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SEPG 1747 - Date Awarded 2000

Are wader nest scrapes adaptively designed to minimise clutch cooling rate?

Jane M. Reid,
University of Glasgow
January 2001

Abstract

Arboreal avian nests are thought to function partly to insulate clutches.  However, the adaptive reasons for the construction of nest scrapes, and the extent to which structural designs reflect functional optima, are poorly understood.  Working on arctic-breeding Pectoral Sandpipers, I tested the hypothesis that using a lined scrape reduces the rate of clutch heat loss, and investigated whether scrapes are effectively designed to minimise heat loss rates.

The use of both an unlined scrape and of lining material reduced the rate at which a test object lost heat.  Constructing a lined scrape is therefore likely to serve to insulate a clutch.

The rate of conductive heat loss from within a scrape increased with scrape depth and decreased with lining depth.  Convective heat loss increased with wind speed in shallow scrapes but not in deep scrapes. Mean observed scrape depth approximately equaled that which minimised convective cooling whilst minimising conductive heat loss.  Further, on average, Pectoral Sandpipers used approximately the lining depth that minimised conductive heat loss whilst minimising material useage.  Scrape dimensions therefore approximated those that are likely to minimise overall heat loss, given the conflicting thermal pressures of the environment.

Available lining materials differed in insulative quality both when wet and dry.  Pectoral Sandpipers did not use lining materials in proportions that reflected local availability.  Instead, relative usage was correlated with a material's insulative quality when wet.  Pectoral Sandpipers therefore used lining materials appropriate to minimising heat loss given their damp breeding environment.

Introduction

Allowing eggs to chill during incubation can be costly for parents, because chilling can harm embryos directly, and also because the energetic demand of rewarming a cool clutch is high.  The problem of clutch chilling is particularly severe for species nesting in cold climates in which only one parent incubates, as the clutch is left unattended whilst the caring parent forages, cooling down rapidly.  In such species there should be strong selection for mechanisms that reduce clutch cooling rate.

The construction of a nest has been suggested to serve this purpose amongst others (Collias & Collias 1984, Hansell 2000) and indeed, there is mounting evidence that arboreal nests built by passerines and related species can function to assist thermoregulation.  However, many birds lay their eggs in shallow scrapes on the ground, and the idea that scrapes also play an adaptive role in minimising clutch cooling has received little attention.  This is despite the recent increase in interest in incubation demands, and in their consequences for overall resource allocation patterns (Monaghan & Nager 1997, Reid et al 2000).

Working on Pectoral Sandpipers (Calidris melanotos) breeding in Barrow, Alaska, I investigated the extent to which using a lined nest scrape reduces the rate of heat loss from a clutch, and thus whether scrape construction can be interpreted as an adaptation to regulating the thermal conditions experienced by developing offspring.  Further, by experimentally investigating the relationships between heat loss rates and scrape dimensions, and relating the insulative qualities of possible lining materials to their relative usage patterns, I investigated how effectively Pectoral Sandpipers design their scrapes to reduce heat loss rates.

Further funding for this project was obtained from the British Ornithologists' Union, the Glasgow Natural History Society and the Frank M. Chapman Memorial Fund. Papers were presented at the British Ecological Society Winter Meeting and at the Edward Grey Institute Bird Biology Conference in January 2001, and a manuscript has been submitted for publication.

Results and discussion

Eggs cooled 9% more slowly within bare nest scrapes than when placed immediately adjacent to scrapes, and the presence of the lining material reduced cooling rate by a further 25% on average (Fig. 1).  Thus by constructing a lined nest scrape, sandpipers are likely to substantially reduce the rate at which their clutch loses heat.  This reduction is likely to improve the thermal conditions experienced by developing offspring and reduce incubation demands, changes that may increase offspring and parental fitness (Webb 1987, Reid et al 2001).  Both nest scrape excavation and the use of lining material can therefore be viewed as adaptations by which parents can insulate their clutch (although the possibility that scrapes also serve other adaptive functions is not excluded).

Figure 1.  Cooling rates of artificial eggs in natural nest scrapes, in scrapes with linings removed, and immediately adjacent to scrapes.  Eggs cooled down significantly more slowly in bare nest scrapes than when adjacent to scrapes (mean cooling coefficients of 8.3 O 0.4 x 10^-3 and 9.1 O 0.3 x 10^-3 respectively, paired t-test t₂₄ = 13.1, P < 0.001).  Heat was lost more slowly from within lined scrapes than from within unlined scrapes (mean cooling coefficient of 6.2 O 0.2 x 10^-3, paired t-test t₂₄ = 10.2, P < 0.001) and from adjacent to scrapes (paired t-test t₂₄ = 13.1, P < 0.001).

