The concept of habitat quality is fundamental to the study of ecology, and ecologists have long recognized the importance of vegetation structure and composition in the assessment of wildlife habitat. Vegetative characteristics affect productivity in birds for a variety of reasons (e.g., predator assemblages, nesting sites, song perches, food availability). I investigated the relationship between habitat quality and prey availability and the effect these parameters have on reproductive success in golden-cheeked warblers (Dendroica chrysoparia). My objectives were to 1) determine any differences in pairing and fledging success of warbler territories within two ecosites exhibiting two distinctive tree species composition 2) explore the relationship between tree species composition, arthropod density, and foraging effort, and the effect these parameters have on reproductive success in golden-cheeked warblers and 3) investigate the connection between preferred foraging substrates and changes in arthropod abundance within golden-cheeked warbler territories throughout the breeding season. I mapped individual warbler territories and searched for fledglings, observed foraging behavior, and collected arthropods in each territory to determine productivity, foraging effort, and food availability. I conducted these methods over two seasons in juniper-oak woodlands on Fort Hood, north-central Texas within 347 territories of two vegetative types: those marked by the predominance of post oak (Quercus stellata) and those marked by the predominance of Texas oak (Quercus buckleyi). Pairing and fledging success of territories differed substantially between the two vegetative types. Movement rates differed considerably between the two vegetative types, indicating a difference in prey encounter rate. Foraging data indicated a clear switch in preferred foraging substrates from oak species early in the breeding season, to Ashe juniper (Juniperus ashei) in mid-May. Arthropod sampling revealed a correlation between preferred foraging substrates and arthropod density. Results suggested that Texas oak was an important foraging substrate for golden-cheeked warblers, and territories that lack this tree species generally did not succeed in fledging young. My study can be used to indicate areas that should be targeted for conservation by local, state, and federal government because they provide high quality habitat based on warbler productivity.

Prescribed fire is a common tool used to restore native diversity, control invasive species, and reduce fuel loads. However, fire alone can be insufficient to restore pre-settlement vegetation; other factors such as differences in native and invasive species pools, deer herbivory, seed availability, and the spatial pattern of the fire can influence vegetation trajectories and restoration outcomes. Central Texas is a mosaic of savanna and mixed woodlands co-dominated by Quercus buckleyi (Texas red oak) and Juniperus ashei (Ashe juniper). In a savanna, I studied the joint effects of initial species composition (native-dominated or invasive-dominated) and disturbance (high-intensity fire, clipping, or control) on the ability of native species to establish, survive, and resist invasion by Bothriochloa ischaemum, an invasive C4 grass (ch. 1). Native savanna patches were resistant to invasion following high-intensity fire; fire can be used to selectively control B. ischaemum. In central Texas savanna and woodlands, under fire suppression and overabundant white-tailed deer, Quercus spp. are failing to regenerate, while J. ashei is increasing in abundance. To better understand vegetation trajectories following J. ashei removal in savanna, I studied the soil seedbank along a chronosequence of J. ashei invasion (ch. 5). In woodland, I studied the joint effects of prescribed fire and deer (ch. 2), clearing of J. ashei followed by high-intensity slash-pile burns (ch. 3), and wildfires (ch. 4) on the abundance and size of J. ashei and of hardwoods. Hardwoods resprouted vigorously after fire; J. ashei individuals of all sizes were killed by fire and slow to re-colonize. These management interventions failed to increase Q. buckleyi seedling abundance. It is likely that deer control is necessary to allow fire to have positive effects on the regeneration of oaks in this region, and wherever deer are over-abundant. However, deer can indirectly benefit hardwoods by reducing competition with palatable forbs (ch. 3). In general, these results show that fire suppression in central Texas oak-dominated woodlands is causing a shift not to more mesic-adapted species, as observed in the eastern US, but to J. ashei, which is at least as xeric-adapted as oak, a process I termed 'juniperization'. / text

