Assortative mating by multiple ornaments in northern cardinals (Cardinalis cardinalis)

In positive assortative mating, individuals of similar phenotypemate together more frequently than expected by chance. Assortativemating by a variety of qualities, including ornamentation, iswell documented in birds. Studies of assortative mating by ornamentshave focused on single, highly conspicuous ornaments, but manyspecies of birds possess multiple ornaments in both sexes. Wecompared ornament expressions between mates of northern cardinals(Cardinalis cardinalis) to determine if assortative mating occurredby one or more of the four ornaments displayed by both sexes.All cardinals possess tall head crests and red-orange bills.In addition, males have black face masks and entirely red bodyplumage, whereas females have blackish face masks and red underwingcoverts. We predicted that cardinals mate assortatively by plumagecolor because red plumage expression has been shown to indicatequality in both sexes. We found that cardinals mate assortativelyby plumage and bill color, the two ornaments colored by carotenoidpigments, but not by mask expression or crest length. Whetherthis mating pattern arises by mutual mate choice or intrasexualselection is not known.     

Social factors affect the singing rates of female northern cardinals Cardinalis cardinalis

Social factors, such as the duration of territorial occupancy or of time paired with a mate can affect the rate at which individual birds sing. This study examined the influence of duration of pair-bond and territory occupancy as well as date on the rate of singing by male and female northern cardinals Cardinalis cardinalis . When differences in breeding status were controlled, female cardinals sang at higher rates earlier in the season. Females in newly formed pairs sang at significantly higher rates than those in pairs that had previously bred together. In contrast, male cardinals did not show significant variation in the rate of singing throughout the season. The song rates of males in newly formed and established pairs did not differ significantly. Song rates for males and females in mated pairs were not significantly correlated. This study suggests that social factors have a strong effect on the rate at which female cardinals sing. It is possible that increased intra-sexual aggression by females when they are establishing a new territory with a new mate leads to this higher level of song output. Once females have established a territory and acquired a mate, dear-enemy effects might diminish the need for acoustic territorial defence. Differences in the constraints of nesting or the attraction of extra-pair mates might explain the sexual differences in male and female singing behaviour.     

Snake behavior and seasonal variation in nest survival of northern cardinals Cardinalis cardinalis

Seasonal variation in nest success is well documented for many bird species. Predator behavior has been suggested as a mechanism behind these seasonal patterns, but this hypothesis has received little attention. Here we test the hypothesis that predator behavior produces seasonal patterns of nest success by relating nest success of northern cardinals Cardinalis cardinalis to the activity of Texas rat snakes Elaphe obsoleta. Cardinal nest survival varied over the season and was lower when rat snakes were more active. The probability that a nest survived was associated both with when cardinals nested and with nest height, indicating that both temporal and habitat factors affected predation risk. The increased success of higher nests could be associated with some aspect of rat snakes’ climbing ability. In combination with results for two other species studied previously at the same location, our results for cardinals suggest that the specific seasonal pattern of nest success expected for a given bird species will depend on how its nesting season coincides with predator activity. Determining the generality of seasonal variation in predator behavior as a mechanism for producing seasonal patterns of avian nest success will require additional studies that investigate birds and their nest predators simultaneously.     

Testosterone elevation and response to gonadotropin-releasing hormone challenge by male northern cardinals (Cardinalis cardinalis) following aggressive behavior

There is much discrepancy about the relationship between testosterone (T) and male aggressive behavior. For example, in birds, males of many species significantly elevate T levels during inter-male conflict. However, this is not universal, and in species where males typically do not elevate T during aggressive interactions, concentrations of the hormone are often assumed to be circulating at maximum levels. We examined if male northern cardinals (Cardinalis cardinalis) significantly elevated T during simulated territorial intrusions (STIs). We also examined if individuals had the capacity to further elevate T levels in response to gonadotropin-releasing hormone (GnRH) injections immediately after an aggressive encounter. Our results indicate that male cardinals do not significantly elevate T levels in response to STIs, but have the physiological capacity to significantly elevate T in response to GnRH injections following aggressive interactions. This implies that T levels of individuals captured during STIs were not at maximum concentrations. However, additional findings in this study also suggest the possibility that prolonged social instability could elicit significant elevations in T in males of this species, warranting further investigation.     

The influence of carotenoid acquisition and utilization on the maintenance of species-typical plumage pigmentation in male American goldfinches (Carduelis tristis) and northern cardinals (Cardinalis cardinalis).

