Fission-Fusion Dynamics in the Facultative Cooperative Breeding Placid Greenbul (Phyllastrephus Placidus)

Biology Department Research Group Terrestrial Ecology Unit (TEREC) ______

FISSION-FUSION DYNAMICS IN THE FACULTATIVE COOPERATIVE BREEDING PLACID GREENBUL (PHYLLASTREPHUS PLACIDUS)

Rori Sys Studentnumber: 01302591

Supervisor(s): Prof. Dr. Luc Lens Scientific tutor: MSc Laurence Cousseau

Master’s dissertation submitted to obtain the degree of Master of Science in Biology Academic year: 2018 - 2019

Introduction

Fission-fusion dynamics within social species are a relatively well observed phenomenon (Silk et al.; 2014 (1)). Fission includes behaviours where groups either split up in subgroups or just completely dissipate. Fusion behaviour on the other hand is characterized by the forming of new groups or the growth of a group. Cooperative breeding is a type of breeding-behaviour where more than two individuals provide care at a single nest. In a recent study it was observed that at least 9% of all passerines show this behaviour (Leedale et al.; 2018 (2)). Numerous hypotheses try to explain the origin of this behaviour, including the ‘inclusive fitness hypothesis’ (cf. infra). The role of the environment is also still under debate. We will be looking at this type of behaviour through the lens of fission-fusion dynamics. The breeding pair is the centre of any cooperative breeding group. Changes to it will affect the entire group significantly. Fission-fusion behaviour surrounding the breeding pair encompass the forming of the pair and their reasons for staying together or breaking up. Pairs can stay together for multiple breeding seasons, sometimes even for entire lifetimes, like in the case of the bearded reedling (Griggio and Hoi ;2011 (3)). However, it is not always clear what the main benefits are of choosing to stay together. A general hypothesis for pair fidelity is that with bonding time the familiarity increases between the pair. This familiarity would increase the cooperation between the pair and positively impact their breeding success, which would encourage not switching to a new partner (Gabriel et al.; 2012 (4)). Results of this hypothesis are mixed. Some studies seem to confirm this. In a study by Griggio et al (2011 (3)) an increase in breeding success with pair bond duration of bearded reedlings was found and in another study by Gabriel et al. (2013) (5) similar results in Steller’s jays were observed. Other studies showed increases in breeding success only after the first year, indicating that after the pair is more familiar with each other no further big increases in breeding success happen, as observed in kittiwakes (Naves et al., 2007 (6)) and barnacle geese (Larsson et al., 1991 (7)), which can live together for many years. Breeding success has also been related to site familiarity (Naves et al., 2007 (6)) and the overall experience of the breeding pair (Orell et al., 1993 (8)). It is clear that the reasons for pair stability can be species-specific and very complex.

Divorce, a clear example of fission, seems to be a general strategy to obtain a better breeding position. This principle is titled the “better option hypothesis” (Ens et al.; 1993 (9)). In great tits, the chance of divorce was higher if the clutch size was smaller, indicating either a lack of experience or compatibility of the couple. (Perrins and McCleery; 1983 (10)). The initiation of the divorce is often dependent on the sex of the breeder as it is often the female breeder that actively chooses to divorce. In domestic pigeons, female breeders were observed to choose new mates after failed breeding attempts while males stayed at their nesting site (Wosegien; 1996 (11)), and in Willow Tits, females were observed to choose more experienced males after divorce (Orell et al 1993 (8)). In several temperate bird species, including the mallard (Cezilly and Nager;1995 (12)), divorce was related to extra-pair paternity, possibly as a mean of mate sampling by the female breeders suggesting evidence for the better option hypothesis. However, this female choice-based fission is not always the cause for divorce as it seemed to be more accidental, as in the death of either breeder, like in the case of the barnacle goose (Larsson et al.; 1991 (7)). Alternatively, the divorce of a pair can be forced by other females in species like kiwis, because a strong female bias of the species (Taborsky et al. 1999 (13)).

Another reason for divorce could also be to avoid inbreeding and to increase the heterozygosity of the offspring. (Kempenaers 2007 (14)) Females have been observed to leave their nest after raising young and go to other nesting locations. Several hypotheses try to explain this female promiscuity. Firstly, female breeders might have to choose between direct and indirect advantages for their young. The care that their partner gives to their young would be the direct advantage. While the genetic benefits that can be acquired through extra-pair copulations are indirect genetic benefits to the young. Secondly, extra-pair copulations might also be an attempt to increase the heterozygosity of the young. Thirdly, it can be an effort to reduce inbreeding and thus to increase hatching success. Lastly, in some birds mating with multiple males can cause sperm competition with the best sperm winning the race. Of course, this is all limited by the ability of individuals to make choices as demonstrated in a study by Wheelwright et al.; 2006 (15). Young females could recognize their father but not their brothers since they had only seen them with juvenile characteristics. They were less likely to mate with their fathers than with their brothers as a consequence. Although inbreeding is generally avoided, breeding females were sometimes observed to breed with their sons, generally only for one year. (Wheelwright et al.; 2006 (15))

Furthermore, as the breeding pair is the central unit of any cooperatively breeding group its stability will be a key component of fission-fusion dynamics that the group undergoes. In a study on marmosets (C. Lazaro-Perea et al.; 2000 (16)) the loss of a breeding female had two possible outcomes for the group. If no adult females were present in the group, a replacement female from outside the group was readily accepted. If multiple other females were in the group, they fought for dominance and kept out new immigrants. The number of extra-group copulations rose in this case and ultimately the group could fission along sexual lines. In the pied babbler, longer pair bonds decreased the amount of immigration the group received, showing a clear connection between pair stability and group dynamics, as was observed in a recent study by Wiley and Ridley (2018) (17). They suggest that this happens because groups with pairs that have been together longer are more related on average and consequently they see a decrease in intra-group reproductive competition as well as in intra-group conflict.

In cooperatively breeding species fission-fusion dynamics can be especially complex. Besides the reasons for the breeders to stay together or not, the non-breeding individuals also have their own reasons for staying in a group or dispersing to join or form new groups. The decision to delay dispersal by certain new-borns is especially important for cooperatively living birds. Birds stay in their natal nest until they are physically ready to risk dispersal but often choose to stay regardless. This is linked to several benefits (Lewis et al.; 1990 (18)). Firstly, better chances at survival and a better place in the queue for a breeding position. In siberian jays (Ekman et al.; 2002) (19) it was observed that fathers provided a safe haven for their young. This allowed them to gain an advantage for possible breeding positions in their natal zone. When replaced by a non-related father the subordinates of the old father tended to leave. In superb fairy wren (Lewis et al.; 1990) (20) dispersal by new subordinates and old ones was mostly limited by mate choice and available habitat of good quality. Leaving the natal territory seems to be beneficial when subordinates cannot inherit a breeding position and when habitats are available with the possibility of better breeding positions. In pied babblers (Nelson-Flower et al.; 2018) (21) subordinates that stayed had greater survival rates and could inherit breeding positions if they had a high social ranking. If subordinates did choose to disperse to other groups the motivations were different dependent on their sex. Females joined groups with high productivity while males joined groups with breeding vacancies and to avoid conflict within their old group. The benefit for unrelated subordinates to enter groups will mostly rely on which social status they can acquire in a new group (Nelson-Flower et al.; 2018) (21).

