Local weather conditions have complex effects on the growth of blue tit nestlings

Author’s Accepted Manuscript Local weather conditions have complex effects on the growth of blue tit nestlings Mark C. Mainwaring, Ian R. Hartley

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To appear in: Journal of Thermal Biology Received date: 4 March 2016 Revised date: 23 May 2016 Accepted date: 23 May 2016 Cite this article as: Mark C. Mainwaring and Ian R. Hartley, Local weather conditions have complex effects on the growth of blue tit nestlings, Journal of Thermal Biology, http://dx.doi.org/10.1016/j.jtherbio.2016.05.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Local weather conditions have complex effects on the growth of blue tit nestlings Mark C. Mainwaring*, Ian R. Hartley Lancaster Environment Centre, Lancaster University, Lancaster, LA1 4YQ, UK *

Corresponding author at: Lancaster Environment Centre, Lancaster University, Lancaster, LA1

4YQ, UK, [email protected] (Mark C. Mainwaring)

Abstract Adverse weather conditions are expected to result in impaired nestling development in birds, but empirical studies have provided equivocal support for such a relationship. This may be because the negative effects of adverse weather conditions are masked by parental effects. Globally, ambient temperatures, rainfall levels and wind speeds are all expected to increase in a changing climate and so there is a need for a better understanding of the relationship between weather conditions and nestling growth. Here, we describe a correlative study that examined the relationships between local temperatures, rainfall levels and wind speeds and the growth of individual blue tit (Cyanistes caeruleus) nestlings in relation to their hatching order and sex. We found that changes in a range of morphological characters were negatively related to both temperature and wind speed, but positively related to rainfall. These patterns were further influenced by the hatching order of the nestlings but not by nestling sex. This suggests that the predicted changes in local weather conditions may have complex effects on nestling growth, but that parents may be able to mitigate the adverse effects via adaptive parental effects. We therefore conclude that local weather conditions have complex effects on avian growth and the implications for patterns of avian growth in a changing climate are discussed.

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Keywords: Blue tit Development Maternal effects Rainfall Temperature Weather Wind speed

1. Introduction

Environmental conditions have a broad range of effects on reproducing animals and in birds, they may affect the timing of egg laying, clutch sizes, nestling survival rates, patterns of nestling growth and parental provisioning patterns (Wingfield, 1984; Stenseth et al., 2002; Møller, 2012). Birds are usually able to respond to predictable changes in environmental conditions, such as the temporal increases in spring temperatures that have resulted from anthropogenic climate change through shifts in their breeding ranges or the timing of reproduction (Charmantier et al., 2008; Chen et al., 2011). However, they are less able to respond to unpredictable short-term temporal fluctuations in environmental conditions, such as those associated with changes in local weather conditions, through changes in parental provisioning or brooding behaviours (McCarty, 2001; Greno et al., 2008). One of the most important effects that short-term changes in local weather conditions can have on birds is affecting patterns of offspring growth (Krijgsveld et al., 2003; Dawson et al., 2005). This is because the conditions experienced during ontogeny can impair the development of morphological characters and internal organs which subsequently affects a number of traits with relevance to an individuals’ fitness during adulthood, such as the acquisition of social dominance, the acquisition of breeding territories and reproductive partners, breeding success and lifespan (e.g. Magrath, 1991; Both et al., 1999; reviewed by Lindström, 1999; Metcalfe and Monaghan, 2001). Adverse weather conditions such as low temperatures, high rainfall or high wind speeds or their interactive effects (e.g. Coe et al., 2015) impair growth either directly by chilling them (Dunn, 1975; Bryant, 1978; Quinney et al., 1986, Keller and van Noordwijk, 1994; McCarty and