Song repertoire structure, organization, and use were studied in 68 male Canada Warblers (Cardellina canadensis) in a breeding population in New Hampshire in 2010-2011. On average, males had complex repertoires of 12 phrases and 55 variants. Repertoire sharing was negatively related to distance between territories, and positively related to longer territory tenure, evidence that males learn songs from neighbours. Males used two singing modes: (I) slow, regular delivery of less variable songs, and (II) fast, intermittent delivery of more variable songs interspersed with chips. Males used Mode I when unpaired and when near females, and Mode II at dawn and during territory disputes, a pattern similar to other warbler species with two song categories. Detectability (whether a male sang) differed little between 1-, 3-, 5-, and 10-min count intervals. Song output and detectability were highest at dawn and in unpaired males, and lowest in paired males late in the season.

Phenotypic traits that vary geographically within species are commonly assumed to represent local adaptations to different environments. In order for local adaptation to evolve by natural selection, three conditions must be met: (1) traits must vary geographically, (2) local variants of traits must provide a fitness advantage (increased survival or reproductive success) within the local environment, and (3) local variants of traits must be heritable. In chapter two, we review evidence for local adaptation in birds. Geographic variation among populations is nearly ubiquitous, yet experimental tests of the fitness advantages of local trait variants are rare among populations of birds, presumably because of the difficulties in transporting individuals between populations. Thirty-seven studies have tested the heritability of among population variation in traits. Thirty-three of the 37 studies found some degree of heritability of variation among populations, consistent with traits diverging in response to natural selection. In chapter three, we experimentally test the fitness consequences of divergent nest morphologies of Yellow Warblers (Dendroica petechia) using reciprocal nest transplant experiments between a temperate and subarctic site in Canada. Yellow Warblers breeding at our subarctic site build larger nests constructed with more insulative materials than Yellow Warblers breeding at our temperate site, and these differences are the result of different nest building behaviours. Temperate nests transplanted to subarctic sites experienced significantly colder temperatures, and tended to suffer higher egg and nestling mortality due to climatic conditions (cold temperatures), than locally transplanted subarctic nests. Adult females breeding in subarctic sites that received temperate nests changed their incubation behaviours by taking shorter recesses than females who received locally transplanted subarctic nests. In contrast, subarctic nests transplanted to our temperate site showed no changes in nest temperature, fledgling success, or parental behaviour during incubation. We suggest that divergent selective pressures acting on Yellow Warblers in subarctic and temperate environments results in different nest building behaviours. Cold temperatures in our subarctic site likely favour increased investment in larger, insulative nests, whereas warmer temperatures at our temperate sites likely favour reduced investment in nest building, and consequently smaller nests. / Thesis (Master, Biology) -- Queen's University, 2010-09-28 13:16:38.686

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The proximate causes triggering nest abandonment are unclear for most species, including the Yellow Warbler (Dendroica petechia), which abandons nests parasitized by cowbirds (via burial or desertion). Cowbird parasitism and rejection of parasitism are costly to some hosts; therefore cues affecting their responses have important evolutionary implications. Manipulative experiments showed that experimentally adding a cowbird egg elicited similar rejection frequencies (2008: 31.8%; 2009: 26.1%) as naturally laid eggs (2008: 27.1%; 2009: 20.0%). In 2008, interaction with an egg-removing model increased the probability of abandonment and the most aggressive individuals were more likely to bury the model cowbird egg. In 2009, eggs added to nests before sunrise were rejected at a frequency (29.7%) similar to eggs added to nests after sunrise (22.9%). Warblers returning to nests after egg addition peered significantly longer at their clutch than at control nests, shuffled their bodies more frequently when on the eggs and spent more time probing eggs with their bill once settled on their parasitized clutch. Furthermore, although non-mimetic blue eggs were not abandoned significantly more frequently than cowbird eggs (blue 31.1% versus cowbird 21.4%), only blue eggs were ejected from some nests. Thus, warblers use both tactile and visual cues to detect the presence of a parasitic egg in their nest. Eggs added to nests were not rejected at a lower frequency than naturally parasitized nests, as was recorded in a previous study. It is difficult to know whether this increase in abandonment of experimental eggs is due to phenotypic plasticity, genetic changes, or other factors. Egg recognition abilities may have changed because I have shown that the warblers’ behaviour changes before versus after egg addition, whereas no changes were recorded in an earlier study. Finally, not all individuals that buried eggs for the first time in 2009 (21.4%) buried again after being re-parasitized (5.3%), when less time remained in the breeding season relative to the first parasitism event. This suggests that egg rejection and host responsiveness in warblers, and likely other avian hosts that use abandonment as a form of rejection, is affected by environmental cues which may act as genetic expressers.