The benefits of staying or leaving aside, a proportion of subordinates also choose to help with raising the young of the breeding pair. Both related and unrelated subordinates have been observed helping the young. The main benefit for related helpers could be 'inclusive fitness'. According to the hypothesis of inclusive fitness the helpers raise their own fitness by increasing the breeding success of the related breeding pair. In fairy wrens different types of helping behaviour have been observed (L. Brouwer et al.; 2014) (22). Female helpers feed the young fixed amounts of food, while male helpers compensate when a decrease in the breeding pairs’ feeding rate occurs. In the first case, the females increase the breeding success of the current year while, in the second case, the males increase the survival chance of the breeding pair and thus their lifetime breeding success, because the breeders can put more resources into their own survival and into their future breeding attempts. In both cases related helpers can gain an indirect fitness benefit. However, if the relatedness between the young and the helpers is unclear, as when a large amount of extra-pair paternity takes place, this indirect benefit can be too uncertain for the helpers. Reduced helping rates were observed in promiscuous fairy wrens where the partners were significantly more unfaithful then in other fairy wren species (Kingma et al.; 2010) (23).

Although an indirect benefit can be a reason for related helpers to help raise young, it cannot always be the cause since helping behaviour has also been observed with unrelated helpers. In long-tailed tits (Leedale; 2018) (24) unrelated subordinates also help although less so than related subordinates. Another possible reason for helping behaviour is the ‘pay-to-stay hypothesis’ where subordinates are only allowed to stay in a group if they benefit the group. As previously mentioned, helping the breeding pair can increase their breeding success in one year or over their lifetime so it stands to reason that the breeding pair would prefer all subordinates to help, especially unrelated subordinates. The pay-to-stay hypothesis states that subordinates are forced to help if they want to stay. In chestnut-crowed babblers (Nomano et al.; 2015) (25) unrelated helpers were observed but showed no indication of displaying their helping behaviour to the breeding pair. In certain species of cichlid (Bergmuller et al.; 2005) (26) bigger subordinates were less likely to help raise young, indicating that pay-to-stay is not always enforceable.

Objectives

In this thesis we will examine the fission-fusion dynamics of the Placid Greenbul (Phyllastrephus placidus), a facultative cooperatively living bird species. Our research was located in the Taita Hills, South-East Kenya. It is a good model species for studying fission-fusion dynamics since we have both temporal data on pair stability and data on social group dynamics. In a first step, we will visually explore their group dynamics using network theory. We will discuss the observed dynamics in a fission-fusion context with the focus on the behaviour of the breeding pair and the subordinates. In a second step, we will focus on pair dynamics and the following hypotheses will be tested:

(i) the ‘mate familiarity effect’ hypothesis which predicts an increase in breeding success with pair familiarization. Hence, a pair that has bred before should have an increased breeding success (ii) the ‘pair incompatibility’ hypothesis which states that divorce is the result of a pair being incompatible. We expect pairs to divorce after a failed breeding attempt. (iii) the ‘better option hypothesis’ where we expect divorce after a year with a high amount of extra-pair paternity, indicating that the breeders are looking for other partners with better qualities or better territory.

Finally, we will study the following hypotheses for subordinates:

(i) the hypothesis of 'direct fitness', which states that subordinates and helpers gain direct benefits from staying within a group. We expect that subordinates stay in a group to later inherit the breeding position. (ii) The’ inclusive fitness hypothesis’ predicts that subordinates are more likely to help if they are more related to the breeding pair or if less EPP, extra pair paternity, was observed so we expect reduced feeding rates/helper ratios with decreasing relation to the breeding pair or with increased EPP. (iii) Lastly, according to the 'pay-to-stay' hypothesis the presence of helpers should be beneficial to the breeding pair or that they are forced to help by the breeding pair. Under this hypothesis we expect all subordinates to be forced to help. We also expect that subordinates either compensate for breeder feeding rates, as in breeders feed less, or they should be additive and increase the breeding success. Both of these results would also be in compliance with the inclusive fitness hypothesis if the helpers are related to the breeding pair.

Material and methods

Study area and species

The study area consists of the cloud forests in the Taita Hills, South-East Kenya, located on two mountain isolates separated by a low-altitude valley. It is part of the northernmost range of the Eastern Arc Mountains biodiversity hotspot and is isolated on all sides by low altitude savannah. There are three large forest fragments: Chawia (CH), Ngangao (NG) and Mbololo, although the latter is isolated from the others, as well as several smaller fragments. The 430 ha of indigenous forest have been pressured severely by local agriculture. (Spanhove et al.; 2013) (27)

The Placid Greenbul or Phyllastrephus placidus is a facultative cooperatively breeding bird that inhabits cloud forests from Central to East Africa. It lives in small families consisting of a breeding pair and up to 5 subordinates, 0 to 3 of which actively help the breeding pair in raising the nestlings. Their nests have a cup shape and are hidden in shrubs, circa 1.3m high. Their breeding seasons starts at the onset of the short rain season in mid-October and lasts until the end of March. They tend to care for clutches of 2 eggs for around 17 days. Afterwards it takes around 13 days for the young to become fledglings. Their main food source consists of invertebrates and small vertebrates as well as occasionally fruits. The main predators of the Placid Greenbul are hawks and monkeys. Other predators include several species of small mammals, including rodents, reptiles and other birds. (Van de Loock et al.; 2017) (28)

Data collection

We obtained data from 9 forest fragments (namely CH, FU, NG, MA, MS, SU, YA, ND) for the years 2007-2010 and 2012-2016. For the data of the years 2012-2016 we have information on the total group size, helping and non-helping subordinates included, based on video recording. For the other years we do not have information on the non-helping subordinates. In total we collected data for 795 individual observations over 443 different nests.

In our dataset we counted 237 different pairs from 332 nest observations. We observed the total amount of birds in a nest, as well as the function of the birds, ranging from male and female breeders, helping and non-helping subordinates and young. In total we observed 388 breeding females (BF), 316 breeding males (BM), 209 subordinates (H), 393 young (PU) and 62 observations for which the function was unknown. For each year we also observed variables for the breeding pair at the nest. Extra-pair paternity was checked for in two ways. Firstly, we observed whether at least one young was the result of extra-pair paternity, EPB. Secondly, we looked at the number of young resulting from extra-pair breeding attempts within a nest, NEPY. We measured breeding success in 3 ways. Firstly, as the number of young that were successfully fledged (NPULFLED). Secondly, whether the breeding pair successfully raised at least one young or not, “Failure”. And lastly, partial failure (FailureP), whether any young died or not. We found 6 complete breeding failures, with 115 complete successes, and 16 partial failures, with 105 complete successes.

Network analysis

A network was created with the individual birds as nodes. Edges were created between individuals that occupied the same social group in the same year. 2 different datasets were considered: - A subset for the years 2012-2015 for which we had certainty about the total group size of each group; - the full dataset from 2007-2015 for which only subordinates that helped raise the young of the breeding pair were counted alongside breeders and young because the total group size is not known. Each time, the entire dataset for all the different forest fragments was considered. Additionally, the NG and CH fragments were also analysed separately for each subset. The other fragments had too few recurring observations for this analysis. In this analysis, we calculated the diameter, density, modularity and the degree of the vertices of the graph. The diameter represents the largest chain of edges found between two individuals. The density is the proportion of present edges from all possible edges in the network. It is influenced by the number of observations. Modularity is used to quantify the connections between and within communities. High modularity for a partitioning reflects dense connections within communities and sparse connections across communities. Individuals that occupied the same nest are considered a community for this analysis. The degree is the average number of edges an individual has. All visualization and analysis of the data were done with the the igraph package in Rstudio (Butts; 2008) (29).

