DOI: 10.5061/dryad.crjdfn37f
title: Experimental evidence that nest orientation influences microclimate in a temperate grassland

description: Birds exhibit an assortment of behavioral strategies to cope with variable
 environmental conditions during reproduction, including altering nest
 construction behaviors. In species building enclosed domed nests, the
 microclimate within nests is influenced not only by its structure and the
 surrounding vegetation but also by the orientation of the nest opening.
 Many grassland-dependent birds build dome-shaped nests with clear
 directionality of openings. We studied two species in northeastern Kansas,
 United States that typically orient their nests east to northeast in this
 region. However, in a drought year, both Grasshopper Sparrows (Ammodramus
 savannarum) and Eastern Meadowlarks (Sturnella manga) shifted orientations
 of their nests southward toward prevailing winds. We hypothesized that
 this shift reduced the deleterious effects of heat stress on parents and
 developing young by diminishing morning solar radiation and increasing
 cooling due to the prevailing southerly winds. To test this hypothesis, we
 measured temperature, humidity, and wind speed at pairs of unoccupied,
 field-collected sparrow and meadowlark nests, experimentally placed to
 face south or east (control) in a non-drought year. Nest orientation
 affected the daily microclimate patterns, with south-facing nests warming
 later in the day relative to east-facing nests. The temperature
 differences depended upon humidity, with south-facing nests being
 relatively cooler under more humid conditions. This work provides the
 first experimental evidence of the benefits of plasticity in nest
 construction under challenging thermoregulatory conditions and shows how
 ground-nesting birds may reduce thermoregulatory demands during incubation
 under climate variation.
description: We conducted the experiment at the Konza Prairie Biological
 Station (KPBS), a 3487-ha native tallgrass prairie located 15 km south of
 Manhattan, Kansas (Knapp et al. 1998). The KBPS is managed experimentally
 as part of the Long-Term Ecological Research program in units subject to
 multiple combinations of prescribed fire and grazing. The site has a
 mid-continental climate with mean daily temperatures in recent years of
 ~13 °C (Macpherson et al. 2008) and the mean maximum daily temperatures
 during July range from 29.4 °–35.6 °C (Craine et al. 2012). At KPBS, the
 sun rises 58.63 degrees from the north and sets 301.36 degrees from the
 north during peak summer, defined in this study as June–August (NOAA Solar
 Position Calculator, https://gml.noaa.gov/grad/solcalc/). Prevailing winds blow from south to north (Lysenko et al. 1994). There is high interannual variability in summer (May–September) precipitation with a 30-yr rainfall average of 65.2 cm (CV of 29.8%; Knapp et al. 2015). During May–July 2019, a total of 52.9 cm of precipitation fell at KPBS; in 2018, a drought year, 20.5 cm of precipitation fell over the same time period (Nippert 2022). Grasshopper Sparrows nest in grazed native prairies of moderate height, little-to-no wooded areas, and patchy bare ground mixed with clumps of vegetation (Shaffer et al. 2021). Eastern Meadowlarks build nests away from habitat edges and select somewhat taller, denser vegetation than Grasshopper Sparrows (Hubbard et al. 2006). Both species construct enclosed, domed nests from dead grasses on the ground (Roseberry and Klimstra 1970; Slater 2004), often placing nests beneath clumps of vegetation (i.e., <em>Amorpha canescens</em>, <em>Baptisia australis</em>, <em>Psoralidium tenuiflorum</em>, and <em>Andropogon gerardii</em>). In this experiment, we manipulated naturally constructed nests, as opposed to artificial nests, to approximate realistic conditions to the extent possible. Since neither species reuses nests, we carefully collected completed (i.e., fledged or failed) nests in 2017 and 2019 from both species. We dug and cut nests out of the surrounding vegetation using scissors and trowels, placing individual nests in small cardboard boxes, which we stored in plastic tubs until experimental initiation. We paired nests by mass, a subjective measure of light penetration, and general appearance to minimize differences in microclimate due to slight differences in nest construction. We conducted the experiment from 12 June–3 August 2019, spanning the hottest and driest parts of the breeding season and capturing times when the thermoregulatory costs of overheating presumably peak at our site. We compared nest microclimate in eight experimental trials; each trial lasted four–six days. We compared microclimate in four–six nest pairs during