Effects of Mesopredators on Nest Survival of Shrub-Nesting Songbirds

ABSTRACT Although nest predation is often the single largest source of mortality in avian populations, manipulative studies to determine predator impacts on nest survival are rare, particularly studies that examine impacts of mid-size mammalian predators (hereafter, mesopredators) on nest survival of shrub-nesting birds. We quantified nest survival and identified nest predators of shrub-nesting songbirds within 4 large (approx. 40-ha) exclosures and 4 control sites within a longleaf pine (Pinus palustris) ecosystem. During 2003–2006, we located and monitored 535 shrub nests (222 with videography) for 4,804 nest-days to quantify daily nest survival and document predation events. We found no support for a treatment effect, suggesting mesopredators had little impact on daily nest survival (0.9303 in controls and 0.9260 in exclosures) of shrub-nesting songbirds. For the 5 most commonly monitored species, daily nest survival within species was constant. Our analysis suggested that shrub nests were most vulnerable during the nestling stage and presence of cameras on nests increased survival with the increase in survival being more pronounced during the incubation stage. We filmed 107 nest predation events, identifying predators at 88 nests. Of these 88 nests, snakes caused 33%, red imported fire ants (hereafter fire ants, Solenopsis invicta) 28%, raptors 17%, corvids 8%, mesopredators 6%, and small mammals 8% of nest predations. Cause-specific nest predation in controls and exclosures did not differ from expectation, providing evidence that compensatory predation did not occur. Nest predators differed from expectation with regard to nest stage; fire ants and raptors only depredated nests during the nestling stage. Presence of cameras had no effect on nest abandonment. Fire ants were the most prevalent nest predator, and nest predation by fire ants was only observed on nestlings, potentially reducing likelihood of renesting. Magnitude and timing of fire ant predation suggests that fire ants may be the most influential nest predator of shrub-nesting birds within the longleaf pine ecosystem. Our data suggest that controlling mesopredators will have no effect on nest success of shrub-nesting birds within longleaf pine forests.     

Ant body posture: gaster curling increases ant speed

1. Body postures adopted by an animal can serve behavioural functions, homeostasis, or energy balance. 2. We investigated the function of holding the gaster curled forward under the thorax in acacia ants, Pseudomyrmex spinicola Emery, by testing whether ants adopted this posture for defence, thermoregulation, or for efficient locomotion. 3. For the defence hypothesis, we expected an increase in the proportion of ants with curled gasters after a visual threat, a vibrational disturbance of a branch, or the release of nestmate's alarm pheromones. Our data did not support these predictions. 4. For the thermoregulation hypothesis, we found a positive correlation between temperature and proportion of curled‐gaster ants. However, we did not find a reduction in the proportion of curled‐gaster ants after shading them, as predicted by this hypothesis. 5. Our data supported the locomotion hypothesis: curled‐gaster ants walked 1 cm s^?1 faster than ants with the gaster held straight. Straight‐gaster ants walked with the thorax closer to the surface, a posture that likely shifts the centre of gravity closer to the surface in a manner similar to gaster curling. 6. Studying the role of the body posture in acacia ants and other insects will provide a better understanding of the kinematics of walking in challenging angles with respect to gravity.     

Effect of queen number on the production of sexuals in natural populations of the fire ant, Solenopsis invicta

ABSTRACT. To determine whether the production of sexuals in the fire ant, Solenopsis invicta Buren, is related to queen number in the field, the numbers of sexuals in monogyne (M) and polygyne (P) colonies of this species were compared. Large colonies ( n =25 M and 25 P) were collected at random during spring, summer and fall when sexuals are actively produced, and the numbers of sexuals present were counted. The numbers of alates plus sexual pupae and larvae in M colonies far exceeded those in P colonies in each season. Significant differences between M and P colonies were also found when the numbers of alates and sexual immatures (pupae + larvae) were compared separately in each season. In addition, M colonies contained significantly more female sexuals (alates + pupae) than did P colonies in the summer and fall, and significantly more males in all three seasons. The negative relationship between queen number and number of sexuals provides evidence that queen control over the production of sexuals, previously established in laboratory experiments, also occurs under natural conditions.     