Obligate brood parasites rely solely on other species, the hosts, to incubate their eggs and raise their offspring, which often reduces the host’s reproductive output. This reproductive cost has led to the evolution of anti-parasite adaptations among hosts, which in turn, has led to better trickery by parasites, a process termed an evolutionary arms race. The objective of this thesis was to investigate host-parasite coevolutionary arms races to address questions of host-use diversity. Host diversity varies dramatically among brood-parasitic species, but reasons for variations in host-use among brood parasites are not well understood. In Chapter 2, I address questions on host diversity specifically, whereas I address questions about coevolutionary interaction between hosts and parasites in Chapters 3, 4 and 5 using two host-parasite systems, one in New Zealand and one in North America. Chapter 2 investigates if host diversity is constrained by aggressive nest defence behaviour. I compared the nest defence behaviour of the exclusive host of the shining cuckoo Chrysococcyx lucidus lucidus on the main islands of New Zealand, the grey warbler Gerygone igata, to two other potentially suitable hosts that are not currently parasitised, the fantail Rhipidura fuliginosa and the silvereye Zosterops lateralis. The results suggest that grey warblers are as aggressive as fantails and silvereyes towards shining cuckoos at the nest and thus, host specialisation in shining cuckoos in New Zealand, at least, does not appear to be the result of nest-defence constraints imposed by potential but unused host species. Chapter 3 investigates if red-winged blackbirds Agelaius phoeniceus, a species that typically accepts the eggs of parasites, recognises, as indicated by changes in incubation behaviour, when they have been parasitised by brown-headed cowbirds Molothrus ater. Recognition without rejection suggests that rejection may be context-dependent but the results suggest that red-winged blackbirds do not recognise when their nests have been parasitised by brown-headed cowbirds, at least at the egg stage. This study was the first to investigate if hosts that almost invariably accept the eggs of parasites recognise when they have been parasitised. Chapter 4 investigated the possibility of coevolutionary arms races occurring through olfactory channels in contrast to earlier work that focussed only on visual and auditory cues. Recent research has revealed that olfactory abilities in birds are more common than previously thought. Uropygial gland secretions are posited to be a key source of avian body odour and its composition has been found to vary among species and individuals as well as between the sexes. I compared gas-chromatography (GC-FID) traces of shining cuckoo preen wax to the GC-FID traces of the grey warbler, the only host of the shining cuckoo in mainland New Zealand, as well as the preen wax of seven other species for evidence of mimicry. Preliminary results suggest there is evidence for mimicry and the potential for odour-based nestling discrimination in grey warblers. Further tests recording the response of grey warblers to odour-manipulated nestlings are necessary. Finally, in Chapter 5, I investigated the response of the song thrush Turdus philomelos, a species that rejects the eggs of the common cuckoo Cuculus canorus and conspecifics at intermediate and low frequencies, respectively, to nest-odour manipulations using the preen wax of conspecifics and heterospecifics. The results suggest song thrush do not use odour to assess the risk of parasitism at least as indicated in terms of changes in incubation behaviour. Investigations of the role of olfaction in avian brood parasite systems can provide a better understanding of brood-parasite coevolution. Only by considering all channels of communication can we be sure to completely understand the coevolutionary dynamics between brood parasites and their hosts.

