Survival of juvenile Florida Scrub-Jays is positively correlated with month and negatively correlated with male breeder death

Juvenile survival in birds is difficult to estimate, but this vital rate can be an important consideration for management decisions. We estimated juvenile survival of cooperatively breeding Florida Scrub-Jays (Aphelocoma coerulescens) in a landscape degraded by fire suppression and fragmentation using data from marked (n = 325) and unmarked juveniles (n = 1,306) with an integrated hierarchical Bayesian model. To assess the combined analyses, we also analyzed these datasets separately, with a Cormack–Jolly–Seber model (marked) and a young model (unmarked). Our data consisted of monthly censuses of territorial family groups from Florida Scrub-Jay populations in East Central Florida collected over a 22-year period. Juvenile survival was estimated from July when young Florida Scrub-Jays begin developing independence to March when they become first-year individuals and grouped according to the habitat quality class of their natal territory that were based on shrub height (with intermediate shrub heights being optimal and short and tall shrub heights being suboptimal) and the presence of sandy openings (the preferred open having many sandy openings; closed not having enough). Parameter estimates in the combined analysis were intermediate to the separate analyses. Notable differences among the separate analyses were that suboptimal habitat survival was lower in the unmarked analysis, the unmarked analysis showed a linear effect of time not seen in the marked analysis, and there was an effect of male breeder death in the marked but not unmarked analysis. The combined data analysis provided more inference than did either dataset analyzed separately including juveniles in optimal-closed territories unexpectedly had higher survival than those in optimal-open, survival increased through time, and male breeder death had a negative effect on survival. This study suggests that optimal-closed habitat may play an important role in juvenile Florida Scrub-Jay survival perhaps by providing better cover from predators and warrants further investigation for management implications.

Methods
Habitat quality within territories was classified annually by overlaying territory maps on digital orthorectified aerial photography. Visual classification specified textures that corresponded to vegetation height (Breininger and Carter 2003); we binned territories into the defined habitat classifications. Sociobiological covariates for each territory were quantified from annual demographic data. Helper count was taken in March (at beginning of breeding season), sibling count and cohort count were taken in July (at beginning of capture history). Male death and female death that occurred between March and the following July was coded as binary (1 = death during year). Two types of survival data were utilized. In the first type (hereafter the marked dataset), all nestlings were banded 11 days after hatching using a numbered metal leg band and a unique combination of color bands allowing identification of individuals. The marked dataset was collected from 1988 to 2009 as part of a long-term demographic monitoring effort of the scrub-jay population  occurring on Merritt Island National Wildlife Refuge/John F. Kennedy Space Center (MINWR/KSC). During this period, the populations under study were intensively monitored, and we attempted to locate all nesting attempts before the penultimate egg was laid, and thus clutch size and age of nestlings were known (Carter et al. 2011). Nests were visited weekly to determine status and count eggs or nestlings until failure or fledging. Thereafter, we conducted a monthly census within each territory to record observations of banded birds. In the second data type (hereafter the unmarked dataset), young were not banded in the nest, but a monthly census like that for the marked dataset was conducted within each territory to observe the total number of unmarked young alive with the family group. The unmarked dataset was collected from 1988 to 2014 as part of a larger and less intensive monitoring effort (i.e. no nest monitoring) of the scrub-jay metapopulation in East Central Florida (Breininger et al. 1995) and on MINWR/KSC (Figure 1).

鸟类幼鸟存活率的估算颇具挑战性，但这一关键生命率对于野生动物管理决策而言至关重要。本研究针对受火灾抑制与生境破碎化影响的退化景观，利用标记（n=325）与未标记幼鸟（n=1306）的监测数据，通过集成分层贝叶斯模型（integrated hierarchical Bayesian model），估算了合作繁殖的佛罗里达灌丛鸦（Aphelocoma coerulescens）的幼鸟存活率。为评估联合分析的有效性，我们还分别采用Cormack-Jolly-Seber模型（Cormack–Jolly–Seber model，适配标记数据集）与幼鸟专属模型（young model，适配未标记数据集）对两类数据进行了单独分析。

本研究的数据来源于佛罗里达州中部东部地区灌丛鸦种群的22年长期监测，内容为领地家族群的月度普查。幼鸟存活率的估算时段为7月（佛罗里达灌丛鸦幼鸟开始独立发育的时期）至次年3月（幼鸟成长为1龄个体的时期），并根据出生地领域的生境质量等级进行分组：生境质量等级基于灌丛高度（中等灌丛高度为最优生境，过矮与过高灌丛均为次优生境）与沙地开阔地存在情况（偏好生境拥有大量沙地开阔地，封闭生境则缺乏此类区域）划分。

联合分析得到的参数估计值介于两项单独分析之间。不同单独分析间存在显著差异：未标记数据集的分析结果显示次优生境的存活率更低；未标记分析呈现出时间的线性效应，而标记数据集分析未发现该效应；标记数据集分析中存在雄性繁殖者死亡的影响，未标记分析则无此效应。联合数据分析相较于单一数据集单独分析，能够提供更丰富的推断结果，其中包括一项意外发现：最优封闭生境中的幼鸟存活率高于最优开阔生境；存活率随时间推移呈上升趋势；雄性繁殖者死亡对幼鸟存活率具有负面影响。本研究表明，最优封闭生境可能通过为幼鸟提供更优的天敌躲避条件，对佛罗里达灌丛鸦的幼鸟存活率发挥重要作用，其管理启示值得进一步探究。

方法
领地内的生境质量每年通过将领地地图叠加至数字正射校正航空影像进行分类。视觉分类依据对应植被高度的纹理特征（Breininger与Carter, 2003），将领地划分为预设的生境类别。每个领地的社会生物学协变量从年度种群动态数据中量化得出：繁殖季初期（3月）统计帮手数量，捕获历史起始时段（7月）统计同胞数量与同期幼鸟群数量。将3月至次年7月间发生的雄性繁殖者死亡与雌性繁殖者死亡编码为二元变量（1=年内发生死亡）。

本研究采用两类存活数据：第一类（下称标记数据集）：所有雏鸟于破壳11日后佩戴编号金属脚环与独特彩色脚环组合，以实现个体识别。标记数据集采集于1988年至2009年，是梅里特岛国家野生动物保护区/约翰·F·肯尼迪航天中心（MINWR/KSC）灌丛鸦种群长期种群动态监测项目的一部分。此期间研究种群受到高强度监测：我们尝试在倒数第二枚卵产下前定位所有筑巢巢址，因此可获知窝卵数与雏鸟日龄（Carter et al., 2011）。每周对巢穴进行探访以确认巢况、统计卵或雏鸟数量，直至巢穴失败或雏鸟离巢。此后，每月对每个领地进行普查，记录带环鸟类的观测情况。

第二类（下称未标记数据集）：幼鸟在巢中未佩戴脚环，但采用与标记数据集一致的月度普查方式，记录每个领地家族群中存活的未标记幼鸟总数量。未标记数据集采集于1988年至2014年，是覆盖佛罗里达州中部东部地区灌丛鸦复合种群（Breininger et al., 1995）以及MINWR/KSC区域的低强度大规模监测项目（无需开展巢址监测）的一部分，对应图1。