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Our understanding of when natural populations are regulated during their annual cycle is limited, breeding season 6.1 for migratory species. This information is needed for parametrizing models that can inform management and conservation. Here, we use 14 years of data on colour-marked birds to investigate how conspecific density and habitat quality during the tropical non-breeding period interact to affect body condition and apparent annual survival of a long-distance migratory songbird, the American redstart Setophaga ruticilla.
Body condition and survival of birds in high-quality mangrove habitat declined as density increased. By contrast, body condition improved and survival did not vary as density increased in adjacent, lower quality scrub habitat, although mean condition and survival were almost always lower than in mangrove.
High rainfall enhanced body condition in scrub but not in mangrove, suggesting factors such as food availability outweighed consequences of crowding in lower quality habitat. Thus, survival of overwintering redstarts in mangrove habitat, disproportionately males, appears to be regulated by a crowding mechanism based on density-dependent resource competition.
Survival of individuals in scrub, mostly females, appears to be limited by density-independent environmental factors but not regulated by crowding.
The timing of birds' breeding seasons: a review of experiments that manipulated timing of breeding
The contrasting effects of density and food limitation on individuals overwintering in adjacent habitats illustrate the complexity of processes operating during the non-breeding period for migratory animals, and emphasize the need for long-term studies of animals in multiple habitats and throughout their annual cycles. Many factors limit animal populations [ 12 ], but the processes that regulate abundance are less understood [ 3 — 5 ].
Population regulation occurs when density-dependent mechanisms such as intraspecific competition for food or territory sites [ 67 ] or interspecific interactions with natural enemies [ 89 ] cause vital rates to covary negatively with population size. Research on population regulation has typically emphasized detecting density dependence. However, we have insufficient information about how and when during the annual cycle density affects demographic rates [ 71011 ]. This knowledge is needed for effective management and conservation of wildlife populations [ 12 ].
Resource competition associated with crowding is the basis of density-dependence as described by Lack [ 13 ] and is perhaps the most ubiquitous regulatory mechanism for breeding season 6.1 animal populations.
At high densities, resources become limited due to smaller territories, more time spent in costly interactions, or both. Individuals suffer reduced fecundity or survival as density increases [ 14 ].
The effects of crowding are probably strongest where individuals exist at high densities in relatively homogeneous environments [ 61516 ]. Empirical research on the strength and form of density dependence in birds has focused primarily on population studies during their breeding season. Much of this work has investigated the relationship between density and reproductive success [ 817 ]. Studies deed to examine density-dependent effects on adult or juvenile survival have typically been conducted with relatively sedentary, small or recovering populations [ 18 — 20 ].
Investigations of migratory birds have considered the effects of density-dependent annual survival in large-bodied species with relatively slow life histories, such as great cormorant Phalacrocorax carbo [ 21 ] and black-tailed godwit Limosa limosa [ 22 ]. In a recent breeding example with a small, long-distance migratory bird, the American redstart Setophaga ruticillayears of higher density were associated with fewer offspring fledged per female, a reduced mean population rate of fledging success and a lower relative contribution of extra-pair fertilizations to male fitness [ 23 ].
We know less about the effects of breeding season 6.1 processes during the non-breeding period. Theoretical work suggests that density dependence outside the breeding season should be widespread and important to avian population dynamics [ 2425 ] and is supported by some empirical evidence [ 2226 — 28 ]. The form and strength of density-dependent survival in the Eurasian spoonbill Platalea leucorodia leucorodia varied with age and season [ 29 ]. In this case, annual survival of subadult and adult birds declined with increasing population size, and density dependence occurred in early winter for subadults and late winter for adults.
What was the weather like in ?
However, no study of a long-distance migratory passerine, to our knowledge, has been deed to detect non-breeding period density dependence, or the mechanisms driving it. Many species of migratory songbirds are declining [ 30 ], emphasizing the need to understand when and how their populations are limited and regulated. Anthropogenic habitat destruction is the primary factor responsible for these declines [ 31 ], but density-dependent processes are central to population dynamics.
For example, if overwinter survival is regulated by conspecific density, loss of high-quality tropical habitats could shape the strength and form of density dependence [ 24 ]. Management of migratory bird populations requires population models that encompass the full annual cycle, parametrized with estimates of seasonal density dependence [ 32 — 34 ]. Here, we investigate how conspecific density breeding season 6.1 habitat quality during the tropical non-breeding period interact to affect body condition and apparent annual survival of a long-distance migratory songbird.
We used 14 years of data from our long-term study of American redstarts S. Both males and females hold and vigorously defend non-breeding territories [ 36 ]. In addition, territories are acquired and maintained via behavioural dominance, with older males typically securing sites in high-quality habitat and forcing females and many younger males to occupy sites in lower quality habitat [ 35 ].
How do we monitor the breeding season?
These considerations provided a unique opportunity to assess how density dependence varies breeding season 6.1 demographic groups of a migratory animal during the non-breeding period. If density dependence operates during this season, we predicted that individual physical condition would decline over winter, and that annual survival would decline following years of high redstart density, regardless of sex or habitat type. American redstarts are small 6—9 g entirely insectivorous songbirds that occupy non-breeding habitats which vary in quality primarily owing to differences in arthropod availability [ 3738 ].