Pectoral Sandpiper scrapes contained 19 O 2mm of lining material on average, a depth approximating that at which the addition of more lining material scarcely reduced conductive heat loss rate (Fig.2).  Hence Pectoral Sandpipers used the lining depth that approximately minimised conductive heat loss whilst minimising the amount of material used.

Figure 2. Relationship between egg cooling rate and scrape lining depth.  Regression equation: C = 0.1+0.15exp^{(-1.3x scrape depth)}.  The solid line indicates the average depth of lining material in Pectoral Sandpiper scrapes (dashed lines O 1SE).

Scrapes averaged 52 O 2mm deep and thus with the natural lining in position, eggs rested approximately 33mm below ground level.  At this depth, clutches were poorly protected from forced convective cooling, and are likely to have lost heat rapidly when left unattended on breezy days.  As rates of heat loss from artificial eggs in deep scrapes were not significantly affected by wind speed, Pectoral Sandpipers could have reduced convective cooling by positioning their eggs further below the ground.  Given that reducing lining depth is likely to have increased conductive heat loss, sandpipers would have had to dig deeper scrapes in order to reduce convective cooling without simultaneously increasing conductive cooling. However, 3cm is approximately the maximum depth at which ground temperature remained relatively high (Fig.3).  As the rate of egg cooling in still air was tightly correlated with ground temperature, a result that is predicted by heat transfer theory (Winterton 1997), this depth is also likely to be the maximum at which the rate of conductive heat loss from a clutch remained low.  Thus Pectoral Sandpipers positioned their eggs at approximately the depth that is likely to have minimised rates of forced convective cooling given the minimisation of conductive heat loss to cold ground.

Figure 3. Relationships between scrape depth, and egg cooling rate and ground temperature.  With wind excluded, egg cooling rate (_) was correlated with scrape depth (r₁₃ = 0.94, N = 15, P < 0.001).  Ground temperature (_) decreased non-linearly with depth (Best fit logistic equation; Ground temperature = 5.13-(5.13/(0.07 +4.99exp^{(-0.74x scrape depth)})), R² = 0.99).

Seven potential lining materials were available in the Pectoral Sandpiper's breeding habitat: grass, moss, Salix leaves, feathers, and three species of foliose lichen (Dactylina arctica, Cetraria cucullata and Thamnolia vermicularis).  The insulative qualities of the seven materials differed significantly when materials were both dry (1-way ANOVA, F_6,38 = 30.35, P < 0.001) and wet (1-way ANOVA F_6,35 = 20.2, P < 0.001).  Waders predominantly collect lining material from the immediate vicinity of their scrape, often gathering material whilst sitting within the scrape itself.  Hence a material's abundance within 30cm of a nest is likely to be a reasonable estimate of its availability as lining material.  However, Pectoral Sandpipers did not use lining materials in the proportions expected from their local abundance (Fig.4).

Figure 4. Mean proportions of nest scrape linings and surrounding habitat that consisted of grass, Salix leaves, moss, Dactylina arctica, Thamnolia vermicularis and Cetraria cucullata.

The extent to which lining materials were used relative to their local abundance was correlated with their insulative quality when wet (Fig. 5). Thus we suggest that Pectoral Sandpipers selected lining materials that minimised rates of conductive heat loss from within scrapes, given that they were nesting in a damp environment.  Pectoral Sandpiper scrape design can therefore be quantitatively explained in terms of the minimisation of heat loss rates.

Figure 5. Relationships between the ratios of material abundances within linings to their abundance within the habitat and a material's insulative quality.  Useage ratio was significantly correlated with a material's insulative quality when materials were wet (r_s6 = -0.94, n = 7, P = 0.002) but not when dry (r_s6 = -0.10, n = 7, P = 0.83).

Acknowledgements

David Norton, Dave Ramey and the Barrow Arctic Science Consortium were of great help during the fieldwork. Professor Jim Dickson and Dr. Brian Corpins kindly assisted with lichen identification.  Graeme Ruxton provided useful discussion during the project and helpful comments on the manuscript.

References

Collias, N.E. & Collias, E.C. (1984)Nest Building and Bird Behaviour. Princeton University Press, Princeton.

Hansell, M.H. (2000) Bird Nests and Construction Behaviour. Cambridge University Press, Cambridge.

Monaghan, P. & Nager, R. (1997) Why don't birds lay more eggs? Trends in Ecology & Evolution 12, 270-274.

Reid, J.M., Monaghan, P. & Ruxton, G.D. (2000) Resource allocation between reproductive phases: the importance of thermal conditions in determining the cost of incubation. Proceedings of the Royal Society London Series B. 267, 37-41.

Reid, J.M., Monaghan, P. & Nager, R.G. (2001) Incubation and the costs of reproduction. In: Avian Incubation: Ecology, Evolution & Energetics.  Ed D.C.Deeming, Oxford University Press, Oxford. In press.

Webb, D.R. (1987) Thermal tolerance of avian embryos: a review.  Condor 89, 874-898.

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