Birds display a tremendous variety of carotenoid-based colors in their plumage, but the mechanisms underlying interspecific variability in carotenoid pigmentation remain poorly understood. Because vertebrates cannot synthesize carotenoids de novo, access to pigments in the diet is one proximate factor that may shape species differences in carotenoid-based plumage coloration. However, some birds metabolize ingested carotenoids and deposit pigments that differ in color from their dietary precursors, indicating that metabolic capabilities may also contribute to the diversity of plumage colors we see in nature. In this study, we investigated how the acquisition and utilization of carotenoids influence the maintenance of species-typical plumage pigmentation in male American goldfinches (Carduelis tristis) and northern cardinals (Cardinalis cardinalis). We supplemented the diet of captive goldfinches with red carotenoids to determine whether males, which are typically yellow in color, were capable of growing red plumage. We also deprived cardinals of red dietary pigments to determine whether they could manufacture red carotenoids from yellow precursors to grow species-typical red plumage. We found that American goldfinches were able to deposit novel pigments in their plumage and develop a striking orange appearance. Thus, dietary access to pigments plays a role in determining the degree to which goldfinches express carotenoid-based plumage coloration. We also found that northern cardinals grew pale red feathers in the absence of red dietary pigments, indicating that their ability to metabolize yellow carotenoids in the diet contributes to the bright red plumage that they display.     

Nest predation,food, and female age explain seasonal declines in clutch size

The selection pressures responsible for intra- and interspecific variation in avian clutch size have been debated for over half a century. Seasonal declines in clutch size represent one of the most robust patterns in avian systems, yet despite extensive research on the subject, the mechanisms underlying this pattern remain largely unknown. We tested a combination of experimental and observational predictions to evaluate ten hypotheses, representing both evolutionary and proximate mechanisms proposed to explain seasonal declines in avian clutch size. In line with long held life-history theory, we found strong support for both an evolved and proximate response to food availability for young. We also found evidence consistent with predictions that proximate level experiential nest predation influences seasonal declines in clutch size. Finally, older females appear to invest more in reproduction (initiate nests earlier and lay larger clutches) and choose better territories than younger females. Our results highlight the importance of examining multiple hypotheses in a theoretical context to elucidate the ecological processes underlying commonly observed patterns in life history.     

Multiple Ornaments in Male Northern Cardinals, Cardinalis cardinalis, as Indicators of Condition

Investigations of male ornaments in the context of sexual selection have tended to focus on single ornaments, although many species of birds possess multiple ornaments. Understanding the evolution of multiple ornaments requires knowledge of correlations among ornaments in the same individual and the extent to which ornament expression reflects individual condition and behavior. Variation in four male ornaments in socially monogamous, biparental northern cardinals (Cardinalis cardinalis) was related to body size, indices of condition, level of paternal care, and reproductive success. Redness of breast plumage positively predicted body size and negatively predicted nestling feeding rate. Bill color predicted current body condition, with birds with redder bills in better condition. Birds with smaller black face masks had greater reproductive success. These results are consistent with the hypothesis that different ornaments in male cardinals provide information on different aspects of condition and behavior.     

Carotenoid-Based Plumage Coloration Predicts Leukocyte Parameters during the Breeding Season in Northern Cardinals (Cardinalis cardinalis)

Dichromatism in songbirds is often associated with polygyny and dimorphic parental investment, and is thought to arise via sexual selection. Northern cardinals (Cardinalis cardinalis) are not only dichromatic but also monogamous and biparental, suggesting that plumage coloration in this species may serve different functions than in more typical dichromatic species. In order to explore the role of sexual selection in the evolution of plumage coloration in cardinals, we used reflectance spectrophotometry to investigate whether two carotenoid‐based ornaments, the male’s red breast and the female’s underwing coverts, contain information that potential mates or competitors could use to assess condition. We found that whereas coloration was not related to body condition (measured as the residual body mass from a regression of body mass on wing chord), more saturated carotenoid coloration was associated with higher heterophil to lymphocyte ratios in males, and with higher white blood cell counts in females. Thus, in both sexes, carotenoid coloration was positively linked to immune measures normally associated with higher levels of stress and infection. These results do not indicate that carotenoid‐based coloration functions as a signal of low levels of stress or disease in this species. We propose instead that because plumage coloration may be related to competitiveness, the more saturated individuals increase their risk of injury, stress, and infection by engaging in more competitive behavior or by secreting more testosterone, or both. Our finding that carotenoid pigmentation is positively associated in males with the size of the cloacal protuberance, an androgen‐sensitive sex character, supports this hypothesis.     

