Egg cooling in nests and attentiveness during incubation in British Passerines

Rosetta Blackman, Charles Deeming, Paul Eady & Sarah Barton

Avian incubation is reliant on birds building their nests in the appropriate place and at the appropriate time. Warming eggs is not just a function of the presence of the incubating adult but is also inter-related with the nest characteristics. However, small songbird species regularly leave their nest during the day to forage. Hence, the bird can have a relatively low attentiveness to the eggs (i.e. the period on the eggs divided by the period on the eggs plus the period off the eggs). However, the patterns of attentiveness for songbirds laying eggs of the same initial mass can vary from 60–100%. This implies that, whilst small egg mass is important in allowing birds to leave from incubation to forage, it is not the whole story. There must be another factor (or factors) that is forcing some species to incubate for more of the day in order to keep their eggs warm.

Nest construction varies between species as does location and time of breeding (Hansell, 2000). Our idea was that for species with different rates of attentiveness it is the insulative properties of the nest that dictate the behaviour of the adults. Our prediction was that the thermal conductance of the nest (the inverse of insulation) will correlate positively with attentiveness of the adults – nests that allow high rates of heat loss would require the adults to attend the eggs for a greater percentage of the day.

Figure 1. Experimental setup showing Blackbird nest suspended in netting with pieces of cloth covering the “Blu-tak” egg. Temperatures were recorded from the mock egg and the ambient conditions using the digital thermometer shown

 

We measured the rates of cooling of mock eggs made of “Blu-tak”, the thermal properties of which we showed to be comparable to that of eggs. Cooling rates in nests were determined by heating up to 60°C a mock egg, a ball of Blu-tak of the same mass as the egg of the species concerned. With a thermister embedded within, it was then placed into the nest (suspended on plastic netting) and covered with eight layers of cloth to mimic the presence of the bird and minimise any convectional heat loss (Figure 1). The time that the mock egg cooled from 50°C to 30°C was recorded for each nest three times and 3-5 nests of each species were tested. Heat flux and thermal conductance were calculated from each cooling measurement. Data on % attentiveness, initial egg mass, female body mass, clutch size and incubation period (days) were all obtained from the literature. Nest dimensions and mass were also measured.

 

  
Chiffchaff nest                                Reed warbler nest

Summary data on the reproductive biology of the bird species.

Species	%Attentiveness	Initial egg mass (g)	Incubation period (days)	Eggs in clutch	Clutch mass (g)	Female body mass (g)
Blackbird	85.2	7.2	12.6	4	28.8	99.8
Blue Tit	75	1.2	14.2	12	14.4	10.8
Chaffinch	82.6	2.7	12.6	4.6	12.42	21.5
Chiffchaff	73.8	1.2	13	5.4	6.48	22.9
Dunnock	66.5	2.13	12	5	10.65	21.1
Great Tit	74.4	1.6	13.9	9.9	15.84	17.6
Meadow Pipit	80	2	13	5	10	19.5
Pied Wagtail	85.3	2.3	12.6	5.1	11.73	19.9
Reed Warbler	68.9	1.75	10.6	8.4	14.7	12.1
Robin	73.4	2.4	13.7	5	12	16.5
Willow warbler	74.3	2	13	6.5	13	8.6

Our results showed that there was no correlation between thermal conductance of the nest and % attentiveness (r = –0.090, P > 0.005) i.e. nests with high attendance did not have low thermal conductance (= high insulation) and vice versa. Thermal conductance did not correlate with initial egg mass, female body mass, clutch mass nor incubation period. Nor did thermal conductance significantly correlate with the mass or thickness of the nests.

Investigations into the thermal characteristics of nests are uncommon. Within a species factors such as altitude can affect nest insulation (Kern & van Riper, 1984). However, our initial prediction that thermal conductance would correlate with attentiveness does not hold true. This would confirm other data on nest thermal conductance. Skowron & Kern (1980) described thermal conductance of some North American Passerines but our analysis shows that there is also no significant relationship with attentiveness in these species (N = 7, r = -0.390, P > 0.05).

Our investigations are on-going to determine whether the non-significant negative correlation would be maintained with the addition of more species. In addition, it is unclear how using a whole clutch of mock eggs, which fill the nest cavity, will affect the determination of thermal conductance. Given that some of the species studied nest in cavities or boxes we are also interested in to what extent this decreases thermal conductance. Finally, other nests are from open sites and so the effect of air movement is of interest. On-going research will clarify these relationships.

This study was funded by a Nuffield Vacation Scholarship awarded to Rosetta Blackman. Many thanks go to Professor Mike Hansell of Glasgow University for supplying nests from the national collection housed at the Hunterian Museum, Glasgow University. Many thanks go to Dr Scott Turner and Professor Mike Kern for his invaluable advice on the physics of heat exchange and to Dr Glenn Baggott for supplying the thermisters used in this study.

A more complete report of this study has been submitted to Ibis.

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

Kern, M. and van Riper, C. (1984). Altitudinal variations in nests of the Hawaiian honeycreeper, Hemignathus virens virens. Condor, 86, 443-54.

Skowron, C. and Kern, M. (1980). The insulation in nests of selected North American songbirds. Auk, 97, 816-24.

Turner, J.S. (1985). Cooling rate and size of birds' eggs - a natural isomorphic body. Journal of Thermal Biology, 10, 101-104.