Marked reduction in demographic rates and reduced fitness advantage for early breeding is not linked to reduced thermal matching of breeding time

Abstract

Warmer springs may cause animals to become mistimed if advances of spring timing, including available resources and of timing of breeding occur at different speed. We used thermal sums (cumulative sum of degree days) during spring to describe the thermal progression (timing) of spring and investigate its relationship to breeding phenology and demography of a long‐distant migrant bird, the northern wheatear (Oenanthe oenanthe L.). We first compare 20‐year trends in spring timing, breeding time, selection for breeding time, and annual demographic rates. We then explicitly test whether annual variation in selection for breeding time and demographic rates associates with the degree of phenological matching between breeding time and thermal progression of spring. Both thermal progression of spring and breeding time of wheatears advanced in time during the study period. But despite breeding on average 7 days earlier with respect to date, wheatears bred about 4 days later with respect to thermal spring progression. Over the same time period, selection for breeding time changed from distinct within‐season advantage of breeding early to no or very weak advantage. Furthermore, demographic rates (nest success, fledgling production, recruitment, adult survival) and nestling weight declined markedly by 16%–79%. Those temporal trends suggest that a reduced degree of phenological matching may affect within‐season fitness advantage of early breeding and population demographic rates. In contrast, when we investigate links based on annual variation, we find no significant relationship between either demographic rates or fitness advantage of early breeding with annual variation in the degree of phenological matching. Our results show that corresponding temporal trends in phenological matching, selection for breeding time and demographic rates are inconclusive evidence for demographic effects of changed phenological matching. Instead, we suggest that the trends in selection for breeding time and demographic rates are due to a general deterioration of the breeding environment.

Factors influencing plasticity in the arrival‐breeding interval in a migratory species reacting to climate change

“…Thus, the historical phenology of a population and its phenological responses to climate change will depend on the relative timing and change of these processes (Kristensen et al, ). Previous findings from this population questioned whether earlier breeding but declining demographic rates in this population result from “mismatched” phenologies or a general deterioration in environmental quality (Arlt & Pärt, ). However, without individual arrival data this picture is incomplete, as it is unclear whether some (or all) of the change in the timing of breeding relative to spring progression is related to constraints associated with arrival (e.g., Kristensen et al, ).…”

“…Thus, birds arrived earlier during the course of the study and there was no requirement for birds to further adjust their breeding time, and hence arrival‐breeding interval, beyond what was normal for this population (i.e., the arrival adjustment “pulled” the breeding time along with it). One potential problem with this explanation is that the arrival and breeding dates have not been keeping up with advances in spring progression at the breeding ground (Arlt & Pärt, ), meaning that birds are arriving progressively later relative to the advancement of spring, but then taking almost the same amount of time before they decide to breed. Assuming that spring progression influences the timing of initiating breeding, this suggests there are other factors influencing breeding decisions.…”

“…To address this issue, we study a long‐distance migratory bird, the northern wheatear ( Oenanthe oenanthe ) that breeds in Sweden and overwinters in Africa (Figure ). We investigate whether recent advances in its timing of breeding (Arlt & Pärt, ) were achieved by adjustment of arrival time, and/or the length of the arrival‐breeding interval. We also examine variation in the length of the arrival‐breeding interval using a Cox proportional hazards model to determine the factors related to the probability of initiating egg laying after arrival.…”

“…We examine these factors not only to gain insights into recent phenological changes in this species, but also to gain a general understanding of the factors that influence or constrain the timing of breeding in migratory birds. Thus, we ask the following questions: (a) How are recent changes in the patterns of breeding time in the northern wheatear (Arlt & Pärt, ) reflected in changes in its arrival times and the arrival‐breeding interval; (b) What factors are related to the probability of beginning egg laying after arrival (e.g., we expect female age, the timing of arrival and the degree of spring progression to influence the arrival‐breeding interval); and (c) How does the relative importance of these factors change throughout the breeding season? Like many seasonally breeding organisms the wheatear displays a seasonal decline in fitness related to breeding time (Öberg, Pärt, Arlt, Laugen, & Low, ; Pärt, Knape, Low, Öberg, & Arlt, ); thus, we would expect factors determining the length of the arrival‐breeding interval to vary in their importance depending on the timing of arrival.…”

Comparing two measures of phenological synchrony in a predator–prey interaction: Simpler works better

“…Naturally, this will not necessarily be true in all study systems: in species that are not highly dependent on a single food type, or whose food does not exhibit a well‐defined seasonal distribution, demographic processes will either depend more strongly on MO or on neither MD nor MO (Dunn, Winkler, Whittingham, Hannon, & Robertson, ; Durant et al, ). However, studies reporting fitness and demographic consequences in this context so far have generally used (proxies of) MD to quantify phenological mismatch and reported reduced fitness in years when temporal mismatch was high (Arlt & Pärt, ; Marrot et al, ; Plard et al, ; Regular et al, ). Durant et al (), on the other hand, quantified effects of MD and food abundance on population indices of reproductive success in three study systems and found that in two of them food abundance was a better predictor than MD.…”