Vertebrate-dispersed species in a fire-prone environment

Abstract It is hypothesized that plant species that produce vertebrate-dispersed seeds (fleshy fruits and brightly coloured arillate seeds) would not be common in fire-prone vegetation because seeds are deposited on the soil surface and are unlikely to survive fires. They have not previously been known to have any means of burial that would enable them to await the next fire as a buried soil seed-bank. We studied vertebrate-dispersed species in fire-prone sclerophyll vegetation to look at mechanisms used by vertebrate-dispersed plants for persisting in these environments. Seeds do not survive heating to 150°C, although species from fire-prone environments are more likely to tolerate moderate heating (80°C) without affecting viability than vertebrate-dispersed species from environments where fire is rare. Most vertebrate-dispersed species have the capacity to regenerate vegetatively, although they are not more likely to have this strategy than ant-dispersed species. Finally, ants do take vertebrate-dispersed seeds and fruits (particularly small ones) indicating the potential for the formation of a soil seed-bank. Some species only persist in fire-prone environments by recolonizing from unburnt areas.     

Diel changes in forager size, activity, and load selectivity in a tropical leaf-cutting ant, Atta cephalotes

Abstract. 1. The leaf-cutting ant Atta cephalotes (L.) in a Costa Rican tropical moist forest showed diel changes in foraging activity. In most colonies studied, foraging was primarily nocturnal, although in a few colonies it was primarily diurnal.
2. In all colonies studied, mean forager mass was larger at night than during the day.
3. At night, most foragers carried freshly cut leaf fragments, whereas during the day a large proportion carried dried fragments and other vegetable matter collected from along their trail.
4. Along one trail, where foraging was primarily nocturnal, the match between ant mass and load mass was compared for laden ants at night and during the day. Laden ants at night were larger, carried relatively heavier loads, and showed a higher degree of matching between their mass and load mass than those foraging during the day.
5. A comparison of load masses of ants coming down a local tree and of ants picking up marked fragments from along their trail suggested that the diel difference in load mass and in the match between ant mass and load mass were related to the greater proportion of ants carrying freshly cut leaf fragments at night. Fresh fragments weighed more due to higher water content, and the match between ant mass and load mass was greater for ants cutting fresh fragments than for ants picking up abandoned fragments from along their trail.
6. Possible explanations for the diel changes in forager size and activity are discussed.     

Comparing longevity of Pseudacteon species of different sizes: effect of sugar feeding

Sugar feeding is known to enhance lifespan in many parasitoid species. Several species of phorid flies in the genus Pseudacteon (Diptera: Phoridae) have been introduced in the Southern U.S.A. for the biological control of imported fire ants of Solenopsis species (Hymenoptera: Formicidae). Pseudacteon species are short‐lived flies and little is known about their nutritional ecology. Results from previous studies in our laboratory show that one of the introduced species, Pseudacteon tricuspis Borgmeier, is capable of feeding on sugar sources with a significant increase in lifespan. However, in Pseudacteon phorid flies, the degree to which sugar feeding can enhance the lifespan of other species is not clear, nor is it known whether there is a relationship between body size and longevity. In the present study, the effect of sugar feeding on the survival of three phorid fly species of different body sizes, Pseudacteon cultellatus Borgmeier, Pseudacteon curvatus Borgmeier and Pseudacteon obtusus Borgmeier, is investigated. For all three species, flies continuously provided with 40% or 20% sucrose solution live longer than individuals provided water only or 40% sucrose solution only on the first day of adult life. However, the degree to which sugar feeding enhanced longevity varies by species and is highest for P. obtusus. The data also indicate that longevity in Pseudacteon phorid flies is related to body size: the largest species (P. obtusus) lives significantly longer than the smaller species (P. cultellatus and P. curvatus). These results suggest that adults of all three Pseudacteon species are capable of feeding on sucrose solution and that sugar feeding can enhance their longevity. The significance of these results is discussed in relation to the field performance of Pseudacteon phorid flies as biological control agents of imported fire ants.