each trial for a total of 18 Eastern Meadowlark and 16 Grasshopper Sparrow comparisons (n = 34). Distributing trials throughout the season allowed us to detect consequences of nest orientation associated with specific weather conditions and to explore the magnitude of microclimatic differences between differently oriented nests temporally. We performed the experiment on a single, flat region of our study area to control for potential microclimate and habitat differences within the study site (Fig. 1). We placed 34 pairs of nests in suitable habitat along parallel ~50-m transects, spaced 10 m apart using a handheld GPS unit (GPSmap 60CSx; Garmin, Olathe, Kansas). At the start of each transect or 10-m mark, we selected and flagged the nearest plant species commonly used by these species under which to place nests. If there were no plants of those species within a meter of the point, we continued to a new point another 10 m northward. Under the selected plant, we parted the grass and fastened two similar conspecific nests to the ground with a nail, with the backs of the nests no more than six cm apart. Within each pair, the two nest cups faced a different direction based on the most common orientation from drought and non-drought years: east (90°; Grasshopper Sparrow) or northeast (45°; Eastern Meadowlarks) to represent typical years, and south (180°; both species) to represent the drought year orientation (Smith et al. 2023). We assessed the consequences of orientation on microclimatic variation by measuring the temperature, humidity, and wind speed at each nest. We attached a small datalogger (DS1923-F5# Hygrochron; iButton®) using Blu-Tack Reusable Adhesive (Bostic) to the underside of small plastic pizza savers and embedded the legs securely in the middle of each nest cup, suspending the dataloggers a couple of centimeters above the ground to measure conditions a nestling or parent would experience inside the nest. The shade from the pizza savers was unlikely to influence the recorded microclimate because the inside of these enclosed nests was already shaded from above. We programmed the dataloggers to record temperature (°C) and relative humidity (%RH) in 10-minute intervals throughout the day and night. We calculated mean hourly temperature and humidity values by averaging the six measurements per hour. We recorded wind speed (m/s) once per day, taking measurements during the hottest part of the day, between 10:30 and 13:00, using a handheld anemometer (Benetech, GM8908) at two locations relative to the nest: (1) immediately in front (at the height) of the nest entrance and facing the direction of the opening, parallel to the ground and perpendicular to the opening, and (2) a single set of measurements at 1 m above the nest pairs. We averaged eight measurements for each set of wind measurements, recorded every 10 seconds, to minimize the effects of gusts on mean wind speed comparisons. Our response variables represented the differences between paired nests in temperature, humidity, and wind speed, calculated by subtracting the values of south-facing nests from their east-facing counterparts. We presented the difference values as opposed to the raw microclimatic variables to account for variation in the raw data unrelated to our experimental treatment. Thus, response variables reflect how birds mitigated expected costs that could occur if birds adopted standard orientations under drought conditions. Negative values indicated that south-facing nests were warmer, more humid, or windier than east-facing nests, whereas positive values indicated that east-facing nests were warmer, more humid, or windier than south-facing nests. We used an information theoretical approach to evaluate four sets of models explaining variation in the following response variables: differences between nest pairs in (1) temperature (°C) and (2) humidity (% relative humidity; %RH), both measured from 09:00–17:00 which we anticipated would encompass peak daytime temperatures relevant to bird thermoregulation. To evaluate how humidity might modulate temperature differences, we again modeled (3) temperature differences, this time including raw humidity values measured in the south-facing nest as a covariate. We modeled differences in (4) wind speed, measured directly in front of nest cups. Because we only collected a single, mid-day set of wind measurements, we calculated mean differences in temperature and humidity between 11:00 and 13:00 to match the temporal resolution of the wind speed in analysis 4. Finally, we evaluated the consequence of wind speed (measured at 1 m above the nest pair) on temperature differences (5a) and humidity differences (5b) in the absence of temporal variables.
