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2017 Brown-headed Nuthatch Winter Social Dynamics and Predictability of Extra-Pair Helpers at the Nest Chente Ortiz

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[Brown-headed Nuthatch Winter Social Dynamics and Predictability of Extra-Pair

Helpers at the Nest]

By

[CHENTE ORTIZ]

Thesis submitted to the Department of Earth, Ocean, and Atmospheric Sciences is in partial fulfillment of the requirements for graduation with Honors in the Major

Degree Awarded: [Spring 2017]

The members of the Defense Committee approve the thesis of Chente Ortiz,

defended on April 6, 2017.

Signatures are on file with the Honors Program office

Dr. Jefferey P. Chanton: Thesis Director

James A. Cox: Outside Committee Member

Dr. Mariana Fuentes: Committee Member

Dr. Emily H. DuVal: Committee Member

Brown-headed Nuthatch Winter Social Dynamics and Predictability of Extra-Pair Helpers at the Nest

1, 2 1 2 Chente Ortiz ,​ James A. Cox ,​ Emily H. DuVal ​ ​ ​

1 2 Tall​ Timbers Research Station, ​ Florida State University, Dept. Biology ​

Abstract

Cooperatively breeding species often have complex group dynamics, where behavior can be heavily influenced by social context. Analyzing the details in these relationships may help unravel the intricate mechanisms underlying cooperative breeding behavior. I document group composition in the non-breeding season in 10 large social groups, and the effect group size (a social element) may have on territorial aggression and juvenile dispersal decisions in the Brown-headed Nuthatch (Sitta pusilla). Eighty percent of groups retained ≥ 1 natal young as late ​ ​ as five months post-fledging, though only 20-30% of nests contain extra-pair members in the spring, suggesting dispersal decisions for some individuals occur later in the year. Group size varied between observations, ranging from zero-nine birds for a single territory. I found that larger groups were proportionally more likely to exhibit territoriality. Comparing predictions of two hypotheses, “safety in numbers” vs. “increased vigilance”, I identify possible social mechanisms that may increase aggression in larger groups. Finally, to understand the social mechanisms that drive helper abundance and distribution, I test 1) the relationship between group size and recruitment of helpers, 2) how individual aggression scores relate to helper status for yearling juveniles, and 3) the relationship between nestling weight and helper status. I found no significant influence from these characteristics on helper status, indicating that further work is still needed to identify the factors leading to helper status in first year juveniles. This work emphasizes social context in animal behavior and highlights a sparsely explored portion of the Brown-headed Nuthatch life cycle. Introduction

Cooperative breeding in the Brown-headed Nuthatch (Sitta pusilla) was first documented by ​ ​ Norris (1958). This behavior has been expanded on by Cox and Slater (2007), which has provided an excellent source of statistical analysis. During the breeding season most nests are typically tended by two adults that make up the breeding pair. Other nests (ca. 20-30%) are tended cooperatively, by breeding groups that include a breeding pair and a single helper (Cox and Slater 2007). Infrequently, cooperative breeding groups can contain up to three helpers in addition to the breeding pair. Such helpers provide food for nestlings and help to defend territories, but they do not appear to breed directly with either of the mated individuals (Cox and Slater 2007). Nuthatches spend most of their time high in the tops of pines, which makes observational studies challenging. However, during the spring, nuthatches nest very low to the ground in dead pine snags (Norris 1958). Perhaps because of this, early research has focused on the breeding season, typically late February to early April, (noted in Norris 1958; Miller and Jones 1999) when individuals are more easily observed (Withgott and Smith 1998). This project focuses on behaviors outside of the breeding season, which are necessary to explain the mechanisms behind well-documented behaviors observed during the breeding season, such as extra-pair helping.

