Long-tailed tit case study

Question 1

Long-tailed tits are one of the few UK birds species that..

Answer 1

breed cooperatively

Question 2

Study site is…

Answer 2

Rivelin valley, Sheffield

Question 3

How many pairs being studied?

Answer 3

40-76 (every year 1994-2020)

Question 4

What does this involve?

Answer 4

Catch adults when building their nests, give them all a unique combination of rings. Find and monitor all nests.

Question 5

Breeding occurs between…

During this period, birds primarily live in …

During they rest of the year they live in ….

Answer 5

march and the end of may/early june

pairs

flocks

Question 6

From June onwards, you are most likely to see these birds in a flock. Typically these consist of between … and … birds, composed of … and …-…. They … and … together.

Answer 6

6, 20, kin, non-kin, forage, roost (at night to save energy and preserve warmth - particular problem in midwinter)

Question 7

These non-breeding flocks persist until the breeding season, at which point they break up and each bird seeks a member of the opposite sex, with whom they can breed. At the start of the breeding season all birds breed in independent pairs (there is a 1:1 sex ratio). The pairs construct a nest and the female lays a clutch of typically around … eggs. The female … these eggs while the male occasionally feeds her, and this goes on for at least 2 weeks. The chicks hatch, get fed by both parents, and in another couple of weeks a brood of chicks will fledge. If all goes to plan that is. Very often the nests, despite their licheny camouflage, get detected by predators and are destroyed. Typical culprits are … and small … (such as weasels and stoats). There are multiple causes of breeding failure, meaning up to … … of nests in a given year get destroyed.

Answer 7

10, corvids, mammals, 3 quarters

Question 8

What happens to these failed breeders?

Answer 8

If nest failure occurs early on in the year (before April) the pair will make another breeding attempt with a new nest. If nest failure occurs after the beginning of may, the pair will abandon breeding for the season and move to the nest of another pair, becoming helpers (feeding chicks and fledglings).

Question 9

… of broods have helpers. Generally between … and … helpers, but sometimes up to …

Answer 9

50%, 1, 3, 8

Question 10

This is an unusual cooperative breeding system as instead of staying home for a period of time before dispersing independently…

Answer 10

all individuals start of by attempting to breed independently, and the decision to breed again or help depends on date

Question 11

This phenomenon is called…

Answer 11

redirected helping

Question 12

There are two ways in which helpers may benefit from this behaviour. The first is by increasing … fitness (by helping relatives). Collecting pedigree data or using genotyping allows … among individuals to be estimated. Find that … of helpers help close kin, usually a … (usually males help). This could be helpers choosing to help relatives or could be chance as relatives tend to live close by and helpers help at the nearest nest.

Answer 12

indirect, relatedness, 77%, sibling

Question 13

To show that helpers really do prefer to help kin, an experiment must be conducted, to test the preference of failed breeders. Failed breeders were given a choice between two nests within their home range, one of which belonged to a … and one to a …-… (both of these nests were protected from predation). Critically, these two nests were … from the failed breeders nest.

Answer 13

relative, non-relative, equidistant

Question 14

In … out of … cases, the failed breeders chose to…

Answer 14

16, 17 (94%), help the relative (so definitely not random, very significant)

• long-tailed tit helpers do show a very significant preference for their relatives when making decisions of whether to help or not

Question 15

Subsequent analysis showed that not only do helpers prefer to help relatives, but they also…

Answer 15

work harder the more closely related they are to the recipient. (positive relationship between provisioning rate of helper and relatedness - mechanism not yet understood)

Question 16

In order to increased inclusive fitness, the helpers must not only help kin, but also…

Answer 16

increase the productivity of those kin

Question 17

Do helpers increase productivity?

Answer 17

Yes - the probability of recruitment (that a fledged chick will survive from the time at which it fledged to become a breeder in the following season) increases with number of helpers at the nest.

Question 18

Helpers also increase … ….

Answer 18

breeder survival (due to reducing reproductive costs) - which means they can breed more successfully in future too, which also benefits related helpers - another way in which indirect fitness could be increased

• evidence that helpers ‘lighten the load’ of reproduction, specifically for males (very little difference in females - work at same rate regardless - males make fewer visits to nest to feed)
• this reduction in the investment that males make feeding their brood seems to have consequences for their survival. Males who have helpers have a significantly higher survival rate (females don’t)

Question 19

Are there direct benefits of helping for long-tailed tit helpers?