description: # Experimental evidence that nest orientation influences microclimate in a
 temperate grassland We
 present the south- and east- facing nest temperature (degrees Celsius),
 humidity (% relative humidity), and wind speed (m/s) for experimentally
 placed Grasshopper sparrow and Eastern meadowlark nests (n= 34). These
 data also include the difference between east- and south- facing nests for
 each of these variables (east - south). ## Description of the data and
 file structure ##### AllTempHumid - all temperature and humidity values
 over a 24 hour period during the experiment (47 days during summer 2019)
 Variable definitions: * hour: The hour of the day when the measurement was
 taken. We extracted this information without the minutes to reduce noise.
 Measurements were taken every ten minutes; in the models, we used average
 hourly values for each day. * Nest_ID: Identification of specific nest
 pairs. The number at the beginning denotes the placement order of the nest
 pairs. The four letters at the end indicate the species that constructed
 those nests. Each unique pair of south- and east-facing nests has a unique
 Nest_ID. This variable was used as a random effect in all models. *
 Species: The alpha code for the species that constructed the nest pair.
 EAME = Eastern Meadowlark; GRSP = Grasshopper Sparrow. We included this
 variable as a fixed effect in all models to account for differences among
 nest pairs due to the species. * Date: The date the measurement was taken
 in month/day/year format. * Time: The time the measurement was taken in
 hour:minute format. * Temp_E: Temperature of east-facing nests in degrees
 celsius. East-facing nests are defined as domed nests whose entrances we
 placed facing due east. There is one measurement every ten minutes. *
 Temp_S: Temperature of south-facing nests in degrees celsius. South-facing
 nests are defined as domed nests whose entrances we placed facing due
 south. There is one measurement every ten minutes. * TempDiff: South -
 East temperature difference in degrees celsius within a nest pair. This
 column served as our response variable in all temperature models that used
 this dataset. * humid_E: Humidity of east-facing nests with units %
 relative humidity. East-facing nests are defined as domed nests whose
 entrances we placed facing due east. There is one measurement every ten
 minutes. * humid_S: Humidity of south-facing nests with units % relative
 humidity. South-facing nests are defined as domed nests whose entrances we
 placed facing due south. There is one measurement every ten minutes. *
 HumidDiff: South - East humidity difference with units % relative humidity
 within a nest pair. This column served as our response variable in all
 humidity models that used this dataset. * ordinal: The day of the year
 expressed numerically, often expressed as ordinal or Julian date. We used
 this variable as a fixed effect in the model selection process. #####
 Daily- one daily measurement of temperature, humidity, and wind speed
 Variable definitions: * Nest_ID: Identification of specific nest pairs.
 The number at the beginning denotes the placement order of the nest pairs.
 The four letters at the end indicate the species that constructed those
 nests. Each unique pair of south- and east-facing nests has a unique
 Nest_ID. This variable was used as a random effect in all models. * Date:
 The date the measurement was taken in month/day/year format. * Species:
 The species that constructed the nests in each nest pair. Potential values
 include Grasshopper Sparrow and Eastern Meadowlark. * temp_E: Temperature
 of east-facing nests in degrees celsius. East-facing nests are defined as
 domed nests whose entrances we placed facing due east. There is one
 measurement every day. * temp_S: Temperature of south-facing nests in
 degrees celsius. South-facing nests are defined as domed nests whose
 entrances we placed facing due south. There is one measurement every day.
