Natal dispersal = permanent movement an individual makes from its birth
site to the place where it reproduces or would have reproduced if it had
survived
Breeding dispersal = movement of adults between breeding attempts
Effective dispersal = when a successful breeding attempt follows either
type of dispersal
Diagram of predator model. The first nest (solid circle at bottom) is not located within a predator's home range, and the
second nest (solid circle, upper right) would have failed on the day that the kill area contained the nest site. Powell and Frasch
(2000) simulated predation by randomly placing a predetermined density of predators
within a 100-km2 nesting area. Each predator
hunted within a home range defined by maximum movements from a central
location and a "daily kill area" (shaded boxes). Each day, each
predator moved from its central location to a point determined by x- and y-axis values randomly selected from a uniform distribution. The
predator caused nest failure if the nest was located within a fixed area surrounding the hunt location, the "daily kill area." After hunting,
the
predator
returned to the central location at the end of the day.
Nest predation and breeding dispersal -- Many types of predators depredate bird nests and thus potentially influence the spatial distribution of their prey. Powell and Frasch (2000) used asimulation model of a double-brooded songbird's nesting seasonto test three predictions about the selective advantage ofdispersing different distances after nest predation by predatorswith varying home range sizes. Their results supported the predictions that (1) dispersing birds had higher success than nondispersingbirds after predation of the first nest, (2) dispersing beyondthe home range of the nest predator increased the success ofthe second nest, and (3) birds whose first nests were depredatedearly in the nesting cycle did better by dispersing fartherthan birds whose nests were depredated later in the nestingcycle. These results provide evidence that predation and predatorcharacteristics may cause variation in adult dispersal distancesduring the breeding season. However, Powell and Frasch (2000) did not find an advantagefor long-distance dispersal when predators with small- or medium-sizedhome ranges were responsible for the predation event. The criticaldecisions of dispersal and dispersal distance made by adultbirds are complex, but it appears that predationevents can create a selective advantage to disperse.
Why do individuals disperse?
Proximate vs. ultimate causation
Proximate causation:
Environmental dispersal - individuals disperse in response to environmental
factors, e.g., absence of, or competition for, suitable resources or parental
aggression
'Innate' dispersal - individuals are genetically predisposed to disperse
Experimental evidence for environmental dispersal:
Spanish Imperial
Eagles (Ferrer 1992) - duration of pre-dispersal period for young eagles
is correlated with blood levels of urea and uric acid (increased levels
= reduced food intake & increased need to metabolize available proteins
= poorer condition = delayed dispersal)
Corticosterone, body condition and locomotor activity: a model for dispersal in screech-owls -- Belthoff and Dufty (1998) proposed a dispersal model based on interactions among hormonal changes, body condition and social stimuli, predicting that corticosterone, an adrenal glucocorticoid known to stimulate locomotor and foraging activity, increases in blood plasma prior to dispersal through a combination of endogenous and exogenous events. This mediates the locomotor activity that underlies dispersal behaviour. Juveniles in good body condition (i.e. those with sufficient fat reserves) will disperse when corticosterone increases. Birds in poor body condition will not, but they will increase their foraging activity under the influence of corticosterone. Dispersal of siblings will reduce aggression and/or competition for food, enabling the remaining juveniles to improve their body condition and disperse. Initial studies on screech-owls, Otus asio and O. kennicottii, have produced results that are generally consistent with the model. For example, captive juvenile screech-owls showed increased locomotor activity in the weeks leading up to the time when free-living juveniles are dispersing, and activity levels declined thereafter. Peaks in corticosterone corresponded with periods of high locomotor activity (i.e. at the time of dispersal) in captive owls. Finally, field studies indicate that dominant juveniles that are presumably in better physical condition, initiated dispersal before their more subordinate siblings.
Ultimate factors responsible for dispersal:
1) Inbreeding avoidance - juveniles disperse from their natal site to prevent breeding with relatives
2) Competition for mates
3) Competition for resources (e.g., breeding territories)
Which is correct??
