DOI: 10.5061/dryad.8sf7m0cr5
title: Overlooked costs of coloniality: Mislaid eggs and the double Incubation of separate nests

description: 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.
description: 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. 
 DOUBLE INCUBATION
 OBSERVATION (2011)<br>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.
 DOUBLE
 INCUBATION OBSERVATION (2016)<br>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.  
description: 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.