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Winkler, 1999; Ritz et al., 1995; Dawson et al., 2005; Pérez et al., 2008; Ardia et al., 2009. 2010; Winkler et al., 2013) and/or indirectly by reducing the ability of parents to catch or transfer prey to the nest (Konarzewski and Taylor, 1989; Becker and Specht, 1991; Boersma and Parrish, 1998; Ritz et al., 1995; Finney et al., 1999; Geiser et al., 2008). In altricial nestlings, these patterns likely represent a trade-off between growth and thermoregulation (Visser, 1998; Rauter and Rever, 2000) because naked young are susceptible to adverse weather conditions. In altricial species however, the female parent often spends the first few days of the nestlings’ lives brooding them whilst the male parent searches for food but as the food requirements of the brood increase, then females spend increasing amounts of time foraging until both parents continually search for food. Nevertheless, the trade-off between growth and thermoregulation means that in adverse weather conditions, nestlings may have to allocate their limited resources towards sustaining their own body temperatures rather than towards developmental functions such as growth (Skagen and Yackel Adams, 2012; Pérez et al., 2016). Despite studies showing that adverse local weather conditions result in impaired offspring growth, other studies have provided no, or weak, support for such a relationship (Dunn, 1975; Murphy, 1985; Johnston, 1993; McCarty and Winkler, 1999; Bradbury et al., 2003). For example, local weather conditions were poor determinants of growth in farmland passerine birds (Bradbury et al., 2003) and the amount of sunshine had no effect on the growth of roseate tern (Sterna dougallii) chicks, although wind speeds were negatively related to growth (Dunn, 1975). It is unclear why adverse weather conditions do not always impair growth but it may be that the effects are masked by parental effects (Dawson et al., 2005) or by resource allocation patterns whereby nestlings shift resources between various morphological characters (Mainwaring and Hartley, 2012). Further studies are thus required to increase our understanding of how weather conditions influence growth. Further, it may provide useful insights into the effects of climate change on patterns of avian growth. Globally, ambient temperatures (IPCC, 2001), rainfall levels 3

(Marvel and Bonfils, 2013) and wind speeds (Vautard et al., 2010; Young et al., 2011) are expected to increase from anthropogenic climate change (Garcia et al., 2014). There is a need to document the effects of global climate change on ecological systems as whilst there has been a great deal of research into the effects on changes in laying dates in passerine birds in temperate environments (Charmantier et al., 2008; Møller, 2012), the effects of local weather conditions on other aspects of the life histories of birds are less well understood (Dawson et al., 2005). Here, we study how variation in local temperature, rainfall and wind speeds affected the growth of individual blue tit (Cyanistes caeruleus) nestlings in relation to their hatching status and sex. Blue tits are a useful study species because their growth is likely to be affected by adverse weather conditions as it constrains the parents’ ability to forage for caterpillars. Blue tit broods hatch with varying degrees of asynchrony with some broods hatching within one day and others hatching out over two or three days, and in these cases the majority of the brood hatches on the first day (Cramp and Perrins, 1993; Stenning, 2008). Early hatched nestlings are larger than their late hatched siblings throughout the growth period (Mainwaring et al., 2010) whilst male nestlings are larger than females throughout (Mainwaring et al., 2011; Mainwaring et al., 2012). Meanwhile, whilst egg constituents, such as lipid content and fatty acids decline (Bourgault et al., 2007), egg mass and volume do not vary through the laying sequence in some populations (Bourgault et al., 2007) but increase through the laying sequence in other populations (Stenning, 2008) thereby enabling late hatched nestlings to mitigate the adverse effects of hatching asynchrony. In this study, we test the following two predictions. First, we predict that the growth of nestlings will be positively affected by high temperatures but negatively affected by high levels of rainfall and wind speeds because high temperatures promote effective foraging and reduce the need to brood the nestlings whilst the converse applies when rainfall and wind speeds are high. Second, we predict that the growth of later hatched and female nestlings will be

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affected to a greater extent than earlier hatched and male nestlings because they are smaller and hence more likely to lose out when competing for parentally provided food.