Most studies indicate that resource competition is an important
factor, but others support mate competition or inbreeding avoidance
Predation and the cost of dispersal-- Ecologists often assume that dispersing individuals experience increased predation risk owing to increased exposure to predatorswhile moving. To test the hypothesis that predation risk isa function of movement distance or rate of movement, Yoder et al. (2004) usedradio-telemetry data collected from 193 Ruffed Grouse (Bonasaumbellus) in southeastern Ohio. They found evidence indicating that increased movement rates mayincrease the risk of predation for adult birds but not juveniles.They also found juvenile and adult birds inhabiting unfamiliarspace were consistently at a much higher risk of predation (threeto 7.5 times greater) than those in familiar space. These resultsindicate that although movement itself may have some effecton the risk of being preyed upon, moving through unfamiliarspace has a much greater effect on risk for Ruffed Grouse. Individuals moving through unfamiliar space may suffer fromdecreased foraging efficiency (which also may affect energeticcondition) or a decreased ability to avoid predators. A defense mechanism used by Ruffed Grouseis concealment in dense brush and undergrowth. Being in unfamiliarspace may compromise this defense mechanism. Grouse moving throughunfamiliar space may be more vulnerable to predation duringforaging. Lack of experience in a new area may lead to difficultylocating food sources that also provide good cover. Although Yoder et al. (2004) based their analysis on distinguishingbetween familiar and unfamiliar space, thepossibility that new territory is not only less familiar butalso of lower quality (i.e., with less available cover or foodsources) than is an individual's original home range cannot be ruled out.Overall, these results support the hypothesis that increased predation risk may bean important cost of dispersal for birds.
Sex-biased dispersal
Among birds, females are the predominant dispersing sex. In contrast, among
mammals, males are the predominant dispersing sex.
Reasons why dispersal may be more costly to female mammals:
Female reproductive success is limited primarily by nutritional constraints,
while males are limited by number of females they can inseminate. So, females
may benefit more than males from familiarity with food resources, denning
sites, & neighbors (i.e., philopatry)
Because male reproductive success depends more on access to mates, they
may benefit by moving to areas or groups where there are larger numbers
of mates (e.g., lions & some primates)
Because intrasexual competition for mates is likely to be more intense
among males in polygynous species, males are more likely to be evicted
& have to disperse to find mates
What about birds?? Possible asymmetry in costs & benefits:
Avian mating systems are largely based on resource defense (males compete
for & defend resources needed for successful reproduction). So, males
might be more successful in establishing territories in their natal area
because familiarity may permit higher feeding rates & reduced predation
rates.
Females might benefit from the potential to choose between the resources
of different mates &, if inbreeding is costly, females rather than
males should disperse.
Plan of the ‘novel environment’ room in which the exploratory behavior of wild-caught Great Tits was tested by Dingemanse et al. (2003).
The room has dimensions 4.0 × 2.4 × 2.3 m. Along each 4-m wall were eight sliding doors, in two rows of four above each other, connecting the holding
cages to
the room. The front 2.4m wall had a 0.9 × 2.0 m door at the left side and a 1.1 × 0.16 m one-way screen through which the birds could be observed.
The room contained five artificial trees made of wood with a trunk of 4 × 4 cm and a height of 1.5 m. Each tree had four cylindrical branches 20 cm long. The
upper two branches (5 cm below the top) were on opposite sides of the trunk, perpendicular to the lower branches (25 cm below the top). Birds entered
the room through one of the sliding doors (Dall 2004).
Natal dispersal and personalities in Great Tits (Parus major) --
Dispersal is a major determinant of the dynamics and genetic structure of populations, and its consequences depend not only on average dispersal rates and distances, but also on the characteristics of dispersing and philopatric individuals. Dingemanse et al. (2003) investigated whether natal dispersal correlated with a predisposed behavioural trait: exploratory behaviour in novel environments. Wild Great Tits were caught in their natural habitat, tested the following morning in the laboratory using an open field test and released at the capture site. Natal dispersal correlated positively with parental and individual exploratory behaviour, using three independent datasets. First, fast-exploring parents had offspring that dispersed furthest. Second, immigrants were faster explorers than locally born birds. Third, post-fledging movements, comprising a major proportion of the variation in natal dispersal distances, were greater for fast females than for slow females. These findings suggest that parental behaviour influenced offspring natal dispersal either via parental behaviour per se (e.g. via post-fledging care) or by affecting the phenotype of their offspring (e.g. via their genes). Because this personality trait has a genetic basis, these results imply that genotypes differ in their dispersal distances. Therefore, the described patterns have profound consequences for the genetic composition of populations.