 * TempDiff: South - East temperature difference in degrees celsius within
 a nest pair. This column served as our response variable in all
 temperature models that used this dataset. * humid_E: Humidity of
 east-facing nests with units % relative humidity. East-facing nests are
 defined as domed nests whose entrances we placed facing due east. There is
 one measurement every day. * humid_S: Humidity of south-facing nests with
 units % relative humidity. South-facing nests are defined as domed nests
 whose entrances we placed facing due south. There is one measurement every
 day. * HumidDiff: South - East humidity difference with units % relative
 humidity within a nest pair. This column served as our response variable
 in all humidity models that used this dataset. * speed_E_Entrance: Wind
 speed at entrance to east-facing nests in meters per second (m/s). We took
 eight of these measurements once per day and averaged them prior to
 construction of this dataset to obtain one daily measurement of entrance
 wind speed. Measurements were taken with a handheld anemometer held
 parallel with the ground and perpendicular to the entrance of the nest,
 directly in front of the entrance without touching the nest with the
 anemometer. * speed_S_Entrance: Wind speed at entrance to south-facing
 nests in meters per second (m/s). We took eight of these measurements once
 per day and averaged them prior to construction of this dataset to obtain
 one daily measurement of entrance wind speed. Measurements were taken with
 a handheld anemometer held parallel with the ground and perpendicular to
 the entrance of the nest, directly in front of the entrance without
 touching the nest with the anemometer. * winddiff: South - East entrance
 wind speed difference in meters per second (m/s) within a nest pair. This
 column served as our response variable in all wind models that used this
 dataset. * speed_Above: Wind speed (m/s) one meter above the nest pair. We
 held the handheld anemometer above each nest pair for at least two
 minutes, a period during which we took eight measurements that we averaged
 prior to the construction of this dataset. * ordinal: The day of the year
 expressed numerically, often expressed as ordinal or Julian date. We used
 this variable as a fixed effect in the model selection process. #####
 Daytime - hourly temperature and humidity values from 09:00 to 17:00.
 Variable definitions: * hour: The hour of the day when the measurement was
 taken. We extracted this information without the minutes to reduce noise.
 Measurements were taken every ten minutes; in the models, we used average
 hourly values for each day. * Nest_ID: Identification of specific nest
 pairs. The number at the beginning denotes the placement order of the nest
 pairs. The four letters at the end indicate the species that constructed
 those nests. Each unique pair of south- and east-facing nests has a unique
 Nest_ID. This variable was used as a random effect in all models. *
 Species: The species that constructed the nests in each nest pair.
 Potential values include Grasshopper Sparrow and Eastern Meadowlark. *
 Date: The date the measurement was taken in month/day/year format. * Time:
 The time the measurement was taken in hour:minute format. In this dataset,
 it ranges from 09:00 to 17:00 * temp_E: Temperature of east-facing nests
 in degrees celsius. East-facing nests are defined as domed nests whose
 entrances we placed facing due east. There is one measurement every hour
 from 09:00 to 17:00. * temp_S: Temperature of south-facing nests in
 degrees celsius. South-facing nests are defined as domed nests whose
 entrances we placed facing due south. There is one measurement every hour
 from 09:00 to 17:00. * TempDiff: South - East temperature difference in
 degrees celsius within a nest pair. This column served as our response
 variable in all temperature models that used this dataset. * humid_E:
 Humidity of east-facing nests with units % relative humidity. East-facing
 nests are defined as domed nests whose entrances we placed facing due
 east. There is one measurement every hour from 09:00 to 17:00. * humid_S:
 Humidity of south-facing nests with units % relative humidity.
 South-facing nests are defined as domed nests whose entrances we placed
 facing due south. There is one measurement every hour from 09:00 to 17:00.
 * HumidDiff: South - East humidity difference with units % relative
 humidity within a nest pair. This column served as our response variable
 in all humidity models that used this dataset. * ordinal: The day of the
 year expressed numerically, often expressed as ordinal or Julian date. We
 used this variable as a fixed effect in the model selection process. ##
 Code/Software We provide the data analysis in R, in which we used the
 package lme4 to run the analyses. We include all necessary packages in the
 code.