Answer 19

Apparently not - helpers in year x do not have a higher probability of future survival and reproduction than non-helpers (both 28%)

Female helpers (rare anyway) do not lay eggs in the current reproductive attempt, no male helpers get EP paternity with females (bc helpers are not around when females are fertile anyway as they are trying to raise their own broods)

Question 20

What are potential direct future benefits to helping?

Answer 20

• help to ‘buy’ flock membership to increase winter survival
• to gain breeding experience
• to find/raise future mates
• to recruit future helpers

(as previously established, none of these are likely to be true in these tits)

Question 21

So there are substantial … benefits to helping but no … benefits

Answer 21

indirect, direct

Question 22

Testing Hamilton’s rule:
Two potential costs to actor (C):
i) … cost (period during which could be potentially breeding)
ii) … cost

Do long-tailed tit helpers suffer an opportunity cost?

Do long-tailed tit helpers suffer a survival cost?

Answer 22

opportunity, survival

No - they tried to breed and failed - no evidence that helpers give up any opportunity to breed

Yes - parental investment is often costly, helpers have a 23% lower survival rate than do non-helpers. We can translate this into a common currency of cost (adult recruits i.e. number of offspring produced) which is -0.029 adult recruits

Question 23

Testing Hamilton’s rule:
Two potential benefits to recipient (B):
i) …
ii) … lightening

Answer 23

productivity (convert to effect of single helper - find that marginal effect of one helper is +0.29 adult recruits)

load (more likely to reproduce in future, marginal effect of one helper on male survival translates to +0.01 adult recruits)

Question 24

Testing Hamilton’s rule:

mean r between helper and brood (when assessing genotypes) =

mean r between helper and male breeder =

Answer 24

0. 16

0. 2 (female relatedness much lower at 0.07, showing that it is mainly male relatedness driving helping behaviour)

Question 25

Testing Hamilton’s rule:

Is rB > C ?

Answer 25

Helping is costly, so it is altruistic. (C = 0.029)

Helping benefits recipients:
- current productivty: rB = 0.16 x 0.29 = 0.047
- future productivity: rB = 0.20 x 0.01 = 0.002
Together = 0.049

rB must > C
0.049 > 0.029

so hamilton’s rule is satisfied

Question 26

Processes generating kin structure:

Individuals living closer to each other are very significantly…

How does this kin structure come about?

Answer 26

more likely to be related than expected by chance
- relatives are clustered together within the population - strong degree of kin structure

Dispersal is limited (don’t disperse very far - only a few hundred metres from where born - females generally move a little further). Those individuals who do disperse also coordinate dispersal with their relatives (53% disperse in kin groups). Additionally, small effective population sizes.

Question 27

Long-tailed tits in … are … yet still cooperative

Answer 27

Estonia, migratory

• help kin despite migration
• stay in kin groups - coordinated dispersal of kin over vast distances

Question 28

What is a small effective population size?

Answer 28

Only a small fraction of a population passes on its genes to the next generation, due to low nest success and high recruitment (chance of survival to following year)
- high relative probability of relatedness compared to other species

Question 29

To summarise, the kin structure comes about due to:

• … dispersal
• … dispersal with kin
• small … … size

Answer 29

limited, coordinated, effective population

Question 30

Why are long-tailed tits cooperative whereas other species are not? What are the constraints on breeding?

Answer 30

Comparing cooperation across many years (1994-2011), we find that there are 2 key ecological drivers of cooperation:

• high nest predation rate (varies between 40-80% of nests among years) - important as determines numbers of potential helpers and recipients in the population (help peaks at intermediate predation rate)
• length of the breeding season (varies between 13-33 days among years) - can try more times if breeding season starts earlier - determines opportunity for independent breeding. As breeding season length increases, cooperation frequency decreases

Question 31

Fitting this into the ECH, the key constraints are high nest .. and … breeding seasons. In combination with the close kin structure, this leads to failed breeders helping to rear related broods (for kin-selected fitness gains)

Answer 31

predation, short

Question 32

Relavant to earlier information: long-tailed tits display … kin ….

Answer 32

active, discrimination