Nest site attributes and temporal patterns of northern flicker nest loss: effects of predation and competition

To date, most studies of nest site selection have failed to take into account more than one source of nest loss (or have combined all sources in one analysis) when examining nest site characteristics, leaving us with an incomplete understanding of the potential trade-offs that individuals may face when selecting a nest site. Our objectives were to determine whether northern flickers (Colaptes auratus) may experience a trade-off in nest site selection in response to mammalian nest predation and nest loss to a cavity nest competitor (European starling, Sturnus vulgaris). We also document within-season temporal patterns of these two sources of nest loss with the hypothesis that flickers may also be constrained in the timing of reproduction under both predatory and competitive influence. Mammalian predators frequently depredated flicker nests that were: lower to the ground, less concealed by vegetation around the cavity entrance and at the base of the nest tree, closer to coniferous forest edges and in forest clumps with a high percentage of conifer content. Proximity to coniferous edges or coniferous trees increased the probability of nest predation, but nests near conifers were less likely to be lost to starlings. Flickers may thus face a trade-off in nest site selection with respect to safety from predators or competitors. Models suggested that peaks of nest predation and nest loss to eviction occurred at the same time, although a competing model suggested that the peak of nest loss to starlings occurred 5 days earlier than the peak of mammalian predation. Differences in peaks of mammalian predation and loss to starlings may constrain any adjustment in clutch initiation date by flickers to avoid one source of nest loss.     

Melatonin delays clutch initiation in a wild songbird

The hormone melatonin is known to play an important role in regulating many seasonal changes in physiology, morphology and behaviour. In birds, unlike in mammals, melatonin has thus far been thought to play little role in timing seasonal reproductive processes. This view is mainly derived from laboratory experiments on male birds. This study tests whether melatonin is capable of influencing the timing of clutch initiation in wild female songbirds. Free-living female great tits (Parus major) treated with melatonin-filled implants prior to the breeding season initiated their first clutch of the season significantly later than females carrying an empty implant. Melatonin treatment did not affect clutch size. Further, melatonin treatment did not delay the onset of daily activity in the wild nor adversely affect body mass in captivity compared with controls. These data suggest a previously unknown role for this hormone in regulating the timing of clutch initiation in the wild.     

Nest predation patterns in ground-nesting passerines on the Iberian Peninsula

The idea that ground-nesting birds have a high risk of predation is widely accepted This paper analyzes the nest predation patterns of ground-nesting passerines on the Iberian Peninsula The nest predation rates are higher m open land species x= 71%) than m countryside farmland (10%) or forest birds (29%) The latter species have no differences in daily survival between ground and above-ground nests The higher nest predation in ground-nesting open land species was similar m peninsular Spain and North America We conclude that it is incorrect to generalize that all ground nesters have high nest predation rates, and we discuss the possible link of the high nest predation rates of open land birds to the decline observed in their population trends in Spain     

Nest desertion by Grey Fantails during nest building in response to perceived predation risk

ABSTRACT Grey Fantails (Rhipidura albiscapa), a common Australian flycatcher, commonly desert their nests before egg‐laying. We tested the hypothesis that Grey Fantails desert incomplete nests in response to the attention of predators by placing a mounted Pied Currawong (Strepera graculina), a common nest predator, near fantail nests that were under construction. As a control, we placed a mounted King Parrot (Alisteris scapularis), a nonpredatory bird similar in size to Pied Currawongs, near other fantail nests. Four of six female fantails (67%) deserted incomplete nests in response to the presentation of the Pied Currawong. In contrast, none of the seven females presented with a mounted King Parrot deserted. Female Grey Fantails may use the attention of a predator at the nest during the building stage as a cue to desert. Such desertion may be adaptive for Grey Fantails because currawongs are large predators, making successful nest defense unlikely, and they also present considerable risk to adults. In addition, fantails may raise multiple broods during a breeding season and, therefore, have a high renesting potential.     

Nest predation and trampling as management risks in grazed coastal meadows

Livestock grazing is an important management tool of agri-environment schemes initiated within the European Union to maintain and restore biodiversity of grassland birds. However, grazing can affect bird populations negatively by depressing reproduction through nest trampling and increasing nest predation. These effects are, however, considered low when using recommended stocking rates. By simulating wader nests, we experimentally quantify and examine the causes of variation in trampling rates on managed Baltic coastal meadows. Secondly, we examine whether livestock presence increases nest predation of one management target, the critically endangered southern dunlin (Calidris alpina schinzii). Trampling rates of experimental nests were high. Only 21% of nests would have survived a three week incubating period early in the grazing season. Trampling rates were most severe at the onset of grazing and decreased with time. Thus, timing of grazing plays a crucial role in determining breeding success on managed meadows. Predation rates of dunlin nests were moderate and did not depend on livestock presence suggesting that incubating dunlin are not disturbed by cattle. While grazing is vital in habitat restoration and in conserving grassland biodiversity, our results suggest that grazing may also threaten the viability of populations if negative effects are underestimated. Therefore, management plans, especially for endangered species, should not only rely on general recommendations on stocking rates but instead planners need to evaluate the significance of negative effects in terms of local conditions (timing of breeding and grazing, space use of cattle and birds, measured trampling rates) and adjust grazing practises accordingly.     