2. Material and methods 2.1 Study site and quantifying reproductive parameters Data were collected from blue tits breeding in deciduous woodland interspersed with small patches of coniferous trees in Lancashire, UK (54º0’N, 02º47’W) during 2004-2006 (Lambrechts et al., 2010). A total of 66 nestboxes were available for hole breeding passerines to occupy and although not all nestboxes were occupied, we only recorded blue tits breeding in the nestboxes. Regular nestbox checks from the beginning of April established the date on which the first egg was laid, assuming that one egg was laid per day (Cramp and Perrins, 1993). Nests were then checked on a daily basis after the sixth egg was laid to establish when incubation began. Nests were left undisturbed during incubation. Then, a couple of days before the predicted hatching date, nests were checked for hatching on a daily basis. We individually marked nestlings on the day that they hatched with an indelible marker pen, meaning that we were able to quantify the exact date of hatching for all of the nestlings. As female blue tits usually begin to incubate their clutch one or two days prior to clutch completion, then broods hatch asynchronously over a period of 2-3 days. Following previous studies (Mock and Forbes, 1995; Mainwaring et al., 2010), we defined those nestlings that hatched on the first day of hatching as early hatched nestlings and those nestlings that hatched on later days as late hatched nestlings.

2.2 Quantifying nestling growth changes

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A total of 614 nestlings from 66 broods were measured every two days until the eldest nestlings were 14 days old. We individually tracked individual nestlings by marking them with an indelible marker pen daily until they were six days old when they were fitted with individually numbers metal rings. At each visit, one of us (MCM) quantified the body mass (± 0.1 gram; electronic balance), head-bill length (± 0.05 mm; dial callipers), tarsus length (from the depression in the angle of the intertarsal joint to the end of the folded foot) (± 0.05 mm; dial callipers) and right fourth primary length (± 0.5 mm; fixed rule) of all nestlings. Note that whilst body mass and head-bill length were measured every two days from day 2 onwards, tarsus length and fourth primary length were measured every two days from day 6 onwards. Nestling growth changes may vary systematically with nestling age, independent of changes due to local weather conditions and so we included a ‘growth period’ factor in the analyses with 1 being the growth change between days 2-4, 2 being the change between days 4-6, 3 being the change between days 6-8, 4 being the change between days 8-10, 5 being the change between days 10-12 and 6 being the change between days 12-14. We have previously calculated the repeatability of values for each morphological character and found them all to be repeatable (Mainwaring et al., 2010). For each morphological character, we quantified growth rate changes by calculating the change in the value obtained at a particular visit from the value obtained at the previous visit. After the nestlings were measured at day 14, the nests were left undisturbed for 6 days because nest visits may have caused the nestlings to fledge prematurely. Nests were then checked again at day 20 (± 1) in order to establish fledging success. We only included nestlings that fledged, and as 47 nestlings never fledged, then 567 nestlings from 66 broods were included in the study.

2.3 Molecular techniques

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We determined the sex of individual nestlings using standard molecular techniques, as we have previously described elsewhere (Mainwaring et al., 2011, 2012).

2.4 Quantifying local weather conditions

Local weather conditions were quantified using values obtained from Lancaster University’s Hazelrigg meteorological station located within 1 km of the study area. We used mean temperature (maximum plus minimum value divided by two; °C), rainfall (mm) and wind speed (miles per hour) to denote local weather conditions. As we quantified nestling growth every two days, we used the mean average of the two weather values from the two days preceding the day of measurement to denote local weather conditions.

2.5 Statistical analyses

Data were analysed using the spss v21.0 (SPSS, Chicago, IL, USA) statistical package. Mixed models were used to analyse the data as they allow the analysis to take account of the nonindependence of data points, and in these analyses, nestling identity nested within brood identity were fitted as a random term throughout (Pinheiro and Bates, 2000). We analysed variation in the change of four morphological characters between successive measurements by using four linear mixed models which had ‘mass change’, ‘head-bill length change’, ‘tarsus length change’ and ‘fourth primary length change’ as dependent variables. Each of the four models had temperature (°C), rainfall (mm), wind speed (miles per hour) and first egg date (days after April 1) as fixed covariate terms and nestling type (early or late hatched), nesting sex (male or female), brood size (4-10), growth period (1-6) and year (2004, 2005, 2006) as fixed factorial terms. All of the fixed explanatory variables and the two-way interaction terms between ‘temperature’, ‘rainfall’ and 7