Territoriality: an important aspect of group living BHNU’s

Brown-headed Nuthatches are non-migratory (Withgott and Smith 1998), socially monogamous, and maintain pair-bonds across multiple breeding seasons (Cox and Slater 2007). This suggests that territorial defense is essential to maintaining year-round territories for pre/post breeding season activities. I test assumptions from two hypotheses on the role of social grouping in territorial encounters. The “Safety in Numbers” hypothesis assumes that a threat is spread proportionately among group members, and that the threat is lessened for the individual as the group size increases. This effect is a common idea in behavioral studies, originally the “dilution effect” coined by Hamilton 1971 to provide an explanation for aggregation in large groups. The “Increased Vigilance” hypothesis stems from the idea that group vigilance increases while individual vigilance decreases in large groups, an observation documented in Powell 1974. Assuming that if individual vigilance is decreased, then individuals must be communicating information amongst themselves in a way that the overall vigilance of the group increases with group size. These hypotheses suggest that threats are decreased, and the specific threats more easily identified, in larger social groups. I predict that a response to territorial intrusion by conspecifics will follow these predictions because there is risk associated with responding to intraspecific aggressors (injury, territory loss, etc), that is lessened in larger groups, and the intruder is detected more easily via increased group vigilance. Based on these assumptions response to playback should increase proportionately with group density.

Cooperation in social groups

True altruism is a hard sell; individuals should act selfishly to increase their own reproductive success (As noted by Darwin). Fitness benefits helpers might accrue in cooperatively breeding species are the subject of ongoing debate. Cockburn (1998) listed a variety of mechanisms by which cooperative birds may benefit from helping behavior. These include territory inheritance, access to nesting group resources, and even the possibility that helped individuals will one day become helpers themselves and return the favor. Cox and Slater (2007) noted that the costs and benefits of helping in the Brown-headed Nuthatch are still largely unknown. More recently, blood work has shown that, although nuthatches are socially monogamous, extra-pair copulations are not uncommon (Han et al 2015). This means extra-pair helpers may accrue fitness benefits by mating with the monogamous pair. I test for several factors in the non-breeding season that may influence resulting helper positions the following spring.

Seasonal variation in nuthatch group composition

As a field technician at the Tall Timbers Research Station (TTRS) in North Florida, I observed large groups of nuthatches (ranging from 4-8 individuals) following the spring breeding season. These groups include parents and offspring but also unrelated individuals that join the groups following juvenile dispersal events. The presence of young that remained in natal territories and other individuals that have dispersed create situations with complex social alliances and interactions. The decision making process for these individuals often leads to some groups having significantly more members than others. The presence of these large groups is interesting because Cox and Slater (2007) note that in 70% of observed cooperative breeding nests in their study period, only one helper is present. This suggests that established winter groups may lose members through mortality, competition, and other means. The formation of these large groups and the mechanisms that hold them together are questions that do not receive much attention in this species. Case studies suggest that such winter groups can be held together by resources that do not hold the same value in the spring. Dickinson and McGowan (2005) found that in juvenile male Bluebirds, delayed dispersal is more common in areas with high winter mistletoe densities, which can lead to a positive correlation between group size and winter resource availability that does not exist in the spring; when mistletoe is absent. Such environmental determinants may be the primary mechanisms behind dispersal decisions, yet social elements may also play a role.

A competitive market for helpers

Siberian Jays (Perisoreus infaustus) exhibit a competitive mechanism for winter group ​ ​ formation. Living in sub-zero temperatures, juveniles compete for non-helper positions within their natal territory in order to benefit from “prolonged brood care”, such as reduced aggression and privilege at feeding sites (Ekman et al. 2002). In this study researchers found that dominant young were more likely to secure group positions in the breeding season than their less dominant siblings. In Red-cockaded Woodpeckers (Leuconotopicus borealis) delayed dispersal is also very ​ ​ ​ ​ common and helpers stand to benefit from territory acquisition. Pasinelli and Walters 2002 indicates that competition may play a large role in dispersal behavior in this species. They report an increase in dispersal probability with an increase in number of male brood-mates. As this difference was independent from the effect of the number of older male helpers already on the territory, the authors conclude that the interaction between siblings influences dispersal behavior and is likely an indication of competitive processes.

