Methods
We captured female wild turkeys using rocket nets baited with corn from
January – March 2014–2019. We aged each individual using the presence
of barring on the 9th and 10thprimaries (Pelham and Dickson 1992). We fitted all females with a
uniquely identifiable aluminum rivet tarsal band and backpack style
GPS/VHF transmitter (Biotrack Limited, Wareham, Dorset, UK; Guthrie et
al. 2011). We programmed GPS units to collect data at 1-hour intervals
(Cohen et al. 2018) between 05:00 to 20:00 daily with one location at
night (23:59:58) to identify roosts until the battery died or the unit
was recovered. We immediately released individuals at the capture
location following processing. Capture, handling, and marking procedures
were approved by the Louisiana State University Agricultural Center
Animal Care and Use Committee (Permits A2015-07 and A2018-13). We
monitored live-dead status daily during the reproductive season using
handheld Yagi antennas and Biotracker receivers (Biotrack Ltd., Wareham,
Dorset, UK). We downloaded GPS locations once per week via a VHF/UHF
handheld command unit receiver (Biotrack Ltd., Wareham, Dorset, UK).
When winter flocks disband, social groups of wild turkeys alter space
use and focus efforts on reproduction (Badyaev et al. 1996b ,
Thogmartin 2001). We assumed that all females within a social group had
access to the same mates, and presumably the same dominant males.
Therefore, we defined a social group as a group of females captured
together during January to March as GPS data indicated that turkeys did
not disperse from wintering flocks before reproduction started, contrary
to suggestions in Badyaev et al. (1996a ). While we defined
females captured together as a social group, we acknowledge that we may
not have captured all of the females in the same breeding group. We
assumed, based on estimates of daily movements by females (Conley et al.
2016, Bakner et al. 2019), that individuals captured within 2 km of each
other were members of the same social group as these individuals
regularly interacted as detailed herein. To further ensure we accurately
defined social groups, we used a dynamic Brownian Bridge movement model
(dBBMM) to create 99% utilization distributions (UDs) for each
individual (Byrne et al. 2014) for the 21 days before the first female
in each group laid the first egg at an eventual nest site. We chose a
21-day period because we were interested in overlap in space use during
the time immediately preceding initiation of the first nest in the
social group, under the assumption that this individual was the dominant
female in the group (Watts and Stokes 1971). We calculated all UDs in
program R version 3.2.5 (R Core Development Team 2020) using package
move (Kranstauber and Smolla 2013). We used a window and margin size
equal to 21 and 9 respectively, and a location error of 10 m (Byrne et
al. 2014). Individuals that share space may constitute a single social
unit (Brown 1975), therefore we calculated the percentage of utilization
distributions that overlapped at least one other UD within a defined
social group during the 21-day period to quantify shared space use
(Kernohan et al. 2001). We assumed that any females who did not maintain
an overlapping range with at least one other female within a social
group or individuals within subgroups were of lower rank, and as such
should subsequently nest later (Ringgenberg et al. 2015). We defined
smaller groups that contained 2-3 individuals with ranges that
overlapped each other, but did not overlap with the main social group,
as a subgroup (Figure 2).
We determined locations of each nesting attempt for each female when an
individual’s locations became concentrated around a single point for
several days (Guthrie et al. 2011, Conley et al. 2015, Yeldell et al.
2017, Wood et al. 2019). We defined the first date of nest incubation as
the first day we recorded the nightly roost location at the nest site,
indicating the female continued incubation during the night (Bakner et
al. 2019). To determine the first date of egg laying (hereafter nest
initiation), we evaluated GPS locations to determine when a female
initially visited the nest site as female wild turkeys do not visit
their nest site until they lay their first egg (Conley et al. 2016,
Collier et al. 2019). We monitored each nesting attempt following Bakner
et al. (2019) and after nest termination, located nest sites using VHF
telemetry and GPS data to confirm the nest location and determine nest
fate. We considered a nest to have been depredated or abandoned if the
female left the nest ≤25 days into incubation, or if only intact eggs,
no eggs, or egg fragments were found at the nest bowl. We considered a
nest successful if ≥1 live poult hatched, and was confirmed visually
during subsequent brood surveys following methods outlined in
Chamberlain et al. (2020).
We scaled the initiation date of the first nest attempt to each social
group, where the date of the first nest initiation was noted as day 1.
We delineated subsequent nest attempts based on the number of days after
the first nest was initiated. We subtracted the initiation day of the
second nest from the initiation day of the first nest, and then
subtracted the initiation day of the third nest from the initiation day
of the second nest, and so on for each first nest attempt within each
social group. We then calculated mean number of days between each nest
initiation attempt within each social group. We speculated that social
groups with more individuals would have more days between subsequent
nest attempts compared to smaller groups. Presumably, larger groups
would contain more females competing to copulate with dominant males
(Orbach et al. 2015), whereas smaller groups would have less competition
and thus be able to copulate in a shorter temporal window, resulting in
a narrower time window during which nests were initiated by females in
that group (Dewsbury 1982, Foster 1983, Avery 1984, Trail 1985, Gratson
et al. 1991, Möller 1992).
Females that attempt reproduction earlier within a season are expected
to have greater annual reproductive success compared to later breeding
individuals (Lack 1968, Perrins 1970) and previous research has noted
that in lekking birds, dominant females breed first (Robel and Ballard
1974, Foster 1983). Dominant females can presumably select nest sites
that could confer fitness advantages through improved nest success
(Sӕther 1990, Martin 1995a , Martin 1995b ), compared to
subordinate females that nest later and may be forced to nest in
suboptimal parts of their ranges or travel farther distances to find
suitable nest sites. To test the prediction that dominant females would
travel shorter distances within their ranges prior to onset of nest
initiation, we used the distance between a female’s nest location and
the centroid of the UD range of the 21-day period before the first nest
of each social group was initiated as our metric. We measured the
distance between the centroid of each female’s 99% UD range to each of
her nest attempts in ArcGIS 10.6 (Environmental Systems Research
Institute, Inc., Redlands, California, USA; Figure 1). To locate the
centroids of each 99% UD, we calculated the x and ycentroid of each UD in the attribute table. We then created a line
between each nest attempt and the centroid and calculated the distance
between each nest attempt within the 99% UD. To test for differences in
mean distance traveled for females with successful versus failed nests,
we used an independent 2-group t -test with an α=0.05 in R (R Core
Team 2020). Likewise, we used a binomial generalized linear model (GLM)
in R (R Core Team 2020) to estimate nest success as a function of first
nest initiation date. We then used a Poisson GLM to estimate the rate
(in days) at which females left their social groups and initiated their
first and second nesting attempts as a function of group size and year.
Finally, we used linear regression to evaluate the effect of social
group size on the number of days between nesting attempts within social
groups.