‘wind speed’ and those between each of the three weather variables ‘nestling type’ and ‘nestling sex’ were initially entered into the models and were assessed for significance when they were the last terms in the models. Terms were then sequentially dropped if their inclusion did not increase the explanatory power of the models, thereby yielding the final models (Crawley, 1993). Note that whilst all of the two-way interactions mentioned above were tested, only those that were statistically significant are presented. All statistical tests are two-tailed, means are presented ± 1 standard error and a critical P-value of 0.05 is applied throughout. Given the large sample sizes in our study, we also report effect sizes by reporting Cohen’s d and Pearson’s r values for grouped and correlative results, respectively, following the suggestions of Nakagawa and Cuthill (2007).

3. Results

3.1 Changes in body mass

Changes in body mass were negatively correlated with temperature and positively correlated with rainfall (Table 1; Figs. 1a-c), whilst a significant interaction between ‘rainfall’ and ‘nestling type’ indicated that late hatched nestlings showed higher levels of mass change when levels of rainfall were low, whilst early hatched nestlings showed higher levels of mass change when levels of rainfall were high (Table 1; Fig. 2). Mass change was not directly related to wind speed (P = 0.440) but got progressively smaller throughout the growing period (Table 1). Mass change was not directly related to first egg date (P = 0.249) nor brood size (P = 0.154) but mass changes were greater in early hatched nestlings than in later hatched nestlings (P < 0.005; Table 1).

3.2 Changes in head-bill length

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Changes in head-bill length were marginally negatively related to temperature (P = 0.046), but were unrelated to either rainfall or wind speed (Table 2; Figs. 1d-f). Changes in head-bill length decreased throughout the growing period but were not influenced by first egg date (P = 0.166) nor brood size (P = 0.194; Table 2). Meanwhile, a significant interaction between ‘rainfall’ and ‘temperature’ indicated that nestlings showed higher head-bill length changes when levels of rainfall were high and when temperatures were low, although the effect was small (Table 2).

3.3 Changes in tarsus length

Changes in tarsus length were positively related to rainfall, but negatively related to both temperature and wind speed (Table 3; Figs. 1g-i). Changes in tarsus length were negatively related to the growth period, but positively related to first egg date (P = 0.021) and brood size (P < 0.001; Table 3).

3.4 Changes in fourth primary length

Changes in fourth primary length were positively related to rainfall and wind speed, but negatively related to temperature (Table 4; Figs. 1j-l), whilst changes in fourth primary length decreased throughout the growth period (P < 0.001). Meanwhile, a significant interaction between ‘wind speed’ and ‘nestling type’ indicated that early hatched nestlings had higher changes at higher wind speeds than late hatched nestlings, although the effect was negligible (Table 4).

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4. Discussion

The main findings of this study were that local weather conditions had complex effects on the growth of blue tit nestlings. We found that changes in the mass, head-bill length, tarsus length and fourth primary length of nestlings were all negatively related to temperature and that whilst changes in mass, tarsus length and fourth primary length were also all positively related to rainfall, changes in tarsus length and fourth primary length were both negatively related to wind speeds. There was also a degree of heterogeneity in the way that local weather conditions affected the growth of individual nestlings as, for example, the hatching status of nestlings interacted with local weather conditions to affect nestling growth. Changes in mass, head-bill length, tarsus length and fourth primary length were all negatively related to local temperatures, meaning that nestling growth rates were lower when ambient temperatures were high. Although the effect was relatively weak in most cases, these findings nevertheless suggest that the findings of our study contradict the findings of both observational (Murphy, 1985; McCarty and Winkler, 1999; Winkler et al., 2013; Pérez et al., 2016) and experimental (Dawson et al., 2005; Pérez et al., 2008; Ardia et al., 2009, 2010) studies that have shown that the nestlings of altricial birds grew faster in higher temperatures. Our findings are seemingly paradoxical as when ambient temperatures are high, the nestlings of altricial birds such as blue tits are expected to be able to allocate their available resources towards growth as the need to sustain their own body temperatures through thermoregulation are reduced (Visser, 1998; Dawson et al., 2005). Moreover, the findings of our study also seemingly contradict the findings from a longitudinal study in Belgium where the developmental time of blue tits and great tits declined by about one day over the course of three decades (Matthysen et al., 2011). As spring temperatures at the study site increased throughout the course of the study,