Pair stability

An analysis was done purely on the observed pairs. In total 237 pairs were observed. For each breeding pair we observed separately several additional variables. Firstly, whether they were still together in the next year, breed_after, with a 0-value indicating that one breeder had found another partner and 1 indicating that they were still together in the next year. If we were unsure about one breeder we marked it as NA. The same was done but for the previous year with ‘breed_before’ indicating whether the pair was together in year before. Lastly, we observed whether a member of the breeding pair was found in any of the following years. This was done because we often did not have repeated observations of a pair, or both individuals separately, and were uncertain of what happened to them. If they were found together in any later year we assumed they stayed together in the intervening years. We used this information to see whether a pair separated or divorced. A separation was counted if in later years, we no longer observed a pair together but one paired with a new mate. A divorce was counted as a separation event for which both breeders were found alive in later years but with another mate. In this case we were certain about their fate. In total we had 10 observations where the breeders were found afterwards but were no longer together and 64 cases where they were together the next year. We found 10 clear separations and only 1 clear divorce. To test the hypothesis for breeding pair fidelity we constructed two models based on whether the breeding pairs were found in the next and/or the previous year respectively. For the first model, we constructed a general linear model (GLM) with the presence of the couple in the next year, as a binomial response variable. We used breeding failure, the number of fledged young and extra-pair

paternity as predictor variables. In accordance with our hypothesis of breeding pair fidelity we expect the chance of the couple to stay together in the next year to be positively related to their breeding success. Also, according to the ‘better option’ hypothesis it should be negatively related to the extrapair paternity. We removed the variables with the highest p-value until the only remaining variables had a p-value < 0.05 or until we found no significantly related variables. Next, we constructed two GLMs to test what affected the breeding success chance. We used breeding failure, partial failure and the number of successfully raised young as response variables in the two models. The number of helpers, the amount of extrapair paternity and the couple status in the previous year were chosen as predictor variables.

Subordinate behaviour

For the subordinate analysis we used the dataset containing all observed individuals and a dataset which had information on whether or not the subordinates were related to the breeding female of the territory they resided in. We had genetic information for 194 observations of which 46 were not related to the breeding female; 148 were related. In this dataset some subordinates were observed multiple times per year and also across multiple years. We corrected for within year resightings, by only counting the first observation, and after doing so we found that the maximum number of years an individual was present was 4 years. If the subordinate is related to the breeding female, we assumed it delayed dispersal as it would be unlikely that a subordinate would disperse and come back again to the parental territory. To test the pay-to-stay hypothesis we already modeled the relation between breeding success and whether subordinates helped or not in a previous model. From the 55 subordinates that were observed to help we made a linear model for the relation between the number of helpers, as the explanatory variable, and the feeding rate of the breeders, obtained from video observations, as the response variable. We expected the helpers to either compensate for a decrease in breeder feeding rates or to increase their yearly breeding success. Lastly, we checked for possible indirect and direct benefits for subordinates staying in the territory of the breeders and possibly helping them. We first modeled the relation between helping behaviour and the relation to the breeding female in a GLM, with the chance that a subordinate helped as a binomial response variable. We expected the chance to help to increase if the subordinates are related to the breeding female. Next, to check for a direct benefit of staying in a territory we searched for the subordinates in our dataset that would go on to become breeders themselves.

We calculated the average lowest distance between all nests in a year. Averaged over all years we found an average lowest distance of 260 meters. We used this as a proxy for the territory of the different nests. If a subordinate that becomes a breeder in later years travelled less than 260 meters away from its nest, we assumed it inherited the territory. We then calculated the distance from their first nest as a breeder to their last nest location from when they were still subordinates. If the subordinates moved over 260m we assumed they did not inherit the territory and dispersed instead. Both the average distance was calculated as was the distribution of inherited territories and the dispersal events.

Results

Network analysis

Visualization of the entire dataset without correcting for the ‘Feeding’ status clearly indicated movement between the different fragments. In the underlying figure we can see 4 fragments that are connected by individuals moving to different nests. The network is highly structured, with new communities being formed per breeding season.

Figure 1: Connected forest fragments

Figure 2: Divorce oversight Shown networks

Breeders switching mates We observed one pair divorcing, the couple with breeding female, A91242, and male breeder AA19119. The breeding female remarried with their son AA14486 while staying in the same nesting area. In 0708 the pair is together and has two PU’s in their nest. In 0809 the pair is still together and AA14487 is now acting as a subordinate for the breeding pair. In 0910 the pair has divorced and AA14487 is now a pair with its mother A91242

Figure 3: Divorce 0708 Figure 4: Divorce 0809 Figure 5: Divorce 0910

Figure 6: Fledgling changes The couple AA19259, BM, and A92545, BF, breed over multiple years. Their fledgling AA26907 starts of as a PU in 1213 and becomes a subordinate that helps raise the couple’s new young in later years. It delays it dispersal multiple years to do this.

Figure 7: Fledgling 1213 Figure 8: Fledgling 1314 Figure 9: Fledgling 1415

Statistical parameters 2012-2015 Table 1: Network Parameters

These are the results for the 2012-2015 subset but where representative of the general results. (apart from the diameter parameter)

As expected, the density is higher per year than if looked at for all the years combined. The CH fragment always has a larger density than the NG fragment. This is normal since CH has less vertices and thus fewer potential edges. For the smaller fragments there is not enough data present.

Modularity is smaller when we look at all the years combined. Interestingly, the modularity of NG is smaller than that of CH, suggesting that the individuals in NG have more connections between nests (are more connected in NG.)

For the degree we found that individuals of NG have more edges than those in CH. This seems to suggest either a larger group size or more nests per year on average.

Pair stability

We constructed the most significant model using a backwards stepwise procedure. A significant relation was found, p-value of 0.026, between the chance of the breeding pair staying together in the next year and the number of fledglings resulting from extra-pair copulations. The logit value for the intercept was 3.5968 and the logit value of NEPY was -1.37. After transforming the logit with the expit transformation we found a decrease in the chance of the pair to stay together the next year with an increasing amount of young from extra-pair copulations. No significant relations were found between the breed_after and breeding success or the number of subordinates.

Graph 1: NEPY and Pair Fidelity

Effect of NEPY on pair fidelity 1.2

0.8

0.6

0.4

Chanceto stay togheter 0.2

0 0 1 2 3 NEPY

The GLM models for breeding success and failure were constructed with a backwards stepwise procedure as well. Here, no significant relation was detected between breeding success and the number of subordinates, extra-pair paternity or the fact that a pair had bred together the previous year.

Subordinate behaviour

We found 27 subordinates that were not related to the breeding female and 96 that were related, resulting in a 78% of related subordinates. Around half the subordinates participated in helping behaviour with 57 non-helpers and 55 helpers. 52 of these helpers were related to the breeding female of the nest they were staying in while only 3 were not related. In our linear model, we found no significant relation between the feeding rate of the male breeder and the number of subordinates that helped with the feeding, with a p-value of 0.257. The feeding rates of the breeding females were significantly related to the amount of help, with the number of helpers decreasing the feeding rate (p-value <0.001). The GLM model predicting whether subordinates helped showed a significant relation with the relatedness of the subordinates to the breeding female. The p-value was <0.001, with logit estimates being -2.63 for the intercept, and 2.52 for the relatedness to the breeder. The chance that a

subordinate helped feeding the young was 0.115 if they were not related and increased to 0.619 if they were related. Of the related individuals 52 helped the breeders while 32 did not. Graph 2: Helping and Relatedness

Helping behaviour and Relatedness 0.8 0.7 0.6 0.5 0.4 0.3

Chanceof helping 0.2 0.1 0 Not Related Related Relatedness

We also checked whether the subordinate was related to the last breeder it was with. From our 137 observed subordinates only 23 were observed as breeders in later years. Overall, we found 11 cases of subordinates becoming breeders and travelling less than 260 meters, 9 of which were related to the breeding female of the group they were in before. 7 subordinates traveled further than 260 meters, 4 of which were related.