The proximate causes triggering nest abandonment are unclear for most species, including the Yellow Warbler (Dendroica petechia), which abandons nests parasitized by cowbirds (via burial or desertion). Cowbird parasitism and rejection of parasitism are costly to some hosts; therefore cues affecting their responses have important evolutionary implications. Manipulative experiments showed that experimentally adding a cowbird egg elicited similar rejection frequencies (2008: 31.8%; 2009: 26.1%) as naturally laid eggs (2008: 27.1%; 2009: 20.0%). In 2008, interaction with an egg-removing model increased the probability of abandonment and the most aggressive individuals were more likely to bury the model cowbird egg. In 2009, eggs added to nests before sunrise were rejected at a frequency (29.7%) similar to eggs added to nests after sunrise (22.9%). Warblers returning to nests after egg addition peered significantly longer at their clutch than at control nests, shuffled their bodies more frequently when on the eggs and spent more time probing eggs with their bill once settled on their parasitized clutch. Furthermore, although non-mimetic blue eggs were not abandoned significantly more frequently than cowbird eggs (blue 31.1% versus cowbird 21.4%), only blue eggs were ejected from some nests. Thus, warblers use both tactile and visual cues to detect the presence of a parasitic egg in their nest. Eggs added to nests were not rejected at a lower frequency than naturally parasitized nests, as was recorded in a previous study. It is difficult to know whether this increase in abandonment of experimental eggs is due to phenotypic plasticity, genetic changes, or other factors. Egg recognition abilities may have changed because I have shown that the warblers’ behaviour changes before versus after egg addition, whereas no changes were recorded in an earlier study. Finally, not all individuals that buried eggs for the first time in 2009 (21.4%) buried again after being re-parasitized (5.3%), when less time remained in the breeding season relative to the first parasitism event. This suggests that egg rejection and host responsiveness in warblers, and likely other avian hosts that use abandonment as a form of rejection, is affected by environmental cues which may act as genetic expressers.

Ecological divergence between Blackcaps and Garden Warblers appears to be incomplete. They resemble each other closely in morphology and their foraging behaviour and food (in the breeding season) are at least broadly similar. Nevertheless, they are sympatric and occur together in a wide range of habitats although Garden Warblers are proportionately commoner in lower, denser vegetation. The two species are strongly interspecifically territorial where they occur together. However, Blackcaps are more interspecifically aggressive than Garden Warblers and are clearly dominant to them in interactions. Blackcaps respond just as strongly to playback of Garden Warbler song as they do to that of Blackcap song. They sometimes intrude into Garden Warbler territories and seek- out and chase the territory holders. In contrast, with rare exceptions, any Garden Warblers which intrude into Blackcap territories are attacked and chased until they leave the area. Also, during song playback experiments, Garden Warblers approach the loudspeaker less closely in response to Blackcap song than they do to Garden Warbler song. Many Blackcaps arrive on the breeding grounds before the earliest Garden Warblers do. A removal experiment, in which such established Blackcaps were systematically removed, showed that some of them had been keeping-out potential Garden Warbler settlers, since the latter then readily established territories and bred in a large part of the Blackcap-free zone. Normally, Garden Warblers have their territories outside Blackcat>-occupied habitat, partly because they are prevented by the aggressiveness of the Blackcaps from settling elsewhere. However, observation and song playback experiments have shown that, once established, Garden Warblers do defend their territories against both conspecifics and Blackcaps. Blackcaps have recently increased dramatically in Britain and Garden Warblers have decreased simultaneously. However, although Blackcaps can limit the local breeding densities of Garden Warblers in any one year, it is not yet clear whether Blackcap numbers are a significant factor in determining the total sizes of Garden Warbler populations.