DOI: 10.5061/dryad.000000043
title: Resource allocation effects on the timing of reproduction in an avian habitat specialist

description: Variation in nutrient allocation can influence the timing of breeding and
 ultimately reproductive output. Time and space constraints might exist,
 however, if fewer food resources are available to meet the costs of
 reproduction early during the reproductive season. Here, for the first
 time, we test whether nutrient allocation strategies for reproduction in a
 shrub-dependent avian species differs with timing of breeding in different
 ecoregions: a high-elevation landscape, containing spatially complex
 vegetation (Rocky Mountains) versus a low-elevation, more homogenous
 landscape (Great Plains). We analyzed data collected from radio-telemetry
 and stable isotopes to assess the degree to which endogenous (body)
 reserves are used for reproduction and whether variation in allocation
 strategies was associated with time of year, ecoregion, habitat quality
 (including sagebrush type and plant greenness), or maternal
 characteristics. Using a Bayesian statistical framework, we found that
 females relied on a similar amount of endogenous reserves for reproduction
 in first nesting and renesting attempts. Additionally, endogenous
 contributions declined more rapidly throughout the nesting season in the
 Rocky Mountains than in the Great Plains. Individuals in high- and
 intermediate-elevation sagebrush types in the Rocky Mountains used similar
 amounts of endogenous reserves, whereas females nesting in low-elevation
 sagebrush used less. Females nesting at intermediate elevations, which
 experience the greatest flush of new green vegetation during the nesting
 season, switched their reliance from endogenous to exogenous sources for
 reproduction as green vegetation became available during spring. Our study
 highlights adaptations of a nutrient-allocation strategy across areas with
 varying levels of resources in time and space in a habitat specialist
 bird. Nutrient allocation by individuals residing in high-elevation areas
 favors a strategy that mainly uses nutrients gained from wintering
 habitats, whereas individuals residing in low-elevation areas mainly use
 exogenous sources for reproduction.
description: Data were collected across two sagebrush-dominated ecoregions:
 The Great Plains of eastern Montana, and the Rocky Mountains of southwest
 Montana, USA (Figure 1). The Great Plains site is located near the town of
 Roundup, which contains rolling topography ranging in elevation from 975m
 to 1250m. The dominant sagebrush species are Wyoming big sagebrush
 (<i>Artemisia tridentata</i> ssp.
 <i>wyomingensis</i>) and silver sagebrush (<i>A. cana
 </i>ssp.<i> cana</i>), with an understory of native
 perennial bunch grasses and forbs. Average annual precipitation and
 temperature is 359 mm and 9.2<sup>o</sup>C, respectively
 (National Climatic Data Center 2019). The Rocky Mountain site is located
 near the town of Lakeview in southwest Montana. Sagebrush in this
 ecoregion occurs across an elevation gradient of 2,000 m to 2,900 m.
 Mountain big sagebrush (<i>A. tridentata</i> ssp.<i>
 vaseyana</i>; hereafter; <i>MTSA</i>) occurs at the
 highest elevations where snowmelt and vegetation green-up occur latest
 during the spring. Three-tip sagebrush (<i>A. tripartita</i>;
 hereafter; <i>TTSA</i>) occurs on south-facing mid-elevation
 slopes where snowmelt and vegetation green-up occur earliest. Basin big
 sagebrush (<i>A. tridentata</i> ssp.<i>
 tridentata</i>; hereafter; <i>BBSA</i>) occurs in valley
 bottoms with deep soils where drifting of snow results in a later snowmelt
 date, but the earliest vegetation senescence. Average annual precipitation
 and temperature at Lakeview is 500mm and 1.6<sup>o</sup>C,
 respectively. We captured female
 sage-grouse using dipnets and spotlights. Upon capture, we collected blood
 samples from individual female sage-grouse by brachial venipuncture using
 a 23-gauge needle with 3 mL syringe. We collected blood samples from a
 subsample of individual female sage-grouse from the Great Plains site in
 both 2016 and 2017 (<i>n</i>=10 females each year), and from
 every captured individual at the Rocky Mountain site from 2014 to 2019
 (<i>n</i>=91). We centrifuged blood samples within 12 hours of
 capture and separated the plasma and red blood cells constituents into
 separate vials. Samples were stored frozen until we prepped samples for
 stable isotope analysis. On
 each study site, we tracked individual female grouse 2–3 times per week
 during the pre-nesting season (April to June) until we discovered a nest.
 Once we discovered a nest, we monitored nest status two times per week
 until the nest either hatched or failed. We considered a nest to be
 successful if ≥1 egg hatched as determined by the presence of any of the
 following in the nest bowl: a chick, an intact egg membrane, and/or
 eggshell cap. We counted membranes after a nest hatched and estimated when
 the first egg was laid (hereafter; <i>initiation date</i>) by
 assuming an egg laying interval of 1.5 days and a 28-day incubation period
 to determine the date (Moynahan et al. 2007).  For failed nests, we
 continued to track the female until mid-June to determine whether a second
 nest was initiated. We included only individuals who successfully hatched
 young, since our primary focus was on explaining variation in nutrient
 allocation to offspring production.
 If a nest was successful, we captured chicks when
 they were 2 to 8 days old (average = 3 d) and collected down feathers for
 stable isotope analysis. Downy feathers from the breast are present at the
 time of hatch and are predominantly keratin that reflect nutrient
 (protein) pools used by adult females during egg development (Williams
 2012). We captured chicks by hand in the early morning by flushing the
 brooding female off her dependent young. We stored feather samples from
 individual chicks in separate paper envelopes prior to stable isotope
 analysis.