We looked at specific cases of subordinates becoming breeders to better understand their behaviour. First, AA10933, the subordinate that traveled the lowest distance. She only moved 28 meters from nest PG0910AM028 to nest PG1314PK053; however there was a time gap from 0910 to 1314, which makes it more difficult to interpret. The subordinate was a female that was not related to the breeding female in 0910 but she did help feed the 2 young of the breeding pair.

Secondly, AA14487, the subordinate that traveled the second lowest distance, 68 meters, from nest PG0809BM001 to nest PG0910AM024. This male was found in multiple years from 0708 to 1415 with a gap between 0910 and 1314. In 0708 the individual was first observed as a juvenile of a breeding pair. In the next year it was a subordinate for the same couple and helped feed the new juveniles. In 0910 the subordinate now became a breeding male together with its mother A91242. After the time gap the now breeding male was found together with a different breeding female and stayed together with her for the years 1314 and 1415. The observation that a son bred with its mother for one year was also found in the study by Bart Kempenaers (2007) (14), where they suggest this might be because the young males are not yet competitive enough.

Discussion

To study fission-fusion dynamics we analyzed a population of placid greenbuls in the Taita Hills, Kenya. Firstly, we analyzed the data visually with a network analysis. We then looked at the behaviour of the main pair of breeding groups, focusing on their decision making for staying with the same partner or divorcing and joining new groups. Lastly, we analyzed the reasons why subordinates delay dispersal or go in search of new groups. When they decided to stay as subordinates of a breeding pair we also looked at why they were allowed to stay and what benefits they themselves could reap.

In our network analysis, we showed that the separate groups of breeding pairs with their subordinates are linked across multiple years to other groups, clearly visualizing the fission-fusion dynamics between groups. We also observed multiple occurrences of birds dispersing from one forest fragment to another and joining a new group there. In comparing the different forest fragments, we found that individuals in NG have more edges on average and more edges between nests. This suggests that not only group sizes in NG are larger, but that there is more movement between groups in comparison to other forest fragments.

Pairs

Pairs that had previously bred together did not have a higher breeding success than those that did not. We thus found no support for the ‘pair familiarity’ hypothesis for the Placid Greenbul. This is in contrast with some other bird species, like bearded reedlings (Griggio et al., 2011) (30), that see an increase in breeding success after the pair had been together in the previous year. In addition, we found that breeding success did not increase the chance of the pair to stay together. Breeding site familiarity was not included in the analysis, which might be more important for this species as it was for the kittiwake (Naves et al., 2007) (6). Since a typical aspect of tropical bird species is the lower amount of territory turnover compared to temperate species, this can also be the case for the Placid Greenbul (Loock et al., 2017) (28). Based on GPS data the groups stay in more or less the same territory, so understanding the role of breeding site familiarity could be key to understanding cooperative breeding in the Placid Greenbul. We did not look at breeder experience, with age as a proxy, used in other studies as a predictor for pair success or divorce, as was done for willow tits (Orell et al., 1993) (31). Both of these variables could be interesting to include in further studies to see if more experienced birds have a higher breeding success chance.

Divorce

We observed a decrease in the chance for the couple to be together in the next year with an increasing number of young resulting from extra pair-copulations. This mirrors the results found in the mallard (F. Cezilly and R. G. Nager,1995) (12). Although we have little data for pairs that separated, with only 10 separations for which we at least one of the breeders was found again with another partner in later years. In 9 out of these 10 separations only the breeding female was found with another partner. This seems to support the better option hypothesis. The female breeders could be assessing possible breeding candidates to check if any are present with a higher status/of higher quality. (Grant B.R,

1987) (32) Instead of looking for better genes/quality males, the females might be looking for males with better territories, in this case they should move after a separation. Alternatively, the males could be divorcing the females because their unfaithfulness, although very few studies exist that observed this behaviour. It is also rather unlikely since the males were almost never found again after divorce. If female breeders were assessing better breeding options, we would expect that they pair with a male with which they had an extrapair copulation. We do not have the data to control for this, however we do have data regarding their territories. We expected females that were looking for better territory, to move to a new territory after separating. In all 10 separations the females moved less than 260 meters and even less than 150m away to pair with their new mate. The average travel distance was just 78 meters. There also did not seem to be a real difference in the number of successfully raised young between the old and new pair. To summarize, the females stayed in their own territory and did not increase their breeding success with their new partner. This goes against the better option hypothesis as divorcing would require some sort of benefit. It is still possible that the female breeders were trying to acquire better genes while they were staying with the previous males. Outbreeding can also be a way to prevent inbreeding, however it would be more likely if the females dispersed after a separation to do this. It could be that no actual divorce takes place, but the male breeders are either competed out and don’t breed afterwards or they are killed by predation. The decreased change for couples to stay together might be because the male is weaker and less able to defend its territory from other males and thus an increased amount of EPC can take place. Since the Placid Greenbul tends to be rather territorial and breeders stay within their territory year after year, the importance of the territory should not be underestimated.

Although the few observed separations would in this case be female chosen, male initiated divorce could still be possible. In A. Culina et al., 2015 (33) males paired with lower ranked females and were more likely to divorce, suggesting that the decision to divorce was made far before the actual divorce itself. Assessing breeders at the time they come together could provide insight into this.

Delay

When checking for reasons for helping behaviour, we looked at delayed dispersal of the subordinates and the possible benefits they can have. We found that most subordinates were related to the breeding female of the territory they stayed in, suggesting that they delayed dispersal to stay there. The reason for this delay could be either because off a shortage of nesting/breeding opportunities (Stacey, P. B. et al., 1991) (34)or because of other benefits to staying within the natal territory. One possible benefit could be inheriting the natal territory. (Koenig et al., 1998) (35) Half of the observed subordinates that became breeders in later years stayed within their natal territories, and the majority were related to the previous breeding pair. This suggests they inherited the territory from the previous breeders. A better position for the breeding position would be a clear benefit of delaying dispersal. As mentioned previously, we found a case where a young male bred with its mother for one year. This is also a way that can make staying within the natal area beneficial. In this case the young male might not have been able to compete for breeding areas and stayed in its natal territory. This behaviour was also observed in Savannah sparrows (Wheelwright et al., 2006) (15).

To check for other possible benefits for delaying dispersal it would be beneficial to observe interactions between breeders and subordinates. In studies about the siberian jays (Ek 1994) for instance, the dominant pair were aggressive against non-related subordinates in the feeding area. Related subordinates were released from such pressure, giving them a clear reason to stay in the natal territory. By observing placid greenbuls at their feeding sites either with camera’s or in person we could see if this is also the case for them.

Inclusive fitness Although it seems clear that our results support the direct fitness hypothesis we also have evidence for the inclusive fitness hypothesis. (Hamilton WD., 1963 (36)) The subordinates were far more likely to help the breeding pair if they were related to the breeding female. This mirrors results found in other species like the promiscuous fairy wrens (S.A. Kingma et al 2010) (23). Yet, we found no evidence that helping behaviour increased breeding success which would increase the helper’s inclusive fitness. (Jessica Meade et al., 2010) (37) Helping also did affect the feeding rates of the breeding. With an increasing number of helpers the breeding females reduced their feeding rates. We see this as another way of increasing inclusive fitness as other studies linked a decrease in female breeder female rates with a higher survival chance and thus higher lifetime breeding success. (Kingma et al., 2010) (23) Although relatedness has a clear effect on helping behaviour in our study, we would advise other studies to also look at the lifetime breeding success of breeding females.

