Birds advancing lay dates with warming springs face greater risk of chick mortality

Abstract

In response to a warming planet with earlier springs, migratory animals are adjusting the timing of essential life stages. Although these adjustments may be essential for keeping pace with resource phenology, they may prove insufficient, as evidenced by population declines in many species. However, even when species can match the tempo of climate change, other consequences may emerge when exposed to novel conditions earlier in the year. Here, using three long-term datasets on bird reproduction, daily insect availability, and weather, we investigated the complex mechanisms affecting reproductive success in an aerial insectivore, the tree swallow (Tachycineta bicolor). By examining breeding records over nearly half a century, we discovered that tree swallows have continuously advanced their egg laying by ∼3 d per decade. However, earlier-hatching offspring are now exposed to inclement weather events twice as often as they were in the 1970s. Our long-term daily insect biomass dataset shows no long-term trends over 25 y but precipitous drops in flying insect numbers on days with low ambient temperatures. Insect availability has a considerable impact on chick survival: Even a single inclement weather event can reduce offspring survival by >50%. Our results highlight the multifaceted threats that climate change poses on migrating species. The decoupling between cold snap occurrence and generally warming spring temperatures can affect reproductive success and threaten long-term persistence of populations. Understanding the exact mechanisms that endanger aerial insectivores is especially timely because this guild is experiencing the steepest and most widespread declines across North America and Europe.

Keywords: climate change; climate variability; life history; migration.

Fig. 1.

Patterns in the changes of temperatures and lay dates in tree swallows. (A) Trends in tree swallow lay dates. Over the last 43 y, both earliest and median lay dates have advanced by nearly 2 wk at both study sites (solid symbols, Ithaca, NY; hollow symbols, Newark Valley, NY). (B) Daytime maximum temperatures from 1989 to 2015, compared with those from the previous 25 y (1965 to 1988), have averaged an annual average increase of only 0.51 °C (horizontal dotted line). However, seasonal temperatures immediately preceding egg laying (gray density plot) have been nearly 2 °C warmer, likely driving tree swallows to initiate earlier clutches. (C) Aerial insect biomass is positively correlated with daytime maximum temperature, and below a certain threshold (18.5 °C), tree swallows successfully fledge fewer chicks. A 3 °C decrease in daily maximum temperature from 18.5 to 15.5 °C results in a 50% decrease in the available aerial biomass to feed young. (D) The overall distribution of tree swallow hatch dates (with solid vertical lines at the medians) have shifted earlier from 1972 to 1976 (blue), to 1989 to1994 (gray), and 2011 to 2015 (red). Because random cold snap occurrence has not changed at the same rate as spring warming, the probability of nestlings experiencing a cold snap has almost doubled from once every 10 y to once every 5 y (change in probability from 11.5 to 19.6%) (filled gray line and points). (E) The effects of cold snap events are often dramatic, as even 1- to 2-d events of temperatures less than 18.5 °C can result in mass die-off events for young chicks. For example, 71% of all chicks died in a single day event on June 9, 2016. The white line is the average daily maximum temperature and the gray shading is ±1 SD. The colored points are the date of a peak mortality event. The colored trajectories to the left of each point are the daily maximum temperatures preceding the event and all are at or below 18.5 °C. (F) There is a significant relationship between the mean number of fledglings produced per nest and the difference in dates between the median annual hatch and that year’s last cold snap. Years in the blue region were on average exposed to cold-snap events. Positive numbers on the x axis correspond to the cold snap occurring after the median hatch date, whereas negative numbers are before median hatch (i.e., no cold snap experienced that year). Error bars correspond with ±SE.