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then this suggests that nestling growth was faster in warmer temperatures. However, it is currently unclear why nestling growth was slower at higher temperatures in our study yet was faster at higher temperatures in other studies of altrical birds. Changes in the growth of nestlings was weakly related to wind speeds but changes in both tarsus and fourth primary lengths were negatively related to wind speeds, whilst changes in mass and head-bill length were not related to wind speeds. Studies of seabirds in cold environments have shown that when wind speeds are high, offspring allocate resources towards the maintenance of body temperatures and away from growth (Dunn, 1975; Ritz et al., 1995), although it is unlikely that high wind speeds directly affected the blue tit nestlings in our study as they were inside nestboxes and were consequently buffered from the wind. This may explain why changes in mass were not related to local wind speeds in our study. However, high wind speeds are likely to have impaired the ability of parents to forage for caterpillars effectively, and similar effects on parental foraging behaviours have been implied in other studies (Dunn, 1975; Ritz et al., 1995; McCarty and Winkler, 1999). Changes in tarsus length were greater in nestlings raised in earlier than in later broods and changes in fourth primary lengths were greater in the early stages of the growth period than in the later stages. We suggest that these patterns can be attributed to the process of siblings competing for food as nestlings preferentially allocate resources towards the growth of their tarsi as it enables them to reach higher in the nest and successfully procure food from their parents (Dickens and Hartley, 2007). Changes in mass, tarsus and fourth primary were all positively but rather weakly related to rainfall, whilst head-bill showed no relationship. Nevertheless, the positive correlation between the amount of rainfall and growth changes suggests that birds were able to successfully acquire food either during rainfall or immediately after rainfall (McCarty and Winkler, 1999) and we suppose that some unknown aspect of rainfall makes it easier for blue tits to locate caterpillars. Alternatively, it is possible that the water content of the caterpillars and thus also the nestlings 11

influenced our findings as the caterpillars are porous and thus may have become heavier during periods of rainfall and become desiccated during drier conditions (Speight, 1979) which would explain the observed results, although this remains to be tested. However, other studies have shown that offspring growth is negatively related to the amount of rainfall in ecologically similar great tits (Keller and van Noordwijk, 1994) and in roseate terns (Dunn, 1975). At the withinbrood level, when levels of rainfall were high, mass changes were greater amongst early hatched nestlings than late hatched nestlings. We suggest that when rainfall levels were high, early hatched nestlings grew faster than their later hatched siblings as they were able to procure food from their parents more successfully from their parents (Dickens and Hartley, 2007). We have outlined above how the growth of various morphological characters of blue tit nestlings vary in a heterogeneous manner in relation to local weather conditions, but rather than considering such morphological characters in isolation, it is more logical to consider how the phenotypes of nestlings are affected by weather conditions. Whilst changes in morphological characters were generally negatively related to temperature and wind speeds, they were generally positively related to rainfall levels suggesting that different aspects of ontogenetic development responded differently to varying weather conditions and may the growth of nestlings is likely to have involved trade-offs between different aspects of ontogeny. Further, as the mass gain of nestlings in our study decreased in higher temperatures but increased in other studies (Winkler et al., 2013; Pérez et al., 2016), then weather-influenced growth patterns may vary on geographic scales. And to add a further level of complexity, later hatched nestlings usually grew slower than their early hatched siblings under adverse weather conditions in our study which suggests that parental feeding rates were reduced in adverse weather conditions (Geiser et al., 2008). Thus it is important to consider how the phenotypes of nestlings are affected by ambient weather conditions and also to consider how complex and potentially interacting factors come together to affect their phenotypes during varying weather conditions. 12