I hypothesize that competition may play a role in nuthatch dispersal decisions and the thinning of groups observed in the spring may be a result of competition between extra-pair birds. In Carrion Crows, social groups have stable dominance hierarchies and dominant individuals control access to food resources as well as mating opportunities (Chiarati et al. 2010). Competitive behaviors can be difficult to quantify in Brown-headed Nuthatches due to challenges in prolonged observation. I test for the effect of territorial aggression on helper status; presuming that a willingness to compete in territorial intrusions signifies social dominance within a group and that these more dominant birds will outcompete less dominant siblings for helper positions. I also test for a possible relationship between nestling weight among peers and helper status. Weights have been used in cooperatively breeding Red-cockaded Woodpeckers as a predictor of future social dominance (Pasinelli and Walters 2002), and this relationship may be similar in nuthatches. Finally I test for the effect of group size on the recruitment of helpers across a range of group sizes, predicting higher likelihood of recruitment in smaller groups due to less competition among extra-pair members.

Methods

All field observations were conducted at Tall Timbers Research Station (30°39'22.6"N 84°12'32.5"W), where a color-marked nuthatch population has been monitored since 2006. Territoriality and group composition

As competition for helper status is only expected to be a factor in large groups, only groups with 5 or more individuals were assessed (n=10 groups). Group size here was predetermined using current field data from ongoing research on this population. Individual identification was documented by color-coded leg bands and confirmed by a band combination database. All birds were identified using spotting scopes to read leg-band combinations. To avoid double counts, unbanded birds within the groups were only counted once, unless both birds were seen at the same time. In some cases differences in plumage coloration were enough to discern individuality. All juveniles were subject to background checks to identify pertinent information such as sex, birth weight, and natal nest. Aggression scores were quantified in terms of the presence/absence of territorial response elicited by individuals to speaker playback. Territorial response was recorded as either 0 (presence only) or 1 (aggressive response). Each of the 10 winter groups were observed four times (40 total trials) from 22-Oct to 3-Dec with a minimum of six and a maximum of 17 days between trials for the same groups. Each observation consisted of an initial five minute control trial in which no playback was used, and two 10 minute playback trials, separated by five minutes of silence. Observations were recorded throughout each trial and territorial responses by individuals were taken as a total from both playback trials (meaning that birds who did not respond in the first trial were still given a “1” for territorial response if they responded in the second). Territorial response is defined in this study as a minimum radial distance of 20m from the playback device and/or a calling frequency of >5 total vocalizations within the two playback trials. Any individuals outside of these specifications were given a response value of “0”. Since my analysis for group aggression is focused on the number of total responses, rather than the relative value for certain individuals, it was possible to include responses from unbanded birds. Unbanded juveniles were not considered in the helper analysis as they could not be distinguished from older birds this far into the post-breeding season.

Use of song playback

I prepared four different mixes of conspecific vocalizations from a large variety of recordings; different mixes were used on each subsequent visit in order to avoid potential pseudoreplication (Kroodsma 1989b). Recording mixes were constructed using online sources (Xeno-canto.org) and mixed using Audacity (an audio editing software), which enabled me to insert silence and extend playback elements to meet the ten minute length I chose for trials. Using Audacity to insert silence breaks also improved the reliability of territorial observations; by setting out my speaker device before playback started and remaining at a distance from the speaker until the end of the trial.

Defining helper status

Helper status was determined using the group composition observed at the start of the 2017 breeding season (Feb-Mar). Group compositions determined in 2017 were compared to the group compositions documented during the 2016 breeding season. In addition to my personal observations, data were taken from other field observations as part of an on-going year-round monitoring program at TTRS. Helpers were defined in this study as any extra-pair individuals that made up the breeding group composition; of those birds, only juveniles were included in my helper analysis.