Environmental noise reduces predation rate in an aquatic invertebrate

Noise is one of a wide range of disturbances associated with human activities that have been shown to have detrimental impacts on a wide range of species, from montane regions to the deep marine environment. Noise may also have community-level impacts via predator–prey interactions, thus jeopardising the stability of trophic networks. However, the impact of noise on freshwater ecosystems is largely unknown. Even more so is the case of insects, despite their crucial role in trophic networks. Here, we study the impact of underwater noise on the predatory functional response of damselfly larvae. We compared the feeding rates of larvae under anthropogenic noise, natural noise, and silent conditions. Our results suggest that underwater noise (pooling the effects of anthropogenic noise and natural noise) decreases the feeding rate of damselflies significantly compared to relatively silent conditions. In particular, natural noise increased the handling time significantly compared to the silent treatment, thus reducing the feeding rate. Unexpectedly, feeding rates under anthropogenic noise were not reduced significantly compared to silent conditions. This study suggests that noise per se may not necessarily have negative impacts on trophic interactions. Instead, the impact of noise on feeding rates may be explained by the presence of nonlinearities in acoustic signals, which may be more abundant in natural compared to anthropogenic noise. We conclude by highlighting the importance of studying a diversity of types of acoustic pollution, and encourage further work regarding trophic interactions with insects using a functional response approach.     

Nest predation and the evolution of conspecific brood parasitism: from risk spreading to risk assessment

Conspecific brood parasitism (CBP) is a taxonomically widespread reproductive tactic. One of the earliest hypotheses put forward to explain the evolution of CBP was "risk spreading"; that is, by laying eggs in more than one nest, parasites may increase the likelihood that at least one offspring will survive to independence. However, the risk spreading hypothesis, based on the assumptions of random nest predation and random selection of target nests by parasites, was theoretically refuted soon after its appearance. New results from the common goldeneye (Bucephala clangula) have revealed that nests are not predated at random and that parasites preferentially lay in safe nests. By taking into account these findings and by modifying accordingly the basic assumptions of the earlier model that refuted the risk spreading hypothesis, we built a model to address the role of nest predation in the evolution of CBP. Model simulations revealed that the selective advantage of parasitic laying, related to nest predation, is much higher than previously thought. Furthermore, the invasion probability of parasitic tactic when initially rare was reasonably high within our model framework. We show that the use of risk assessing, instead of random risk spreading, makes parasitic laying evolutionarily advantageous.     

Nest size affects clutch size and the start of incubation in magpies: an experimental study

Nest size has been suggested to be a sexually selected traitindicating parental ability of both males and females. To testwhether a female's reproductive decisions (e.g., clutch sizeand starting incubation) change in relation to experimentalmanipulation of nest size, as would be predicted if nest sizeis a sexually selected signal reflecting the male's parental quality, we manipulated nest size in a population of monogamousmagpies before laying by adding or removing about 20 cm oflarge sticks in the roof of magpie nests. On the one hand,we found that clutch size of reduced nests was smaller thanthat of control or enlarged nests. Moreover, clutch size was significantly related to nest size after manipulation, whichindicates that females adjust clutch size to the final sizeof the nest, nest size thereby being a good candidate for asexually selected trait. On the other hand, number of eggshatched during the first day is hypothesized to be related to the expected available resources during nestling growth, andsubsequent nestlings hatched are likely to die due to broodreduction if resources are not sufficient to raise well-developednestlings. Nest size is hypothesized to inform females abouta male's willingness to invest in reproduction, and we foundthat in broods of experimentally reduced nests, females startedto incubate earlier in the laying sequence than they did inbroods of control or enlarged nests. Moreover, in experimentallyreduced nests, fewer nestlings hatched during the first day,and the difference in body mass between the first and the fourthnestling hatched increased. This result is in accordance withthe hypothesis that the female's decision of when to start incubationin the laying sequence is mediated by nest size, a sexuallyselected trait signaling parental quality. We discuss alternativeexplanations for the results such as the possibility that nestsof different treatments may differ in their thermoregulationproperties or in their protection against predators.