To summarise, we have shown that local weather conditions affect patterns of growth in blue tit nestlings. Changes in the growth of all four of the morphological characters that we quantified were negatively related to temperature, changes in the growth of three of those characters were positively related to rainfall whilst changes in two characters were negatively related to wind speeds. We also found evidence that the hatching status of individual nestlings influenced their patterns of growth in relation to local weather conditions. This suggests that as ambient temperatures, rainfall levels and wind speeds are all expected to increase in a changing climate (Garcia et al., 2014) in a complex manner, then the results are not easily disentangled and further studies that manipulate weather conditions (see Pérez et al., 2008; Ardia et al., 2009, 2010) are needed for a better understanding of the relationship between weather conditions and the growth of birds and other animals. Importantly, our study suggests that the effects of local weather conditions on offspring growth are likely to vary and both large and small geographic scales such as between individuals within a given population. Consequently, there are several avenues where further research may prove useful. First, we urge further studies to examine how local weather conditions influence avian growth as the studies to date report contradictory findings and there is a need to understand the generality of the patterns described thus far. More specifically, it would be useful to examine the influence of fine scale weather patterns on avian growth as it may well be that, for example, high day time and low night time temperatures may influence growth in different ways as parents are likely to be brooding their offspring at night time. Second, studies that experimentally alter the microclimates within nests are rare (reviewed by Mainwaring et al., 2014; but see Dawson et al., 2005; Ardia et al., 2009, 2010) but such studies are likely to highlight the mechanisms underlying the observed growth patterns and thereby increase our understanding of the ways in which local weather conditions affect offspring growth.

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Acknowledgements We thank Megan Dickens for help collecting the data; Ian Owens for useful advice; and the Natural Environment Research Council for funding as a studentship to MCM (NER/S/A.2003/11263) and as a research grant to IRH (NE/E010806/1).

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Table 1: Mass Effect

d.f.

F

P

Effect

value

value

size

temperature 1,3283 40.94

<0.001 0.21a

rainfall

1,3283 6.40

0.011

0.29a

wind speed

1,3283 0.60

0.440

0.12a

first egg date

1,3283 1.33

0.249

0.04a

nestling

1,3283 13.26

<0.001 0.05a

nestling sex

1,3283 1.67

0.189

0.02b

brood size

9,3283 1.47

0.154

0.05b

growth

5,3283 550.15 <0.001 0.62a

type

period year

2,3283 6.86

temperature 1,3120 17.59

0.001

0.04b

<0.001 0.06a

x rainfall rainfall

x 1,3102 6.73

0.010

0.08a

x 1,3102 4.66

0.031

0.17a

wind speed rainfall nestling type

Summary of a linear mixed model examining changes in the mass growth of blue tit nestlings in relation to local weather conditions. The dependent variable was changes in mass; the explanatory variables were temperature (°C), rainfall (mm), wind speed (miles per hour), first egg date (days after April 1), nestling type (early hatched, late hatched), nesting sex (male, female), brood size (4-10), growth period (1-5) and

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year (2004, 2005, 2006); and nestling identity ‘nested’ within brood identity was fitted as a random term. Two-way interaction terms are only shown if they were significant and effect sizes refer to either Cohen’s d (a) or pearsons r (b) values as appropriate. Note that significant variables are highlighted in bold.

Table 2: Head-bill length Effect

d.f.

F

P

Effect

value

value

size

temperature 1,3280 3.97

0.046

0.11a

rainfall

1,3280 1.75

0.185

0.25a

wind speed

1,3280 1.60

0.205

0.08a

first egg date

1,3280 1.92

0.166

0.01a

nestling

1,3280 20.32

<0.001 0.07b

nestling sex

1,3280 2.09

0.124

0.03b

brood size

9,3280 1.37

0.194

0.01a

growth

5,3280 256.33 <0.001 0.56a

type

period year

2,3280 5.76

temperature 1,3099 10.00

0.003

0.03b

0.002

0.18a

x rainfall Summary of a linear mixed model examining changes in the head-bill length growth of blue tit nestlings in relation to local weather conditions. The dependent variable was changes in head-bill length; the explanatory variables were temperature (°C), rainfall (mm), wind speed (miles per hour), first egg date (days after April 1), nestling type (early hatched, late hatched), nesting sex (male, female), brood size (410), growth period (1-5) and year (2004, 2005, 2006); and nestling identity ‘nested’ within brood identity was fitted as a random term. Two-way interaction terms are only shown if they were significant and effect

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sizes refer to either Cohen’s d (a) or pearsons r (b) values as appropriate. Note that significant variables are highlighted in bold.