Statistical analysis

All analyses were done using R package: lme4 (R Core Team 2016). I used a binomial GLMM (Generalized Linear Mixed Model) to test the relationship between group density and likelihood of any response; this was used to test for the expected result that a group would be more likely to exhibit a single response, if it had more potential responders present. Another GLMM was used to test the proportional effect of group density on individual response. By using these GLMM’s I was able to include Group ID as a random variable and isolate its potential effect on my response variable. Similar models were used to test three hypotheses regarding helper status acquisition: 1) the effect of group size on helper recruitment probability, 2) the effect of individual aggression level on helper status (Y/N), and 3) the effect of nestling weight on helper status (Y/N). In each model Group ID was chosen as a random variable.

Results

Group composition and territorial aggression

80% of sample groups contained at least one young who had successfully fledged from the same nest five months prior. The likelihood of a single aggressive response increased as group density increased, Figure 1. (GLMM: N=40, estimate =0.66 +/- 0.24, z=2.8, p=0.005), this relationship ​ was expected, given that if response probability is constant, then more potential responders should increase the chance of any response within the group. I did not observe any aggressive ​ responses during the control trials (n=40). The test for proportional response revealed a ​ significant increase in per-capita likelihood of response as group density increased, Figure 2. (GLMM: N=40, estimate=0.34 +/- 0.16, z=2.1, p=0.034).

Figure 1. This graph shows the response (Y/N) outcome of each territorial trial over the value for group density for each visit. The trend shows a higher response probability for visits where more birds were present. Data points shown are fitted predictions from the GLMM, not raw values.

Figure 2.

This graph shows the proportional response, or per-capita response rate measured for each trial outcome over the group density for each visit. Data points shown are fitted predictions from the GLMM, not raw values.

All tests on helper status acquisition showed a nonsignificant effect of the explanatory variable on the observed helper status: Helper status ~ individual aggression (GLMM: N=22, estimate=1.22 +/- 0.76, z=1.61, p=0.108), helper status ~ fledgling weight (GLMM: N=17, estimate=0.39 +/- 0.54, z=0.73, p=0.466), and helper recruitment ~ group size (GLMM: N=9, estimate=0.48 +/- 0.84, z=0.57, p=0.569).

Discussion

Non-random selection for group monitoring

The sample groups selected here were density biased. I hoped to assess competition among extra-pair juveniles (i.e. groups with > 5 birds), so the groups selected were therefore male biased, as males are more likely than females to delay dispersal and remain in their natal territory (Cox and Slater 2007). The proportion of observed cooperative breeding is much higher in my study than that which has been reported in Cox and Slater 2007, whose data reflect unbiased selection for group monitoring. Additionally, this population is subject to experimental cross-fostering, in which some of the brood members share a common fledge nest, but not necessarily the same parents. My sample size contains only 22 juveniles, for which only 20 have data to show cross-fostered status. I was unable, given this small sample to test for relative likelihood of juveniles to become helpers based on their cross-fostered status (true natal vs kidnapped), which might be an interesting thing to investigate.

Non-breeding season group composition and spring thinning

I document stable group membership for natal juveniles five months past fledging date; 80% of groups retained natal young throughout the winter observation period. This finding suggests that large groups are a stable aspect of nuthatch biology that stem from juvenile dispersal decisions. The decrease in group density between the winter and spring still remains a puzzling phenomenon; my attempts to test for competitive processes were met with insignificant p-values, however competition remains a complex interaction and cannot be disregarded as a potentially important mechanism driving changes in group composition. Due to time constraints, I was not able to continue my observations past (Dec. 3rd), ideally I should have carried out observations until the first initiation of nest building in order to document a more precise timing for the shift observed in group composition between the winter months and the spring breeding season, however it is not clear in my analysis how birds disband from larger groups, it is likely that groups tapper down over time with some individuals leaving sooner or later than others. Further analysis is needed to describe the thinning process of these groups.