We found no clear evidence for the pay-to-stay hypothesis, with only half our subordinates helping the breeding pair. Most helpers were also related to the breeding pair so inclusive fitness seems the more likely reason for helping behaviour.

All together our results show the subordinates gain benefits by staying in a group in two ways. Firstly, there is a direct benefit for them, when they tend to delay dispersal and stay in their natal territory most likely to be either protected by their parents, to inherit the territory or to wait for good breeding opportunities. The second benefit is indirect, the subordinates tend to help feed the young of the breeders if they are related. The female breeders lowering their feeding rates should result in an increase in their lifetime breeding success. Looking at the further live history of the young that were helped by the subordinates might reveal other possible indirect benefits as this help could have benefits that only become clear later on in the lifecycle. This was the case in wolf pups (. Sparkmanet al., 2010) (38).

Conclusion

The main source of fission-fusion processes are the dispersing subordinates. They join groups and by doing so link breeding groups as well as forest fragments together. By dispersing they try to acquire better breeding positions and territory. The main things they have to consider are the benefits of staying within their natal territory versus going to other non-related groups where they might acquire better breeding positions. Young birds choosing to delay dispersal can later inherit their natal territory. The main reason for the delay seems to be the limited available territory and breeding positions. The costs of delaying dispersal can be decreased by aiding, gaining indirect

benefits by helping the breeding pair. This mainly benefits the breeding female and is mostly done if the subordinates are related to her. Divorce of the breeding pair is rather rare. But the pair splitting up is related to the amount of extra- pair young. It is not entirely clear whether actual divorces are happening since in most cases the partners were not found again. It could very well be that they were outcompeted or killed. In the rare cases were we found one breeder with another mate they stayed within their previous territory so territory limitations seem to be a determining factor here aswell.

SUMMARY: FISSION-FUSION DYNAMICS IN THE FACULTATIVE COOPERATIVE BREEDING PLACID GREENBUL (PHYLLASTREPHUS PLACIDUS)

We set out to analyse fission-fusion dynamics in the cooperatively breeding Placid greenbul, Phyllastrephus placidus. Fission-fusion dynamics refer to processes by which groups of animals form and split up. Either completely or partly changing the affected group. We focused on two aspects of the Placid Greenbul groups, the breeders and the subordinates. For the breeding pair we focussed on the processes which keep them together or separate them. While for subordinates we focused on their reasons for dispersal as well as decision making in helping behaviour. All analysis was done using R-studio. Our research area consisted of the forest fragments of the Taita Hills, South-East Kenya. These cloud forests are located on two mountain isolates separated by a low-altitude valley. It is part of the northernmost range of the Eastern Arc Mountains biodiversity hotspot and is isolated on all sides by low altitude savannah. There are three large forest fragments: Chawia (CH), Ngangao (NG) and Mbololo although the latter is isolated from the others, as well as several smaller fragments. The 430 ha of indigenous forest have been pressured severely by local agriculture We examined fission-fusion dynamics in the Placid greenbul because we had both temporal data on pair stability as well as data on their social group dynamics. Phyllastrephus placidus is a facultative cooperatively breeding bird that inhabits cloud forests from Central to East Africa. It lives in small families consisting of a breeding pair and up to 5 subordinates, 0 to 3 of which actively help the breeding pair in raising the nestlings. Their breeding seasons starts at the onset of short rain season in mid-October and lasts until the end of March. They tend to care for clutches of 2 eggs for around 17 days. Afterwards it takes around 13 days for the young to become fledglings. We obtained data from 9 forest fragments (namely CH, FU, NG, MA, MS, SU, YA, ND) for the years 2007-2010 and 2012-2016. For the data of the years 2012-2016 we have information on the total group size, helping and non-helping subordinates included, based on video recording. For the other years we do not have information on the non-helping subordinates. In total we collected data for 795 individual observations over 443 different nests. In our dataset we counted 237 different pairs from 332 nest observation. We observed the total amount of birds in a nest, as well as the function of the birds, ranging from male and female breeders, helping and non- helping subordinates and young. For each year we also observed variables for the breeding pair at the nest. Extra pair paternity was checked for in two ways. Firstly, we observed whether at least one young was the result of extra-pair paternity, EPB. Secondly, the observed number of young resulting from extra pair breeding attempts within a nest, NEPY. We measured breeding success in 3 ways. Firstly, as the number of young that were successfully fledged. Secondly, whether the breeding pair successfully raised at least one young or not, “Failure”. And lastly, partial failure (FailureP), whether any young died or not. We found 6 complete breeding failures, with 115 complete successes, and 16 partial failures, with 105 complete successes.

In a first step we visually explored the different breeding groups spread over the different forest fragments using network theory. Network theory has a relatively long history but has not been used for understanding animal behaviour for that long. Simply put network theory tries to acquire understanding of behaviour and its effects through observing the network qualities in specific. For example, which individuals are key points in the network, how do the interactions between individuals change over time and so on. We found that the nests in NG were the most connected to other nests within that forest fragment. Individuals in NG also had more edges on average. With NG being the largest forest fragment, this was expected. More interestingly we clearly observed movement of individuals between the fragments connecting them over multiple years. We also visualized a specific divorce and juveniles becoming subordinates in their natal territory.

As a second step, we focused on pair dynamics and tested several hypotheses. First, we tested the ‘mate familiarity effect’ hypothesis which predicts an increase in breeding success with pair familiarization. Hence, a pair that has bred before should have an increased breeding success. Our findings did not support this hypothesis as pairs previously bred together did not have an increased breeding success. Secondly, we tested the ‘pair incompatibility’ hypothesis. Following this hypothesis it is expected that pairs divorce after a failed breeding attempt. Or in other words if they are not compatible with each other. Again, we found no support for this hypothesis in the placid greenbul. Thirdly, we examined the ‘better option’ hypothesis which roughly predicts that a breeder breeds with outside the pair to access his/her options and divorces if they find a better partner. The partner that remarries should also increases his/her breeding success with this new partner. We found conflicting evidence for this theory. We found significant evidence that shows that divorce is more likely if a higher number of young resulting from extra-pair paternity were present. We did not have the necessary data to check whether the partners remarry with the individual they were infidelity with. Also, we found very few cases where one of the partners was sighted again but with a new partner, only 10 cases. 9 of these 10 cases only the female breeders were observed in later years and they were classified as separations. In the one remaining case the male was observed as well, and we classified this as a divorce. In the separations we were not certain that the male did not just die. In the 9 separations the females did not increase their breeding success which goes against the better option hypothesis. In all 10 cases the remarrying females did not move more than 150 meters, so we assumed they stayed within their previous breeding territory. Thus, they were also not looking for better territory. Since most tropical bird species tend to have less territory turnover than temperate species, we suggest that separations or divorces are caused by either the death of the other partner or them being outcompeted and not being able to remarry. A higher amount of NEPY might be caused by the male breeder not being able to fend of other males. As a last step we analysed the subordinate’s behaviour. We wanted to know why subordinates stay within their natal dispersal and why they disperse. We also examined the helping behaviour that some subordinates showed, when they fed the young of the breeding pair. We started by testing whether the subordinates could gain a direct benefit. We observed that over half the subordinates inherited the natal territory when they became breeders and most of those inheriting the territory were related to the breeding par. Delaying dispersal has a clear benefit in the chance to inherit territory. And dispersing subordinates also have the option of inheriting territory in other groups. Next, we tested the ‘inclusive fitness’ hypothesis, which predicts that subordinates are more likely to help others if they are more related to them. Almost all the helping subordinates were related to the breeding female and thus had a clear motivation to help. We also looked at whether this feeding behaviour affected the breeders. Male breeders did not adjust their feeding rates, but female breeders reduced their feeding rates with more helpers present. Helpers got inclusive fitness by allowing the female breeders to focus less on feeding and more on survival, which increases their lifetime breeding success. Lastly, we tested for evidence for the ‘pay-to-stay’ hypothesis. It is also possible that the parents force the subordinates, especially the unrelated ones, to help them as payment for staying in their territory and the benefits it entails. As most unrelated subordinates did not feed the young it would seem they are not forced to help. We would advise further research to also observe the placid greenbul in their feeding areas as it might be possible that the non-related subordinates are treated less well as has been observed in other bird species. We also observed a special case, as the one observed divorce had the breeding female staying in her territory and breeding for one year with her son would was born there, became a subordinate, her breeding mate and later mated with another female. We suggest that this constitutes as another direct benefit for birds to delay their dispersal. As previously mentioned, tropical species tend to have less territory turnover than temperate species. With more limited territory and thus possible breeding access for young birds, delaying dispersal could be an attempt to gain as much benefit as possible without breeding. Similarly, territory also seemed to be very important for the female breeders during separations, as they remained in their old territory. Assessing

