Latitudinal variation in day length and working day length has a confounding effect when comparing nest attentiveness in tropical and temperate species

Abstract

During incubation tropical passerines have been shown to have lower levels of nest attentiveness than their counterparts at north temperate latitudes, spending a higher percentage of daylight time off the nest. This difference has been interpreted as evidence of parental restraint; tropical birds allocate more time to daily self-maintenance, perhaps preserving their higher annual survival rates and future breeding potential. But such comparisons are susceptible to the confounding effects of day length variation, because a given amount of time spent off the nest will account for a greater percentage of daylight time near to the equator than at high latitudes during spring and summer. Based on a pattern of increasing day length between 0° and 70°N we show that the impact of this bias is likely to be small where sites are separated by less than 30-40° of latitude, but should increase substantially both with latitudinal span and distance from the equator. To illustrate this effect we compared nest attentiveness in two congeners, breeding at 1°S and 52°N. During incubation, Stripe-breasted Tits Parus fasciiventer in Uganda had a shorter working day (time from emerging to retiring) than north temperate Great Tits P. major, and spent a higher percentage of daylight time off the nest (32%) than Great Tits in the UK (24%). However this difference was almost wholly explained by the latitudinal difference in day length; the amount of time spent off the nest differed by just 10 min d -1 (<1% of the 24-hr cycle). We show that this effect may be moderated by the change in working day length, which increased less rapidly (in relation to latitude) than day length. Although these effects can thus confound latitudinal comparisons of nest attentiveness, accentuating a pattern predicted by life history theory, they are avoidable if attentiveness is expressed simply as the percentage of time or the number of minutes spent incubating per 24 hours.

Life history traits associated with low annual fecundity in a central African Parid: the Stripe-breasted Tit Parus fasciiventer

“…Provisional findings have shown that Stripe-breasted Tits lay smaller clutches than north temperate Great Tits, have slower provisioning rates, sometimes breed cooperatively (Shaw 2003;Yatuha and Dranzoa 2010), and may have a higher annual survival rate (c. 0.75; P. Shaw, unpublished) than is typical of adult Great Tits (c. 0.50; Perrins 1979). Contrary to expectations, the two species show similar levels of nest attentiveness (Shaw and Cresswell 2014).…”

Nest predation research: recent findings and future perspectives

“…Increasingly some research groups are focusing work in tropical areas (e.g. Martin et al 2000Martin et al , 2015Ghalambor and Martin 2001;Shaw and Cresswell 2014), but we urge scientists with access to these and other relatively unstudied areas (i.e. boreal regions or arid habitats) to initiate the collection of baseline nesting and nest predator data.…”

The importance of nighttime length to latitudinal variation in avian incubation attentiveness

“…Incubation attentiveness affects egg temperature, and hence offspring traits, but inferences in comparative studies are sensitive to whether attentiveness is measured during the day or over a 24‐h period (Shaw and Cresswell ). Compared to 24‐h attentiveness, daytime attentiveness more closely reflects the ways in which parents trade‐off investment in reproductive effort versus adult self‐maintenance, particularly when incubation is done by a single sex and eggs cool while the parent forages or is otherwise off the nest (White and Kinney ).…”