Incubation of separate nests

Cite this dataset

Hund, Amanda (2022). Overlooked costs of coloniality: Mislaid eggs and the double Incubation of separate nests [Dataset]. Dryad.


The evolution of colonial breeding remains an outstanding question in evolutionary biology, in part due to our limited understanding of the costs and benefits of group living. We document 85 cases of Barn Swallows Hirundo rustica erythrogaster laying, and subsequently abandoning, eggs in empty, unclaimed nests located adjacent to active nests during a six-year study. The frequency of this behavior was positively correlated with total available nests, a metric that increases with colony size. In addition, two female swallows were observed alternately incubating multiple clutches after mislaying eggs in neighboring nests. We argue that the potential to mislay eggs and allocate parental care across separate nests may represent an overlooked cost of colonial nesting in birds.


This study was conducted at 43 breeding sites in Boulder County, Colorado, U.S.A. (4080601400N, 10581001600W) 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 6 31 nests (mean 6 1 SD) and 10 6 10 breeding females (mean 6 1 SD), with 3.5 6 1.72 available nests per breeding female (mean 6 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 (Møller, 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.

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 (4–6 June), 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 on- and 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 every min for 5 d (22–26 June, n 1⁄4 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.  

Usage notes

There are two types of data included in this repository. 1) Detailed temperature and behavior data from two nests incubated by the same female in 2016. This data comes from temperature readings taken from thermocouple eggs installed in both nests. 2) Summary data collected from longterm nest check data (2011-2016) from barn swallow colonies, quantifying the number of abandoned nests with 1 or 2 eggs and other data about the colony. 

See README for detailed description of each dataset. 


National Science Foundation, Undergraduate Research Opportunities Program, Award: IOS-1602400

Verderber Undergraduate Research Fund at the University of Colorado at Boulder, Award: 1553835