Natal dispersal distances of four species of birds in Great Britain (European Blackbird, Turdus merula;
Reed Warbler, Acrocephalus scirpaceus; Lesser Black-backed Gull, Larus fuscus; and Woodpigeon, Columba palumbus)
(Paradis et al. 1998).
How far do young disperse?
Median dispersal distances for most birds
is less than 10 home range diameters. Why?
Assume:
individuals do not share ranges (or territories)
adults (residents) are dominant to dispersing young
So, dispersing young settle on the first uncontested site they find &
uncontested sites arise through the death of adults.
But, if true, then why do some young disperse substantial distances
& what about young that become floaters & do not obtain and defend
a site? More research is needed.
The evolution of different dispersal distances might involve multiple selective forces operating
at different spatial scales (Ferriere et al. 2000).
Floater home ranges and prospective behavior of European Starlings -- In many bird species, floaters are present on the breeding grounds in one or more years before they breed. There is increasing evidence that they have specific home ranges in which they search for information about current and future breeding opportunities. Tobler and Smith (2004) investigated the role of prospecting in a migratory European Starling (Sturnus vulgaris) population. Radio-tracking showed that male starling floaters use specific home range areas during the breeding period. Nest-box observations demonstrated that non-parental nest intrusion is common in the starling and that it is significantly more frequent during the nestling than during the incubation period. In addition, small groups of nest boxes were more likely to be occupied by starlings if they had been put up during the preceding breeding season. These results suggest that floaters try to acquire information about local breeding communities. One specific type of information may be the location of potential breeding sites.
Number of fledglingsproduced by birds thatgrew up in aparasite-free nest (open circles and dashed line) ora nest infested withectoparasitic hen fleas (solid circles and solid line)in relation to theirnatal dispersal distance. Residualsof the model withoutthe interaction between parasitetreatment and natal dispersaldistance are shown.
Natal dispersaldistance of recruits originatingfrom testosterone-
injected (N =26) and control-injected (N= 19) eggs.
Means+ 1 SE areshown.
Maternal modulation of natal dispersal in Great Tits -- The decision of how far to disperse fromthe natal territory hasprofound and long-lasting consequencesfor young animals, yetthe optimal dispersal behavioroften depends on environmentalfactors that are difficultor impossible to assessby inexperienced juveniles. Naturalselection thus favors mechanismsthat allow the adaptiveand flexible adjustment ofthe offspring's dispersal behaviorby their parents viaeither paternal or maternal effects. Tschirren et al. (2007) showed that different dispersal strategies maximize the reproductive successof young Great Tits(Parus major) originating from aparasite-infested or a parasite-freenest and demonstrate thatdifferential transfer of maternalyolk androgens in responseto parasitism can resultin a modification ofthe offspring's dispersal behaviorthat appears adaptive. These results demonstrate that prenatal maternaleffects are an importantyet so far neglecteddeterminant of natal dispersaland highlights the potentialimportance of maternal effectsin mediating coevolutionary processesin host-parasite systems.
Literature cited:
Belthoff, J. R. and A. M. Dufty, Jr. 1998. Corticosterone, body condition and locomotor activity: a model for dispersal in
screech-owls. Animal Behaviour 55: 405-415.
Dall, S. R. X. 2004. Behavioural Biology: Fortune Favours Bold and Shy Personalities.Current Biology 14: R470-R472.
Keppie, D.M. and J. Towers. 1992. A test on social behavior
as a cause of dispersal of Spruce Grouse. Behav. Ecol. Sociobiol. 30:343-346.
Nilsson, J.A. and H.G. Smith. 1985. Early fledgling mortality
and the timing of juvenile dispersal in the Marsh Tit Parus palustris.
Ornis Scand. 16:293-298.
Ritchison, G., J. R. Belthoff, and E.J. Sparks. 1992. Dispersal
restlessness: evidence for innate dispersal by juvenile Eastern Screech-Owls?
Anim. Behav. 43:57-65.