Redstart habitat quality influences numerous measures of bird performance in both sexes. Individuals in high-quality habitats are typically in better physical condition [ 3940 ] and depart earlier on spring migration [ 3941 ] at the end of the non-breeding period than individuals in lower quality habitat.
Annual survival probability is also related to overwinter habitat quality [ 3642 ]. This site experiences strong seasonality in precipitation typical of many tropical regions. We studied redstarts in two habitats: mangrove forest and adjacent second-growth scrub.
Mangrove forest was dominated by black mangrove Avicennia germinans but also contained some white Laguncularia racemosa and red Rhizophora mangle mangrove. Mangrove stands typically were inundated by 0.
Little understory or ground-level vegetation was present except for mangrove pneumatophores. Black mangroves ranged in diameter at breast height dbh from 8 to 75 cm, were regularly dispersed at intervals of 10—15 m, and had breeding season 6.1 and contiguous canopies averaging about 8 m in height. These trees retained the majority of their leaves through the dry season, keeping this habitat relatively moist and shady throughout the time of redstart occupancy. Second growth scrub habitat contained shrubs and small trees ranging from 2 to 8 cm in dbh and 2—8 m in height, forming a dense understory and ground layer of vegetation.
Although this area is a nature preserve, cattle roamed freely in some years, and trees were often cut for charcoal and fence-posts, creating a mosaic of thickets, vine tangles and grassy patches. Scrub vegetation was dominated by logwood Haematoxylon campechianuma thorny small tree with a fluted trunk and many small leaves introduced into Jamaica.
Less frequent tree species included Bursera simarubaTerminalia latifolia and Crescentia alata. Unlike mangrove forest, trees and other vegetation in scrub habitat dropped most of their leaves facultatively during the spring dry season, especially in the driest years, and standing water was never present. Redstarts were studied for 14 years from mid-January through to mid-April, — and — on four 5 ha study plots, two in mangrove and two in scrub. Plots were separated by — m and were gridded at 25 m intervals to facilitate locating redstarts and mapping territories.
Redstarts were captured and recaptured in mist nets accompanied by vocalization broadcasts and a decoy. We counted the of redstarts of each age- and sex-class in each study plot by intensively mapping the activities of colour-marked and unmarked individuals on each study site over a one to four month period each year. Both males and females vocalize regularly during territory defence and exhibit stereotyped chase behaviours at territory boundaries, making them easy to detect and count.
Territory mapping consisted of recording redstart movements for an average duration of 10 min and occasionally up to 1 h per day. Within each season, each redstart was observed for several hours breeding season 6.1 identify territory boundaries. We converted mapping observations to densities by summing the total of territories per plot divided by its total monitored area. We built annual encounter histories by resighting marked birds that had occupied territories during the year.
When a bird was not resighted on the first visit, we revisited that territory a minimum of three times in a season, often using song playbacks, to confirm that an individual was not present. We also intensively searched habitat within m of each study plot to detect individuals that shifted territory locations. Because redstarts show strong fidelity to winter territories between years [ 36 ], our intensive search efforts produced reliable data on individual presence and absence. Effects of crowding were examined using body condition in relation to conspecific density.
This analysis was done with only 9 years,,and of data, because we did not intensively capture birds in all years. We analysed only birds captured in spring 15 March—15 April. Body condition at this time of year reflected environmental conditions across the non-breeding period [ 3638 ]. We estimated body condition as the standardized residuals from a regression of body mass on tarsus and wing length.
Body condition data were analysed using a linear mixed model with random intercepts fitted for each bird, because some individuals were sampled in more than 1 year. We assessed the ificance of each variable by iteratively removing it from the full model and comparing the reduced to the full model with a likelihood ratio test with 1 d.
Analyses were done with package lme4 [ 43 ] in program R [ 44 ]. Initial analyses suggested that a of birds in mangrove forest could strongly influence parameter estimates as indicated by their large residual values and high leverage. Rather than exclude these data points, we performed robust regression with iterated reweighted least squares to reduce their influence on parameter estimates.
We calculated the absolute breeding season 6.1 of standardized residuals and used Huber weights to down-weight data points of high influence. Analyses were done with the rlm function in package MASS [ 45 ]. We first modelled recapture probability p as a function of age, sex, habitat and study effort. Study effort was determined for each year based on whether fieldwork was a high effort four-month visit or a lower effort one-month visit. Fieldwork was not done inso we fixed the recapture rate to 0 for that year, resulting in a 2 year average survival estimate from to The recapture model with the greatest support was used to examine factors affecting annual survival.
A marking effect was included depending on whether birds were marked in their second year or after their second year. Time dependence was measured by allowing a separate survival estimate each year by including common and separate temporal patterns in each habitat. We considered models where density altered apparent survival equally in both habitats, differently between habitats, and in one habitat but not the other. Density estimates were unavailable inandand we used the overall mean density for survival analyses in those years. We ranked candidate models by second-order Akaike's information criterion AIC c differences and estimated the relative likelihood of each model with AIC c weights w i.
Models within two units of the top model were considered to have equal support, except in cases where they differed by only one parameter and the more parametrized model had a higher AIC c [ 48 ]. Apparent breeding season 6.1 probabilities for each age- and sex-class by habitat type were obtained by model averaging. Most models had annually variable survival owing to covariate effects or time-dependence. Therefore, model averaging generated an annual mean estimate and standard error for each group.