available habitat and the habitat quality could provide more insights into the decision-making processes of the placid greenbul. To conclude, the main source of fission-fusion processes are the dispersing subordinates. They join groups and by doing so link breeding groups as well as forest fragments together. By dispersing they try to acquire better breeding positions and territory. The main things they have to consider are the benefits of staying within their natal territory, like inheritance, versus going to other non-related groups where they might acquire better breeding positions. The main reason for the delay seems to be the limited available territory and breeding positions. The costs of delaying dispersal can be decreased by aiding gaining indirect benefits by helping the breeding pair. This mainly benefits the breeding female and is mostly done if the subordinates are related to her. Divorce of the breeding pair is rather rare. But the pair splitting up is related to the amount of extra pair young. It is not entirely clear whether actual divorces are happening since in most cases the partners were not found again. It could very well be that they were outcompeted or killed. In the rare cases where we found one breeder with another mate, they stayed within their previous territory, so territory limitations seem to be a determining factor here as well.

SAMENVATTING: FISSION-FUSION DYNAMICS IN THE FACULTATIVE COOPERATIVE BREEDING PLACID GREENBUL (PHYLLASTREPHUS PLACIDUS)

Het doel van deze thesis was het analyseren van de ‘fission-fusion’ dynamiek in de ‘cooperatively breeding’ placid greenbul, Phyllastrephus placidus. Kort samengevat omvat ‘fission-fusion’ processen die groepen van dieren vormen en opsplitsen. Deze processen veranderen of ontbinden de groep compleet. De focus ligt op twee specifieke delen van de placid greenbul nest-groep, de subordinaten en de broedende vogels. Voor het koppel bekeken we de processen die hun samenhouden of opsplitsen. Terwijl we voor de subordinaten focusten op de redenen om het nest te verlaten en zich te verspreiden. Ook het proces om te kiezen om mee te helpen werd onder de loep genomen. Alle analysen werden met behulp van R-studio uitgevoerd. De regio van ons onderzoek bestond uit de bosfragmenten van de Taita Hills, Zuid-Oost Kenya. Deze ‘cloudforests’ bevinden zich tussen twee bergen gescheiden door een lage-altitude vallei. Het is een deel van de noordelijke Oostelijk Arc Mountains biodiversiteit hotspot en is geïsoleerd aan alle kanten door een lage altitude savannah. Er zijn drie grote bosfragmenten. Chawia (CH), Ngangao (NG) en Mbololo, hoewel die laatste geïsoleerd is van de anderen alsook van de vele kleinere fragmenten. Het 430 ha bos wordt zeer zwaar belast door de lokale agricultuur. Er werd gekozen voor de placid greenbul omdat we zowel temporele data hadden voor ‘pair-stability’ alsook data voor de dynamieken van de sociale groep. De placid greenbul is een facultatief coöperatief broedende vogel die de ‘cloudforests’ van Centraal tot Oost-Afrika bewoont. Ze leven in kleine families, van een broedend paar met tot vijf subordinaten. Waarvan sommigen (0-3) ervan actief helpen met het grootbrengen van de jongen. Hun broedseizoenen starten bij het begin van het korte regenseizoen in midden oktober en duren tot het eind van maart. Ze zorgen meestal voor clutches van twee eieren en dit voor ongeveer 17 dagen. Nadien duurt het ongeveer 13 dagen totdat de jongen veren krijgen. We hebben data voor 9 bosfragmenten, namelijk CH, FU, NG, MA, MS, SU, YA, ND, voor de jaren 2007-2010 en 2012-2016. Voor de data van de jaren 2012-2016 hebben we informatie over de totale groep-grootte, helpende en niet helpende subordinaten geïncludeerd, en dit gebaseerd op video-recording. Voor de andere jaren hebben we geen informatie voor de niet-helpende subordinaten. In totaal hebben we data verzameld van 795 individuele observaties over 443 verschillende nesten. In onze dataset hebben we 237 verschillende paren van 332 nesten geobserveerd. Voor elke observatie hebben we het ring-nummer van de vogel en z’n functie. Gaande van mannelijke of vrouwelijke broeders, helpende en niet-helpende subordinaten en de jongen. Voor elk jaar hebben we ook aparte variabelen geobserveerd voor het broedend paar. ‘Extra pair paternity’ werd op twee manieren gecontroleerd. Enerzijds of er een jong aanwezig was die het resultaat was van ‘extra pair copulations’(EPC’s). Anderzijds als het aantal jongen die het resultaat waren van EPC aanwezig waren in

het nest, NEPY. Breeding succes werd gemeten op drie manieren. Eerst, aan de hand van het aantal jongen die succesvol grootgebracht werden. Als tweede, ‘complete failure’, of het paar minsten 1 jong succesvol grootgebracht had of niet. Als derde, ‘partial failure’, of een jong gestorven was of niet. We vonden 6 cases met complete faling tegenover 115 met deelse successen. Anderzijds vonden we 16 deelse falingen met 105 complete successen.