A questionable approach to predicting juvenile helper statuses

Originally this project was designed to rank birds based on their intensity of territorial aggression and use these rankings to make predictions about future helper status, however an unexpected decrease in intensity throughout the entirety of my experimental trials lead to a low level of variation among juveniles. Rather than rank birds based on intensity of response, I chose to rank them based on their proportion of response. In this way I could still compare an individual’s opportunities to respond with whether or not they chose to respond. While this method does suggest a likelihood of response for each bird, it does not include any measures for intensity of response, which is an important factor to consider when two birds may have an equal likelihood to respond. For example, in a single trial, a hyper aggressive bird that is constantly calling and flying near to the playback device would be given equal weight to a bird that might only fly close once and didn’t call at all. One possible implication is that the assumption of social dominance predicted by territorial response may not be a fair one in these rare situations. To improve the predictability of extra-pair helping with this method, data should be taken from a larger range of observations, so that ample variation in territorial intensity is available, and proportional response does not need be used as a proxy.

A note on pseudoreplication in playback trials

The conspecific vocalizations used here did do not contain information on the identity of the sampled birds; it is not possible to be sure different recordings in the same area are coming from different focal birds. Additionally, due to quality differences in recordings, some recordings were used more than once across different tapes. For these reasons my playback samples are not true replicates, and this may contribute to experimental error in my territorial aggression trials, such as playback habituation (Kroodsma 1989b), in which familiarity with playback stimulus leads to non-reliable response from focal birds.

Conclusions

Takeaways from group composition on dispersal patterns

While the mechanisms driving delayed dispersal remain complicated, my observations suggest evidence of a strong tendency toward delayed dispersal in large groups. This research also highlights the difficulty in defining delayed dispersal in this species, and suggests that there may be varying levels of dispersal. Dickinson and McGowan (2005) indicate that, while delayed dispersal and helping behavior are often very closely correlated in cooperative species, some species, such as Western Bluebirds (Sialia mexicana) do not follow the same trend (cite a Duckworth paper here somewhere); like male Western Bluebirds, nuthatches quite often do not become helpers (Cox and Slater 2007), despite a tendency to delay dispersal (documented here). When discussing dispersal behavior in the Brown-headed Nuthatch, it may be necessary to identify the primary and secondary levels at which dispersal takes place: 1) following spring fledging, and 2) before the initiation of nest building.

Playback experimentation

I was unable to provide clear replicates for my playback samples, meaning that my sample population may have been at risk of habituation to song playback. Despite this factor I did not record any aggression in my control trials (n=40), in which no playback was used. I only observed aggression during the use of playback, suggesting that the territorial response was in fact a response to the playback device, not my presence in the territory as an experimenter. Results from my group density analysis support the “safety in numbers” and “increased vigilance” hypotheses as individual response likelihood increased with group density. My competition-based predictions on helper status in juveniles were inconclusive, although I provide a potentially useful framework to test certain competition-based predictions on helper status acquisition in the Brown-headed Nuthatch, which may prove useful to other researchers studying this system.

My research discusses the importance of social context in explaining dispersal decisions and variations in territorial aggression, and includes observations of group dynamics during the nonbreeding season; a relatively unexplored area in Brown-headed Nuthatch behavior.

Acknowledgements

I would like to give special thanks to my head mentors and collaborators on this project: Dr. Emily H. DuVal, who was the brains behind the statistical analysis reported in this paper, and James A. Cox, my commander in chief and Brown-headed Nuthatch guru. Their encouragement and careful guidance helped tremendously. Furthermore I have to give credit to the members of the DuVal Lab at FSU for their advice and support, and the BHNU unit at TTRS, whose data collection made this project a reality. Finally to the remaining members of my thesis committee: Dr. Mariana Fuentes, who provided helpful feedback on my preliminary ideas for this project, and Dr. Jeffrey P. Chanton, who helped me from the very beginning of my thesis preparation and was willing to be my head advisor for this project despite its irrelevance to his extensive background in chemical oceanography. I would also like to thank Melina Müller for her advice and support in completing this project

References

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