Table 3: Tarsus length Effect

d.f.

F

P

Effect

value

value

size

temperature 1,2145 29.93

<0.001 0.16a

rainfall

1,2145 8.08

0.005

wind speed

1,2145 50.57

<0.001 0.07a

first

egg 1,2145 5.36

0.021

0.18a

<0.01a

date 1,2145 50.57

<0.001 0.11b

nestling sex

1,2145 0.12

0.890

brood size

9,2145 3.48

<0.001 <0.01a

growth

5,2145 594.48

<0.001 0.73a

2,2145 35.50

<0.001 0.11b

temperature 1,2000 24.61

<0.001 0.09a

nestling type

<0.01b

period year

x

wind

speed rainfall

x 1,2000 16.72

<0.001 0.14a

wind speed Summary of a linear mixed model examining changes in the tarsus length growth of blue tit nestlings in relation to local weather conditions. The dependent variable was changes in tarsus length; the explanatory variables were temperature (°C), rainfall (mm), wind speed (miles per hour), first egg date (days after April 1), nestling type (early hatched, late hatched), nesting sex (male, female), brood size (4-10), growth

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period (1-5) and year (2004, 2005, 2006); and nestling identity ‘nested’ within brood identity was fitted as a random term. Two-way interaction terms are only shown if they were significant and effect sizes refer to either Cohen’s d (a) or pearsons r (b) values as appropriate. Note that significant variables are highlighted in bold.

Table 4: Fourth primary length Effect

d.f.

F

P

Effect

value

value

size

temperature 1,2177 6.69

0.010

0.10a

rainfall

1,2177 6.41

0.011

0.10a

wind speed

1,2177 5.54

0.019

0.01a

first egg date

1,2177 0.85

0.356

0.01a

nestling

1,2177 11.96

0.001

0.06b

nestling sex

1,2177 0.87

0.421

0.02b

brood size

9,2177 2.41

0.010

0.04a

growth

5,2177 13.03

<0.001 0.01a

2,2177 0.79

0.452

0.03b

temperature 1,2033 6.52

0.011

0.12a

0.034

0.02a

type

period year

x rainfall wind speed 1,2033 4.52 x

nestling

type Summary of a linear mixed model examining changes in the fourth primary length growth of blue tit nestlings in relation to local weather conditions. The dependent variable was changes in fourth primary length; the explanatory variables were temperature (°C), rainfall (mm), wind speed (miles per hour), first

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egg date (days after April 1), nestling type (early hatched, late hatched), nesting sex (male, female), brood size (4-10), growth period (1-5) and year (2004, 2005, 2006); and nestling identity ‘nested’ within brood identity was fitted as a random term. Two-way interaction terms are only shown if they were significant and effect sizes refer to either Cohen’s d (a) or pearsons r (b) values as appropriate. Note that significant variables are highlighted in bold.

Fig. 1. Changes in the mass (a, b, c), head-bill length (d, e, f), tarsus length (g, h, i) and fourth primary length (j, k, l) of blue tit nestlings in relation to local temperature, rainfall and wind speed.

Fig. 2. Changes in the mass of early hatched and late hatched blue tit nestlings in relation to local rainfall. Note that the white symbols and the dashed line represent early hatched nestlings and the black symbols and the solid line represent late hatched nestlings.

Highlights  Weather conditions may influence avian growth but studies report mixed findings.  We examine how local temperature, rainfall and wind speed affected blue tit growth.  Changes in morphological characters were negatively related to temperature and wind speed.  Meanwhile, changes in morphological characters were positively related to rainfall.  We conclude that local weather conditions have complex effects on avian growth.

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