Als de eerste stap van onze analyse hebben we de dataset visueel geëxploreerd door gebruik te maken van ‘network theory’. Network theory heeft een lange geschiedenis maar is nog niet zo lang in gebruik bij de analyse van ‘animal behaviour’. Het probeert simpelweg gedrag en de consequenties ervan te begrijpen door observaties te maken van de kwaliteiten van het netwerk van dieren. Bijvoorbeeld, welke individuen er een belangrijke positie in het netwerk behouden, hoe interacties tussen individuen veranderen over tijd etc.. Wij observeerden dat de nesten in NG het meest geconnecteerd waren met andere nesten in dat bosfragment. Individuen in NG hadden, gemiddeld, ook meer ‘edges’ ? met andere individuen. Dit was te verwachten aangezien NG het grootste bosfragment is. We zagen ook dat individuen de verschillende bosfragmenten verbinden over de loop van verschillende jaren. We observeerden ook paren die opsplitsen en juveniele vogels die subordinaten werden in hun geboorte territorium. Als tweede stap focusten we op de gedragingen van het paar en testen we verschillende hypotheses. Eerst testen we de ‘mate familiarity effect’ hypothese die een stijging voorspelt in het broedsucces gepaard gaande met de familiariteit van het paar. Een paar dat voordien tezamen gebroed heeft moest dus een betere kans op succes hebben. Onze data bevestigde deze hypothese niet, onze paren hadden geen hogere succeskans. Ten tweede testen we de ‘pair incompatibility’ hypothese. Volgens deze hypothese zouden paren scheiden na een gefaalde broed poging. Dus als ze niet compatibel zijn met elkaar. Hier vonden we ook geen support voor. Ten derde bekeken we de ‘better option’ hypothese die voorspelt dat een ‘breeder’ paart met vogels buiten het koppel om de opties die het heeft, te bekijken en mogelijks te scheiden als een betere partner gevonden werd. De partner die van partner verandert, moet dus in het broedsucces een stijging hebben met de nieuwe partner. Wij hadden tegenstrijdig bewijs voor deze theorie. Een koppel had minder kans om het volgende jaar tezamen te zijn als er meer jongen afkomstig van ‘extra pair paternity’ aanwezig waren in het nest. We hadden niet de nodige data om te kijken of de partners zich herenigden met degene waar ze ontrouw mee waren geweest. We vonden ook weinig cases waar 1 van de partners achteraf teruggevonden werd met een nieuwe partner, slechts 10 cases. In 9 van deze 10 cases was het de vrouwelijke partner die het volgende jaar met een nieuwe partner was, we classificeerden deze cases als ‘separations’. In de ene overige case was het mannetje ook geobserveerd en we classificeerden dit als de enige echte scheiding. In die 9 separations was het niet zeker of het mannetje niet gewoon gestorven was. In geen enkele van deze 10 cases werd het broedsucces verhoogd wat tegen de ‘better option theorie’ ingaat. In alle 10 cases gingen de vrouwtjes niet verder dan 150 meter van hun vorige nest en nemen we aan dat ze in hun vorige territorium bleven. Dus ze waren ook niet op zoek naar een beter territorium. Aangezien de meeste tropische vogelsoorten meestal minder ‘territorium turnover’ hebben dan gematigde klimaat soorten, denken we dat de separaties en scheidingen veroorzaakt worden door of, de dood van 1 van de partners of doordat die weg geconcureerd worden en zich niet meer verbinden met een andere partner. Een hoge hoeveelheid NEPY kan veroorzaakt zijn doordat het mannetje niet in staat is om andere mannetjes weg te jagen. Als laatste stap hebben we het gedrag van de subordinaten geanalyseerd. We wilden weten waarom ze in hun geboorte territorium bleven en waarom ze zich verspreiden. We keken ook naar het hulpgedrag dat sommigen vertoonden. We starten met het testen, of de subordinaten een direct voordeel konden krijgen door te blijven. Ongeveer de helft van de subordinaten erfden het territorium waar ze voordien in verbleven als ze ouders werden en de meesten hiervan waren gerelateerd aan het vorige paar. Dus dispersie uitstellen heeft een mogelijks direct voordeel. Disperserende subordinaten kunnen ook zoeken naar territorium waar ze kunnen erven. Hierna testen we de ‘inclusive fitness’ hypothese, die voorspelt dat subordinaten zullen helpen als ze gerelateerd zijn aan het ouderlijk paar. Bijna alle helpende subordinaten waren gerelateerd aan het vrouwtje

en hadden dus een duidelijke motivatie om te helpen. We keken ook hoe dit helpgedrag de ouders beïnvloede. Mannetjes bleven even veel voeden als voordien maar de vrouwtjes begonnen minder te voeden naarmate er meer helpers waren. Helpers kregen ‘inclusive fitness’ door de vrouwtjes zich meer te laten focussen op overleving in plaats van het grootbrengen van de jongen, waardoor hun ‘lifetime breeding succes’ verhoogd kan worden. Als laatste testen we de ‘pay-to stay’ hypothese. Het is mogelijk dat de ouders de subordinaten forceren om te helpen, zeker de niet gerelateerde, als voorwaarde om in het territorium te kunnen blijven. Aangezien de meeste niet-gerelateerde subordinaten niet hielpen werden ze dus niet gedwongen. We adviseren om in toekomstig onderzoek, de placid greenbul te observeren in de omgeving waar ze foerageren om te kijken of daar de niet-gerelateerde subordinaten minder goed behandeld worden, zoals werd geobserveerd bij andere vogelsoorten. In de ene echte scheiding observeerden we dat het broedend vrouwtje in haar territorium bleef en voor één jaar tezamen was met haar zoon. De zoon werd daar geboren, werd een helper en broedde een jaar met zijn moeder om daarna met een ander vrouwtje van start te gaan. Dit is mogelijks een compensatiegedrag, voor de gelimiteerde mogelijkheid aan goed territorium en broedmogelijkheden. Zoals voordien vermeld, tropische soorten hebben meestal minder territorium turnover. Met de gelimiteerde hoeveelheid territorium en dus mogelijke ‘breeding access’ voor jonge vogels, kan het uitstellen van dispersie een poging zijn om zoveel mogelijk voordeel te halen zonder te broeden. Bij de vrouwtjes die opnieuw een koppel vormden, was territorium ook zeer belangrijk aangezien ze hun oude territorium hielden. Beschikbare habitat en de kwaliteit ervan inschatten kan meer inzicht verschaffen in het beslissingsproces van de placid greenbul. Onze conclusies: De hoofdbron van fission-fusion processen zijn de disperserende subordinaten. Ze gaan naar nieuwe groepen en linken zo de verschillende nesten alsook de bosfragmenten. Door te disperseren proberen ze betere broedposities en territorium te krijgen. De grootste overweging die ze moeten maken zijn de mogelijke voordelen die ze kunnen krijgen als ze in het ouderlijk territorium blijven, zoals erven, tegenover het disperseren naar niet-gerelateerde groepen waar ze mogelijks een betere positie kunnen krijgen. De hoofdreden voor delay is de gelimiteerde toegang tot territorium en broedposities. De kosten van delay kunnen verminderd worden door het verkrijgen van indirecte voordelen door het hoofdkoppel te helpen. Dit is hoofdzakelijk voordelig voor het vrouwtje en wordt meestal door gerelateerde helpers uitgevoerd. Echte scheidingen zijn redelijk zeldzaam. Hoewel het duidelijk is dat een koppel minder kans heeft om tezamen te zijn afhankelijk van hoeveel NEPY, is het niet duidelijk wat de achterliggende processen hiervan zijn aangezien meestal de partners niet teruggevonden werden. Het zou kunnen, dat ze weg gecompeteerd werden of gedood zijn. In de zeldzame gevallen dat één van de broeders toch een andere partner had gevonden bleven ze in hun vorige territorium en verhoogde hun succes niet. Territorium limitatie lijkt hier dus ook een belangrijke factor.

Acknowledgements I’d like to thank my promoter professor Luc Lens and my supervisor MSc Laurence Cousseau for guiding me through my thesis. As well as Myriam van Imschoot for spellchecking.

References • Amanda M. Sparkman, Jennifer Adams, Arthur Beyer, Todd D. Steury, Lisette Waits and Dennis L. Murray. 2010. "Helper effects on pup lifetime fitness in the cooperatively breeding red wolf (Canis rufus)." (The Royal Society) Online. (ref1)

• Amy E. Leedale, Stuart P. Sharp, Michelle Simeoni, Elva J. H. Robinson and Ben J. Hatchwell. 2018. "Fine-scale genetic structure and helping decisions in a cooperatively breeding bird." (Molecular Ecology) 27. (ref2) • Antica Culina, Camilla A. Hinde and Ben C. Sheldon. 2015. "Carry-over effects of the social environment on future divorce probability in a wild bird population." (the Royal Society) 282. (ref3) • Barbara Taborsky and Michael Taborsky, 1999. "The Mating System and Stability of Pairs in Kiwi Apteryx spp." (Journal of Avian Biology) 30 (2). (ref35) • Bruno J. Ens, Uriel Safriel and Mike P. Harris. 1993. "Divorce in the long-lived and monogamous oystercatcher, Haematopus ostralegus: Incompatibility or choosing the better option?" (Animal Behaviour) 45 (6). (ref5) • Butts, Carter T. 2008. "network: A Package for Managing Relational Data in R." (Journal of statistical software) 24. (ref6) • C. Lazaro-Perea, C. S. S. Castro, R. Harrison, A. Araujo, M. F. Arruda, C. T. Snowdon. 2000. "Behavioral and demographic changes following the loss of the breeding female in cooperatively breeding marmosets." (Behavioral Ecology and Sociobiology) 48 (2). (ref7) • C. M. Perrins and R. H. McCleery, 1985. "The effect of age and pair bond on the breeding success of Great Tits Parus major." (Ibis) 127 (3). (ref26) • Cockburn, Andrew. 2006. "Prevalence of different modes of parental care in birds." (Proc Biol Sci) 273 (1592). (ref8) • Dries Van de Loock, Diederik Strubbe, Liesbeth De Neve, Mwangi Githiru, Erik Mattysen and Luc Lens. 2017. "Cooperative breeding shapes post-fledging survival in an Afrotropical forest bird." (Ecology and Evolution) . (ref9) • Frank Cezilly and Ruedi G. Nager, 1995. "Comparative evidence for a positive asociation between divorce and extra-pair paternity in birds." (The Royal Society) 262. (ref27) • Fumiaki Y. Nomano, Lucy E. Browning, James L. Savage, Lee A. Rollins, Simon C. Griffih and Andrew F. Russell. 2015. "Unrelated helpers neither signal contributions nor suffer retribution in chestnut-crowned babblers." (Behavioral Ecology) 26 (4). (ref10) • Grant, B.R. & Grant, P.R. 1987. "Mate choice in Darwin's finches." (Biological Journal of the Linnean Society) 32. (ref11) • Hamilton, W. D. 1963. "The Evolution of Altruistic Behavior." (The American Naturalist) 97 (896). (ref13) • Jan Ekman and Michael Griesser. 2002. "Why offspring delay dispersal: experimental evidence for a role of parental tolerance." (Royal Society) 269 (1501). (ref12) • Jessica Meade, Ki‐Baek Nam, Andrew P. Beckerman and Ben J. Hatchwell. 2010. "Consequences of ‘load‐lightening’ for future indirect fitness gains by helpers in a cooperatively breeding bird." (Journal of Animal Ecology) 79 (3). (ref15) • Kari Koivula, Kimmo Lahti, Markku and Orell Seppo Rytkönen, 1993. "Prior residency as a key determinant of social dominance in the willow tit (Parus montanus)." (Behavioral Ecology and Sociobiology) 33 (4). (ref32) • Kempenaers, Bart. 2007. "Mate Choice and Genetic Quality: A Review of the Heterozygosity Theory." (Advances in the Study of Behavior) 37. (ref16)

• Koenig WD, Haydock J, Stanback MT. 1998. "Reproductive roles in the cooperatively breeding acorn woodpecker: incest avoidance versus reproductive competition." (The American Naturalist) 151 (3). (ref17) • Liliana C. Naves, Emmanuelle Cam, Jean Yves Monnat. 2007. "Pair duration, breeding success and divorce in a long-lived seabird: benefits of mate familiarity?" (Animal Behaviour) 73. (ref20) • Lyanne Brouwer, Martijn van de Pol and Andrew Cockburn. 2014. "The role of social environment on parental care: offspring benefit more from the presence of female than male helpers." (Journal of Animal Ecology) 83. (ref21) • Markku Orell, Seppo Rytkönen and Kari Koivula. 1994. "Causes of divorce in the monogamous willow tit, Parus montanus, and consequences for reproductive succes." (Animal Behaviour) 48. (ref22) • Martha J. Nelson-Flower, Elizabeth M. Wiley, Tom P. Flower and Amanda R. Ridley. 2018. "Individual dispersal delays in a cooperative breeder: Ecological constraints, the benefits of philopatry and the social queue for dominance." (Journal of Animal Ecology) 87. (ref23) • Matteo Griggio and Herbert Hoi. 2011. "An experiment on the function of the long-term pair bond period in the socially monogamous bearded reedling." (Animal Behaviour) 82 (6). (ref14) • Matteo Griggio, Herbert Hoi. 2011. "An experiment on the function of the long-term pair bond period in the socially monogamous bearded reedling." (Animal Behaviour) 82. (ref24) • Matthew J. Silk, Darren P. Croft, Tom Tregenza & Stuart Bearhop. 2014. "The importance of fission–fusion social group dynamics in birds." IBIS 701-715. (ref25) • Oscar Sánchez-Macouzet, Cristina Rodríguez and Hugh Drummond. 2014. "Better stay together: pair bond duration increases individual fitness independent of age-related variation." (Royal Society) 281 (1786). (ref28) • Pär Forslund, Kjell Larsson. 1991. "The effect of mate change and new partner's age on reproductive success in the barnacle goose, Branta leucopsis." (Behavioral Ecology) 2. (ref29) • Peter B. Stacey and J. David Ligon; 1991. "The Benefits-of-Philopatry Hypothesis for the Evolution of Cooperative Breeding: Variation in Territory Quality and Group Size Effects." (The American Naturalist) 137 (6). (ref19) • Pia O. Gabriel & Jeffrey M. Black; 2012. "Correlates and Consequences of the Pair Bond in Steller's Jays." (Ethology) 119. (ref4) • Ralph Bergmüller, Dik Heg and Michael Taborsky. 2005. "Helpers in a cooperatively breeding cichlid stay and pay or disperse and breed, depending on ecological constraints." (The Royal Society) 272. (ref30) • Ridley, Elizabeth M. Wiley and Amanda R. 2018. "The benefits of pair bond tenure in the cooperatively breeding pied babbler (Turdoides bicolor)." (Ecology and Evolution) 8. (ref31) • S. G. Pruett-Jones & M. J. Lewis; 1990. "Sex ratio and habitat limitation promote delayed dispersal in superb fairy-wrens." (Nature) 348. (ref18)

• Sjouke A. Kingma, Kat Bebbington, Martijn Hammers, David S. Richardson and Jan Komdeur. 2016. "Delayed dispersal and the costs and benefits of different routes to independent breeding in a cooperatively breeding bird." (Evolution) 70. (ref33) • Sjouke A. Kingma, Michelle L. Hall, Elena Arriero and Anne Peters. 2010. "Multiple benefits of cooperative breeding in purple-crowned fairy-wrens: a consequence of fidelity?" (Journal of Animal Ecology) 79. (ref34) • Toon Spanhove, Tom Callens, Caspar A. Hallmann, Petri Pellikka and Luc Lens. 2014. "Nest predation in Afrotropical forest fragments shaped by inverse edge effects, timing of nest initiation and vegetation structure." (J Ornithol) 155. (ref36) • Wheelwright, N. T., Freeman‐Gallant, C. R., and Mauck, R. A. 2006. "Asymmetrical incest avoidance in the choice of social and genetic mates." (Animal Behavior) 71. (ref37) • Wosegien, Angelika. 1996. "Experiments on pair bond stability in domestic pigeons (Columba Livia domestica)." (Behaviour) 134. (ref38)

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